Heme A is not essential for assembly of the subunits of cytochrome c oxidase of Rhodobacter sphaeroides.

The aa(3)-type cytochrome c oxidase of Rhodobacter sphaeroides, a proteobacterium of the alpha subgroup, is structurally similar to the core subunits of the terminal oxidase in the mitochondrial electron transport chain. Subunit I, the product of the coxI gene, normally binds two heme A molecules. A deletion of cox10, the gene for the farnesyltransferase required for heme A synthesis, did not prevent high level accumulation of subunit I in the cytoplasmic membrane. Thus, subunit I can be expressed and stably inserted into the cytoplasmic membrane in the absence of heme A. Aposubunit I was purified via affinity chromatography to a polyhistidine tag. Copurification of subunits II and III with aposubunit I indicated that assembly of the core oxidase complex occurred without the binding of heme A. In addition to formation of the apooxidase containing all three large structural proteins, CoxI-II and CoxI-III heterodimers were isolated from cox10 deletion strains harboring expression plasmids with coxI and coxII or with coxI and coxIII, respectively. This demonstrated that subunit assembly of the apoenzyme was not an inherently ordered or sequential process. Thus, multiple paths must be considered for understanding the assembly of this integral membrane metalloprotein complex.

The home stretch, a first analysis of the nearly completed genome of Rhodobacter sphaeroides 2.4.1.

Rhodobacter sphaeroides 2.4.1 is an alpha-3 purple nonsulfur eubacterium with an extensive metabolic repertoire. Under anaerobic conditions, it is able to grow by photosynthesis, respiration and fermentation. Photosynthesis may be photoheterotrophic using organic compounds as both a carbon and a reducing source, or photoautotrophic using carbon dioxide as the sole carbon source and hydrogen as the source of reducing power. In addition, R. sphaeroides can grow both chemoheterotrophically and chemoautotrophically. The structural components of this metabolically diverse organism and their modes of integrated regulation are encoded by a genome of approximately 4.5 Mb in size. The genome comprises two chromosomes CI and CII (2.9 and 0.9 Mb, respectively) and five other replicons. Sequencing of the genome has been carried out by two groups, the Joint Genome Institute, which carried out shotgun-sequencing of the entire genome and The University of Texas-Houston Medical School, which carried out a targeted sequencing strategy of CII. Here we describe our current understanding of the genome when data from both of these groups are combined. Previous work had suggested that the two chromosomes are equal partners sharing responsibilities for fundamental cellular processes. This view has been reinforced by our preliminary analysis of the virtually completed genome sequence. We also have some evidence to suggest that two of the plasmids, pRS241a and pRS241b encode chromosomal type functions and their role may be more than that of accessory elements, perhaps representing replicons in a transition state.

Identification of the structural subunits required for formation of the metal centers in subunit I of cytochrome c oxidase of Rhodobacter sphaeroides.

Genetic manipulation of the aa(3)-type cytochrome c oxidase of Rhodobacter sphaeroides was used to determine the minimal structural subunit associations required for the assembly of the heme A and copper centers of subunit I. In the absence of the genes for subunits II and III, expression of the gene for subunit I in Rb. sphaeroides allowed purification of a form of free subunit I (subunit I(a)()) that contained a single heme A. No copper was present in this protein, indicating that the heme a(3)-Cu(B) active site was not assembled. In cells expressing the genes for subunits I and II, but not subunit III, two oxidase forms were synthesized that were copurified by histidine affinity chromatography and separated by anion-exchange chromatography. One form was a highly active subunit I-II oxidase containing a full complement of structurally normal metal centers. This shows that association of subunit II with subunit I is required for stable formation of the active site in subunit I. In contrast, subunit III is not required for the formation of any of the metal centers or for the production of an oxidase with wild-type activity. The second product of the cells lacking subunit III was a large amount of a free form of subunit I that appeared identical to subunit I(a)(). Since significant amounts of subunit I(a)() were also isolated from wild-type cells, it is likely that subunit I(a)() will be present in any preparation of the aa(3)-type oxidase isolated via an affinity tag on subunit I.

Cox11p is required for stable formation of the Cu(B) and magnesium centers of cytochrome c oxidase.

Assembly of the core subunits of the aa(3)-type cytochrome c oxidase in mitochondria and aerobic bacteria such as Rhodobacter sphaeroides requires the association of three subunits and the formation of five to seven metal centers. Several assembly proteins are required for the late stages of oxidase assembly in eukaryotes; some of these are also present in Rb. sphaeroides. To investigate the role of one of these proteins, Cox11p, the mitochondrial-like oxidase of Rb. sphaeroides was overexpressed and purified from cells that lacked cox11, the gene for Cox11p. The oxidase that assembled in the absence of Cox11p lacked Cu(B) at the active site and contained greatly reduced amounts of metal at the magnesium/manganese-binding site between subunits I and II. This inactive oxidase, however, did contain hemes a and a(3), Cu(A), and all three subunits. These results indicate that Cox11p is required at a late, perhaps final, step in the assembly of cytochrome oxidase, most likely the insertion of Cu(B). Oxidase which assembled in a strain with a low copy number of cox11 appeared nearly wild type, suggesting that Cox11p is required in substoichiometric amounts for its role in oxidase assembly.

Suicide inactivation of cytochrome c oxidase: catalytic turnover in the absence of subunit III alters the active site.

The catalytic core of cytochrome c oxidase is composed of three subunits: I, II, and III. Subunit III is a highly hydrophobic membrane protein that contains no redox centers; its role in cytochrome oxidase function is not obvious. Here, subunit III has been removed from the three-subunit mitochondrial-like oxidase of Rhodobacter sphaeroides by detergent washing. The resulting two-subunit oxidase, subunit III (-), is highly active. Ligand-binding analyses and resonance Raman spectroscopy show that its heme a(3)-Cu(B) active site is normal. However, subunit III (-) spontaneously and irreversibly inactivates during O(2) reduction. At pH 7.5, its catalytic lifetime is only 2% that of the normal oxidase. This suicide inactivation event primarily alters the active site. Its ability to form specific O(2) reduction intermediates is lost, and CO binding experiments suggest that the access of O(2) to reduced heme a(3) is inhibited. Reduced heme a accumulates in response to a decrease in the redox potential of heme a(3); electron transfer between the hemes is inhibited. Ligand-binding experiments and resonance Raman analysis show that increased flexibility in the structure of the active site accompanies inactivation. Cu(B) is partially lost. It is proposed that suicide inactivation results from the dissociation of a ligand of Cu(B) and that subunit III functions to prevent suicide inactivation by maintaining the structural integrity of the Cu(B) center via long-range interactions.

Heterologous expression of correctly assembled methylamine dehydrogenase in Rhodobacter sphaeroides.

The biosynthesis of methylamine dehydrogenase (MADH) from Paracoccus denitrificans requires four genes in addition to those that encode the two structural protein subunits, mauB and mauA. The accessory gene products appear to be required for proper export of the protein to the periplasm, synthesis of the tryptophan tryptophylquinone (TTQ) prosthetic group, and formation of several structural disulfide bonds. To accomplish the heterologous expression of correctly assembled MADH, eight genes from the methylamine utilization gene cluster of P. denitrificans, mauFBEDACJG, were placed under the regulatory control of the coxII promoter of Rhodobacter sphaeroides and introduced into R. sphaeroides by using a broad-host-range vector. The heterologous expression of MADH was constitutive with respect to carbon source, whereas the native mau promoter allows induction only when cells are grown in the presence of methylamine as a sole carbon source and is repressed by other carbon sources. The recombinant MADH was localized exclusively in the periplasm, and its physical, spectroscopic, kinetic and redox properties were indistinguishable from those of the enzyme isolated from P. denitrificans. These results indicate that mauM and mauN are not required for MADH or TTQ biosynthesis and that mauFBEDACJG are sufficient for TTQ biosynthesis, since R. sphaeroides cannot synthesize TTQ. A similar construct introduced into Escherichia coli did not produce detectable MADH activity or accumulation of the mauB and mauA gene products but did lead to synthesizes of amicyanin, the mauC gene product. This finding suggests that active recombinant MADH is not expressed in E. coli because one of the accessory gene products is not functionally expressed. This study illustrates the potential utility of R. sphaeroides and the coxII promoter for heterologous expression of complex enzymes such as MADH which cannot be expressed in E. coli. These results also provide the foundation for future studies on the molecular mechanisms of MADH and TTQ biosynthesis, as well as a system for performing site-directed mutagenesis of the MADH gene and other mau genes.

Overexpression and purification of cytochrome c oxidase from Rhodobacter sphaeroides.

The aa3-type cytochrome c oxidase of Rhodobacter sphaeroides has been overexpressed up to seven fold over that in wild-type strains by engineering a multicopy plasmid with all the required oxidase genes and by establishing optimum growth conditions. The two operons containing the three structural genes and two assembly genes for cytochrome c oxidase were ligated into a pUC19 vector and reintroduced into several oxidase-deleted R. sphaeroides strains. Under conditions of relatively high pH and maximal aeration, high levels of expression were observed. A smaller expression vector, pBBR1MCS, and a fructose promoter (fruP)5 were found not to enhance cytochrome c oxidase expression in R. sphaeroides. An improved cytochrome c oxidase purification protocol is reported, which combines histidine elution from a nickel affinity column and anion-exchange chromatography, and results in a higher yield and purity than previously obtained.

Factors which stabilize the methylamine dehydrogenase-amicyanin electron transfer protein complex revealed by site-directed mutagenesis.

Methylamine dehydrogenase (MADH) and amicyanin form a physiologic complex within which electrons are transferred from the tryptophan tryptophylquinone (TTQ) cofactor of MADH to the type 1 copper of amicyanin. Interactions responsible for complex formation may be inferred from the crystal structures of complexes of these proteins. Site-directed mutagenesis has been performed to probe the roles of specific amino acid residues of amicyanin in stabilizing the MADH-amicyanin complex and determining the observed ionic strength dependence of complex formation. Conversion of Phe97 to Glu severely disrupted binding, establishing the importance of hydrophobic interactions involving this residue. Conversion of Arg99 to either Asp or to Leu increased the Kd for complex formation by 2 orders of magnitude at low ionic strength, establishing the importance of ionic interactions which were inferred from the crystal structure involving Arg99. Conversion of Lys68 to Ala did not disrupt binding at low ionic strength, but it did greatly diminish the observed ionic strength dependence of complex formation that is seen with wild-type amicyanin. These results demonstrate that the physiologic interaction between MADH and amicyanin is stabilized by a combination of ionic and van der Waals interactions and that individual amino acid residues on the protein surface are able to dictate specific interactions between these soluble redox proteins. These results also indicate that the orientation of MADH and amicyanin when they react with each other in solution is the same as the orientation of the proteins which is seen in the structure of the crystallized protein complex.

Polar residues in helix VIII of subunit I of cytochrome c oxidase influence the activity and the structure of the active site.

The aa3-type cytochrome c oxidase from Rhodobacter sphaeroides is closely related to eukaryotic cytochrome c oxidases. Analysis of site-directed mutants identified the ligands of heme a, heme a3, and CuB [Hosler et al. (1993) J. Bioenerg. Biomembr. 25, 121-133], which have been confirmed by high-resolution structures of homologous oxidases [Iwata et al. (1995) Nature 376, 660; Tsukihara et al. (1995) Science 269, 1069; (1996) 272, 1136]. Since the protons used to form water originate from the inner side of the membrane, and the heme a3-CuB center is located near the outer surface, the protein must convey these substrate protons to the oxygen reduction site. Transmembrane helix VIII in subunit I is close to this site and contains several conserved polar residues that could function in a rate-determining proton relay system. To test this role, apolar residues were substituted for T352, T359, and K362 in helix VIII and the mutants were characterized in terms of activity and structure. Mutation of T352, near CuB, strongly decreases enzyme activity and disrupts the spectral properties of the heme a3-CuB center. Mutation of T359, below heme a3, substantially reduces oxidase activity with only minor effects on metal center structure. Two mutations of K362, approximately 15 A below the axial ligand of heme a3, are inactive, make heme a3 difficult to reduce, and cause changes in the resonance Raman signal specific for the iron-histidine bond to heme a3. The results are consistent with a key role for T352, T359, and K362 in oxidase activity and with the involvement of T359 and K362 in proton transfer through a relay system now plausibly identified in the crystal structure. However, the characteristics of the K362 mutants raise some questions about the assignment of this as the substrate proton channel.

A ligand-exchange mechanism of proton pumping involving tyrosine-422 of subunit I of cytochrome oxidase is ruled out.

The molecular mechanism by which proton pumping is coupled to electron transfer in cytochrome c oxidase has not yet been determined. However, several models of this process have been proposed which are based on changes occurring in the vicinity of the redox centers of the enzyme. Recently, a model was described in which a well-conserved tyrosine residue in subunit I (Y422) was proposed to undergo ligand exchange with the histidine ligand (H419) of the high-spin heme a3 during the catalytic cycle, allowing both residues to serve as part of a proton transporting system. Site-directed mutants of Y422 have been constructed in the aa3-type cytochrome c oxidase of Rhodobacter sphaeroides to test this hypothesis (Y422A, Y422F). The results demonstrate that Y422 is not an essential residue in the electron transfer and proton pumping mechanisms of cytochrome c oxidase. However, the results support the predicted proximity of Y422 to heme a3, as now confirmed by crystal structure. In addition, it is shown that the pH-dependent reversed electron transfer between heme a and heme a3 is normal in the Y422F mutant. Hence, these data also demonstrate that Y422 is not the residue previously postulated to interact electrostatically with heme a3, nor is it responsible for the unique EPR characteristics of heme a in this bacterial oxidase.

Two conformations of the catalytic site in the aa3-type cytochrome c oxidase from Rhodobacter sphaeroides.

Resonance Raman spectra of the carbon monoxy derivative of the aa3-type cytochrome c oxidase from Rhodobacter sphaeroides show two distinct Fe-CO stretching modes (519 and 493 cm-1) at room temperature. The frequency of the mode at 519 cm-1 coincides with that of other terminal oxidases at neutral pH. Two C-O stretching modes, one at 1966 cm-1 and one at 1955 cm-1, are also found. The splitting of the C-O stretching mode is consistent with the FTIR spectra of cytochrome c oxidases at cryogenic temperatures in which two different conformations (alpha and beta) of the catalytic site of the enzyme are present. The splitting of both the Fe-CO and C-O stretching modes under our conditions indicates that these two forms of the enzyme are also present at room temperature, and with the additional information on the Fe-CO modes provided here, a structural origin for the two forms may be postulated. The alpha-form has the same general structure of the active site as mammalian oxidase, a structure in which the copper atom that is the part of the Fe-CuB binuclear site interacts strongly with the bound CO. We postulate that the copper atom exerts a strong polar or steric effect on the heme-bound CO, resulting in either compression of the Fe-CO bond or distortion of the Fe-CO moiety.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of site-directed mutants locates a non-redox-active metal near the active site of cytochrome c oxidase of Rhodobacter sphaeroides.

Substoichiometric amounts of Mn are bound by the aa3-type cytochrome c oxidase of Rhodobacter sphaeroides and appear in the EPR spectrum of the purified enzyme as signals that overlay those of CuA in the g = 2.0 region. The Mn is tightly bound and not removed by a high degree of purification or by washing with 50 mM EDTA. The amount of bound Mn varies with the ratio of Mg to Mn in the growth medium. Oxidase containing no EPR-detectable Mn can be prepared from cells grown in low Mn/Mg, while high Mn/Mg in the growth medium gives rise to near stoichiometric levels (0.7 mol/mol of aa3). Incubation of purified Mn-deficient oxidase with 1 mM Mn does not allow incorporation into the tight binding site, indicating that this site is not accessible in the assembled protein. When bound Mn is depleted by growth in high Mg, there is no change in electron transfer activity, suggesting that Mg may substituted for Mn and maintain protein structure. Analysis of site-directed mutants in an extramembrane loop close to the active site of cytochrome oxidase identifies His-411 and Asp-412 of subunit I as probable ligands of the Mn. Mutation of either residue leads to lower activity and loss of Mn binding, even in cells grown in elevated concentrations of Mn. Since Mn binding correlates with the [Mn] to [Mg] ratio in the culture medium, we propose that Mn competes for the site that normally binds a stoichiometric Mg ion in aa3-type cytochrome c oxidases.

A continuous wave and pulsed EPR characterization of the Mn2+ binding site in Rhodobacter sphaeroides cytochrome c oxidase.

The ligation environment of the tightly bound Mn2+ in cytochrome c oxidase from Rhodobacter sphaeroides has been characterized by electron paramagnetic resonance (EPR) and electron spin echo envelope modulation (ESEEM). The EPR data show that the Mn2+ is six-coordinate and located in a highly symmetric binding site. Analyses of X- and Q-band EPR spectra show that the zero field splitting parameter D is 115 +/- 25 G (0.0107 +/- 0.0023 cm-1) in the fully oxidized enzyme and 125 +/- 15 G (0.0117 +/- 0.0014 cm-1) in the fully reduced enzyme. For both redox forms of the enzyme the value of E is < or = 25 G (0.0023 cm-1). By comparison with crystal structures of Mn2+ binding proteins, the structural changes at the Mn2+ binding site upon redox state change of the enzyme are estimated to be < or = 0.2 A in ligand bond lengths and < or = 10 degrees in bond angle. This analysis indicates that little modification occurs at the Mn2+ site upon redox change at the other metal centers. Considering the proximity of the Mn2+ site to heme a and heme a3-CuB [Hosler, J. P., Espe, M. P., Zhen, Y., Babcock, G. T., & Ferguson-Miller, S. (1995) Biochemistry 34, 7586-7592], we interpret these results to imply also that there is no large protein conformational change near the heme a and heme a3-CuB sites upon a change in their redox states. Multifrequency 3-pulse ESEEM results provide direct evidence for a nitrogen ligand to the Mn2+, which is assigned to a histidine by comparison with ESEEM studies of Mn(2+)-bound lectins [McCracken, J., Peisach, J., Bhattacharyya, L., & Brewer, F. (1991) Biochemistry 30, 4486-4491] and specifically to His-411 in subunit 1 on the basis of mutagenesis studies (Hosler et al., 1995). From these results a partial model of the Mn2+ binding site has been constructed.

A loop between transmembrane helices IX and X of subunit I of cytochrome c oxidase caps the heme a-heme a3-CuB center.

Site-directed mutants were prepared of four consecutive and highly conserved residues (His-411, Asp-412, Thr-413, Tyr-414) of an extramembrane loop that connects putative transmembrane helices IX and X of subunit I of Rhodobacter sphaeroides cytochrome c oxidase. The modified enzymes were purified and analyzed by optical, resonance Raman, FTIR, and EPR spectroscopies. Consistent with our recent model in which both hemes are ligated to histidines of helix X [Hosler, J. P., et al. (1993) J. Bioenerg. Biomembr. 25, 121-136], substitutions for three of these four residues cause perturbations of either heme a or heme a3. Resonance Raman spectra of the mutant Y414F demonstrate that Tyr-414 does not participate in a hydrogen bond with the heme a formyl group, but its alteration does result in a 5-nm red-shift of the alpha-band of the visible spectrum, indicating proximity to heme a. The mutant D412N shows changes in resonance Raman and FTIR difference spectra indicative of an effect on the proximal ligation of heme a3. Changing His-411 to alanine has relatively minor effects on the spectral and functional properties of the oxidase; however, FTIR spectra reveal alterations in the environment of CuB. Conversion of this residue to asparagine strongly disrupts the environment of heme a3 and CuB and inactivates the enzyme. These results suggest that His-411 is very near the heme a3-CuB pocket. We propose that these residues form part of a cap over the heme a-heme a3-CuB center and thus are important in the structure of the active site.

Identity of the axial ligand of the high-spin heme in cytochrome oxidase: spectroscopic characterization of mutants in the bo-type oxidase of Escherichia coli and the aa3-type oxidase of Rhodobacter sphaeroides.

Prokaryotic and eukaryotic cytochrome c oxidases and several bacterial ubiquinol oxidases compose a superfamily of heme-copper oxidases. These enzymes are terminal components of aerobic respiratory chains, the principal energy-generating systems of aerobic organisms. Two such heme-copper oxidases are the aa3-type cytochrome c oxidase of Rhodobacter sphaeroides and the bo-type ubiquinol oxidase of Escherichia coli. These enzymes catalyze the reduction of oxygen to water at a heme-copper binuclear center. Energy conservation is accomplished by coupling electron transfer through the metals of the oxidases to proton translocation across the cellular membrane. The Rb. sphaeroides and E. coli enzymes have previously been utilized in site-directed mutagenesis studies which identified two histidines which bind the low-spin heme (heme a), as well as additional histidine residues which are probable ligands for copper (CuB). However, the histidine that binds the heme of the binuclear center (heme a3) could not be unequivocally identified between two residues (His284 and His419). Additional characterization by Fourier transform infrared spectroscopy of the CO-bound forms of the E. coli enzyme in which His284 is replaced by glycine or leucine demonstrates that these mutations cause only subtle changes to CO bound to the heme of the binuclear center. Resonance Raman spectroscopy of the Rb. sphaeroides enzyme in which His284 is replaced by alanine shows that the iron-histidine stretching mode of heme a3 is maintained, in contrast with the loss of this mode in mutants at His419. These results demonstrate that His284 is not the heme a3 ligand.(ABSTRACT TRUNCATED AT 250 WORDS)

Insight into the active-site structure and function of cytochrome oxidase by analysis of site-directed mutants of bacterial cytochrome aa3 and cytochrome bo.

Cytochrome aa3 of Rhodobacter sphaeroides and cytochrome bo of E. coli are useful models of the more complex cytochrome c oxidase of eukaryotes, as demonstrated by the genetic, spectroscopic, and functional studies reviewed here. A summary of site-directed mutants of conserved residues in these two enzymes is presented and discussed in terms of a current model of the structure of the metal centers and evidence for regions of the protein likely to be involved in proton transfer. The model of ligation of the heme a3 (or o)-CuB center, in which both hemes are bound to helix X of subunit I, has important implications for the pathways and control of electron transfer.

Cytochrome aa3 of Rhodobacter sphaeroides as a model for mitochondrial cytochrome c oxidase. Purification, kinetics, proton pumping, and spectral analysis.

Aerobically grown Rhodobacter sphaeroides synthesizes a respiratory chain similar to that of eukaryotes. We describe the purification of the aa3-type cytochrome c oxidase of Rb. sphaeroides as a highly active (Vmax > or = 1800 s-1), three-subunit enzyme from isolated, washed cytoplasmic membranes by hydroxylapatite chromatography and anion exchange fast protein liquid chromatography. The purified oxidase exhibits biphasic kinetics of oxidation of mammalian cytochrome c, similar to mitochondrial oxidases, and pumps protons efficiently (H+/e- = 0.7) following reconstitution into phospholipid vesicles. A membrane-bound cytochrome c is associated with the aa3-type oxidase in situ, but is removed during purification. The EPR spectra of the Rb. sphaeroides enzyme suggest the presence of a strong hydrogen bond to one or both of the histidine ligands of heme a. In other respects, optical, EPR, and resonance Raman analyses of the metal centers and their protein environments demonstrate a close correspondence between the bacterial enzyme and the structurally more complex bovine cytochrome c oxidase. The results establish this bacterial oxidase as an excellent model system for the mammalian enzyme and provide the basis for site-directed mutational analysis of its energy transducing function.

Definition of the catalytic site of cytochrome c oxidase: specific ligands of heme a and the heme a3-CuB center.

The three-subunit aa3-type cytochrome c oxidase (EC 1.9.3.1) of Rhodobacter sphaeroides is structurally and functionally homologous to the more complex mitochondrial oxidase. The largest subunit, subunit I, is highly conserved and predicted to contain 12 transmembrane segments that provide all the ligands for three of the four metal centers: heme a, heme a3, and CuB. A variety of spectroscopic techniques identify these ligands as histidines. We have used site-directed mutagenesis to change all the conserved histidines within subunit I of cytochrome c oxidase from Rb. sphaeroides. Analysis of the membrane-bound and purified mutant proteins by optical absorption and resonance Raman spectroscopy indicates that His-102 and His-421 are the ligands of heme a, while His-284, His-333, His-334, and His-419 ligate the heme a3-CuB center. To satisfy this ligation assignment, helices II, VI, VII, and X, which contain these histidine residues, must be in close proximity. These data provide empirical evidence regarding the three-dimensional protein structure at the catalytic core of cytochrome c oxidase.

Relationship between Gene Dosage and Gene Expression in the Chloroplast of Chlamydomonas reinhardtii.

Expression of three chloroplast genes encoding proteins of different chloroplast complexes and the rRNA gene has been examined in cells having reduced numbers of chloroplast genomes as a result of growth in the presence of the thymidine analog 5-fluorodeoxyuridine. While accumulation of total mRNA for rpl2 (ribosomal protein L-1), rbcL (ribulose bisphosphate carboxylase large subunit) and atpA (alpha-subunit of ATP synthase) declined with gene copy number, the levels of translatable mRNA and rates of synthesis of these three proteins were largely unaffected. Accumulation of rRNA declined less precipitously than mRNA levels for the three proteins in response to the reduction in chloroplast genome number. Chlamydomonas appears to compensate for reductions in the number of chloroplast genomes at several different levels. Populations of cells with only one-fourth the wild-type amount of chloroplast DNA per cell on average have half the normal level of chloroplast ribosomes and nearly normal rates of CO(2) fixation and levels of specific chloroplast encoded proteins. These results suggest that normal cells accumulate a large excess of transcripts for chloroplast genes and that levels of expression of these genes are regulated by posttranscriptional mechanisms.

mRNAs for two ribosomal proteins are preferentially translated in the chloroplast of Chlamydomonas reinhardtii under conditions of reduced protein synthesis.

Two mutants of the green alga Chlamydomonas reinhardtii, one deficient in the small subunit of the chloroplast ribosome and the other having chloroplast ribosomes with reduced function under certain conditions, show a characteristic syndrome of photosynthetic defects resulting from reduced chloroplast protein synthesis. These include subnormal levels of ribulose 1,5-bisphosphate carboxylase (Rubisco), reduced Hill reaction activity, diminished capacity to fix CO2, and abnormal thylakoid stacking. However, these mutants accumulate normal appearing chloroplast ribosome monomers or large subunits containing normal ribosomal protein components. In this paper, we demonstrate that pulse-labeled cells of these mutants synthesize two large subunit chloroplast ribosomal proteins at about 60% of the wild-type rate, whereas Rubisco large subunit (LSU) and the alpha subunit of CF1 are made at only 4 to 8% of the wild-type rate. No difference in the rate of turnover between ribosomal proteins and Rubisco LSU in mutant and wild-type cells was observed during a subsequent 60 min chase. Differences between the mutants and wild-type cells in the relative synthesis rates of these proteins were not reflected in the relative levels of mRNA (either hybridizable or in vitro translatable). In aggregate, these data suggest that C. reinhardtii preferentially translates chloroplast ribosomal protein mRNAs under conditions of reduced total chloroplast protein synthesis.