Chloroplast DNA methylation and inheritance in Chlamydomonas.

When Chlamydomonas reinhardtii cells mate, a zygotic maturation program is activated, part of which leads to destruction of chloroplast DNA (cpDNA) from the mating type minus (mt-) parent, and, therefore, to uniparental inheritance of mating type plus (mt+) cpDNA. A long-standing model that explains the selective destruction of mt(-) cpDNA in zygotes invokes a methylation-restriction system. We tested this model by using the potent methylation inhibitor 5-aza-2'-deoxycytidine (5adc) to hypomethylate parental cpDNA and found that the pattern of cpDNA inheritance is altered by 5adc in a manner that is consistent with the model. Surprisingly, however, hypomethylated mt+ cpDNA is not destroyed in zygotes as the methylation-restriction model predicts it should be. Destruction of mt- cpDNA is also unaffected when the parental mt+ cpDNA is hypomethylated. Instead, loss of methylation affects the relative rates of replication of residual mt- cpDNA and mt+ cpDNA in germinating zygotes. The mode of action for 5adc on cpDNA replication in germinating zygotes may be via hypomethylation of mt+ cpDNA, but is also consistent with its action as a DNA-damaging agent. Interestingly, 5adc causes reduced cpDNA replication only in germinating zygotes, not in vegetatively grown cells, indicating that cpDNA replication is qualitatively different in these two stages of the life cycle. Our results demonstrate that methylation is not necessary for protection of the mt+ cpDNA in early zygotes and uncover a novel stage of the Chlamydomonas life cycle when replication of cpDNA is highly susceptible to perturbation. Our data support a model in which differential cpDNA replication in germinating zygotes is used as a mechanism to selectively amplify intact and properly methylated cpDNA molecules.

Control of cell division by a retinoblastoma protein homolog in Chlamydomonas.

A key pathway that controls both cell division and differentiation in animal cells is mediated by the retinoblastoma (RB) family of tumor suppressors, which gate the passage of cells from G(1) to S and through S phase. The role(s) of the RB pathway in plants are not yet clearly defined, nor has there been any evidence for its presence in unicellular organisms. Here we have identified an RB homolog encoded by the mat3 gene in Chlamydomonas reinhardtii, a unicellular green alga in the land plant lineage. Chlamydomonas cells normally grow to many times their original size during a prolonged G(1) and then undergo multiple alternating rounds of S phase and mitosis to produce daughter cells of uniform size. mat3 mutants produce small daughter cells and show defects in two size-dependent cell cycle controls: They initiate the cell cycle at a below-normal size, and they undergo extra rounds of S phase/mitosis. Unlike mammalian RB mutants, mat3 mutants do not have a shortened G(1), do not enter S phase prematurely, and can exit the cell cycle and differentiate normally, indicating that the RB pathway in Chlamydomonas has a different role than in animals.

Glycosylated polyproline II rods with kinks as a structural motif in plant hydroxyproline-rich glycoproteins.

Hydroxyproline-rich glycoproteins (HRGPs) are the major proteinaceous components of higher plant walls and the predominant components of the cell wall of the green alga Chlamydomonas reinhardtii. The GP1 protein, an HRGP of the C. reinhardtii wall, is shown to adopt a polyproline II helical configuration and to carry a complex array of arabinogalactoside residues, many branched, which are necessary to stabilize the helical conformation. The deduced GP1 amino acid sequence displays two Ser-Pro-rich domains, one with a repeating (SP)(x)() motif and the other with a repeating (PPSPX)(x)() motif. A second cloned gene a2 also carries the PPSPX repeat, defining a novel gene family in this lineage. The SP-repeat domains of GP1 form a 100-nm shaft with a flexible kink 28 nm from the head. The gp1 gene encodes a PPPPPRPPFPANTPM sequence at the calculated kink position, generating the proposal that this insert interrupts the PPII helix, with the resultant kink exposing amino acids necessary for GP1 to bind to partner molecules. It is proposed that similar kinks in the higher plant HRGPs called extensins may play a comparable role in wall assembly.

Rapid evolution of sex-related genes in Chlamydomonas.

Biological speciation ultimately results in prezygotic isolation-the inability of incipient species to mate with one another-but little is understood about the selection pressures and genetic changes that generate this outcome. The genus Chlamydomonas comprises numerous species of unicellular green algae, including numerous geographic isolates of the species C. reinhardtii. This diverse collection has allowed us to analyze the evolution of two sex-related genes: the mid gene of C. reinhardtii, which determines whether a gamete is mating-type plus or minus, and the fus1 gene, which dictates a cell surface glycoprotein utilized by C. reinhardtii plus gametes to recognize minus gametes. Low stringency Southern analyses failed to detect any fus1 homologs in other Chlamydomonas species and detected only one mid homolog, documenting that both genes have diverged extensively during the evolution of the lineage. The one mid homolog was found in C. incerta, the species in culture that is most closely related to C. reinhardtii. Its mid gene carries numerous nonsynonymous and synonymous codon changes compared with the C. reinhardtii mid gene. In contrast, very high sequence conservation of both the mid and fus1 sequences is found in natural isolates of C. reinhardtii, indicating that the genes are not free to drift within a species but do diverge dramatically between species. Striking divergence of sex determination and mate recognition genes also has been encountered in a number of other eukaryotic phyla, suggesting that unique, and as yet unidentified, selection pressures act on these classes of genes during the speciation process.

Mating type in Chlamydomonas is specified by mid, the minus-dominance gene.

Diploid cells of Chlamydomonas reinhardtii that are heterozygous at the mating-type locus (mt+/mt-) differentiate as minus gametes, a phenomenon known as minus dominance. We report the cloning and characterization of a gene that is necessary and sufficient to exert this minus dominance over the plus differentiation program. The gene, called mid, is located in the rearranged (R) domain of the mt- locus, and has duplicated and transposed to an autosome in a laboratory strain. The imp11 mt- mutant, which differentiates as a fusion-incompetent plus gamete, carries a point mutation in mid. Like the fus1 gene in the mt+ locus, mid displays low codon bias compared with other nuclear genes. The mid sequence carries a putative leucine zipper motif, suggesting that it functions as a transcription factor to switch on the minus program and switch off the plus program of gametic differentiation. This is the first sex-determination gene to be characterized in a green organism.

A sex recognition glycoprotein is encoded by the plus mating-type gene fus1 of Chlamydomonas reinhardtii.

Sexual fusion between plus and minus gametes of the unicellular green alga Chlamydomonas reinhardtii entails adhesion between plus-specific and minus-specific "fringe" proteins displayed on the plasma membrane of gametic mating structures. We report the identification of the gene (fus1) encoding the plus fringe glycoprotein, which resides in a unique domain of the mating-type plus (mt+) locus, and which was identified by transposon insertions in three fusion-defective mutant strains. Transformation with fus1+ restores fringe and fusion competence to these mutants and to the pseudo-plus mutant imp11 mt-, defective in minus differentiation. The fus1 gene is remarkable in lacking the codon bias found in all other nuclear genes of C. reinhardtii.

A mating type-linked mutation that disrupts the uniparental inheritance of chloroplast DNA also disrupts cell-size control in Chlamydomonas.

An intriguing feature of early zygote development in Chlamydomonas reinhardtii is the active elimination of chloroplast DNA from the mating-type minus parent due presumably to the action of a zygote-specific nuclease. Meiotic progeny thus inherit chloroplast DNA almost exclusively from the mating-type plus parent. The plus-linked nuclear mutation mat3 prevents this selective destruction of minus chloroplast DNA and generates progeny that display a biparental inheritance pattern. Here we show that the mat3 mutation creates additional phenotypes not previously described: the cells are much smaller than wild type and they possess substantially reduced amounts of both mitochondrial and chloroplast DNA. We propose that the primary defect of the mat3 mutation is a disruption of cell-size control and that the inhibition of the uniparental transmission of chloroplast genomes is a secondary consequence of the reduced amount of chloroplast DNA in the mat3 parent.

The iso1 gene of Chlamydomonas is involved in sex determination.

Sexual differentiation in the heterothallic alga Chlamydomonas reinhardtii is controlled by two mating-type loci, mt+ and mt-, which behave as a pair of alleles but contain different DNA sequences. A mutation in the mt minus-linked imp11 gene has been shown previously to convert a minus gamete into a pseudo-plus gamete that expresses all the plus gametic traits except the few encoded by the mt+ locus. Here we describe the iso1 mutation which is unlinked to the mt- locus but is expressed only in minus gametes (sex-limited expression). A population of minus gametes carrying the iso1 mutation behaves as a mixture of minus and pseudo-plus gametes: the gametes isoagglutinate but they do not fuse to form zygotes. Further analysis reveals that individual gametes express either plus or minus traits: a given cell displays one type of agglutinin (flagellar glycoprotein used for sexual adhesion) and one type of mating structure. The iso1 mutation identifies a gene unlinked to the mating-type locus that is involved in sex determination and the repression of plus-specific genes.

Domain conservation in several volvocalean cell wall proteins.

Based on our previous work demonstrating that (SerPro)x epitopes are common to extensin-like cell wall proteins in Chlamydomonas' reinhardtii, we looked for similar proteins in the distantly related species C. eugametos. Using a polyclonal antiserum against a (SerPro)10 oligopeptide, we found distinct sets of stage-specific polypeptides immunoprecipitated from in vitro translations of C. eugametos RNA. Screening of a C. eugametos cDNA expression library with the antiserum led to the isolation of a cDNA (WP6) encoding a (SerPro)x-rich multidomain wall protein. Analysis of a similarly selected cDNA (VSP-3) from a C. reinhardtii cDNA expression library revealed that it also coded for a (SerPro)x-rich multidomain wall protein. The C-terminal rod domains of VSP-3 and WP6 are highly homologous, while the N-terminal domains are dissimilar; however, the N-terminal domain of VSP-3 is homologous to the globular domain of a cell wall protein from Volvox carteri. Exon shuffling might be responsible for this example of domain conservation over 350 million years of volvocalean cell wall protein evolution.

The mating-type locus of Chlamydomonas reinhardtii contains highly rearranged DNA sequences.

The mating-type locus of Chlamydomonas reinhardtii exists as two apparent alleles (mt+ and mt-) that control mating in haploid gametes and sporulation and meiosis in diploid mt+/mt- zygotes. Twelve genes, seven unrelated to life cycle transitions, are tightly linked to mt, suggesting that the locus exerts recombinational suppression. A 1.1 Mb chromosome walk from a gene linked to mt demonstrates that the mt+ and mt- loci carry four intrachromosomal translocations, two inversions, and large deletions and duplications within a 190 kb sector, presumably accounting for the recombinational suppression that extends through 640 kb of flanking homologous DNA. The rearranged domain also carries blocks of mt(+)- and mt(-)-specific sequences, at least one of which includes a mt(+)-specific gene. The locus has the properties of an incipient sex chromosome.

A mating type-linked gene cluster expressed in Chlamydomonas zygotes participates in the uniparental inheritance of the chloroplast genome.

A characteristic feature of early zygote development in Chlamydomonas is the selective degradation of chloroplast DNA from the mating type minus parent. The zygote-specific gene cluster ezy-1 is linked to the mating type locus and is transcribed almost immediately upon zygote formation. We show here that the acidic Ezy-1 polypeptide is rapidly transported to both the plus and minus chloroplasts, where it interacts with each chloroplast nucleoid. Expression of ezy-1 is selectively inhibited when plus, but not minus, gametes are briefly ultraviolet irradiated just prior to mating, a treatment known to disrupt the uniparental inheritance of chloroplast traits. We propose that the Ezy-1 polypeptide participates in the destruction of the minus chloroplast DNA in zygotes and thus the uniparental inheritance of chloroplast traits. The ezy-1 gene represents a valuable molecular probe for dissecting mechanisms underlying organelle inheritance.

Isodityrosine cross-linking mediates insolubilization of cell walls in Chlamydomonas.

Enzymatic removal of the cell wall induces vegetative Chlamydomonas reinhardtii cells to transcribe wall genes and synthesize new hydroxyproline-rich glycoproteins (HRGPs) related to the extensins found in higher plant cell walls. A cDNA expression library made from such induced cells was screened with antibodies to an oligopeptide containing the (SP)x repetitive domains found in Chlamydomonas wall proteins. One of the selected cDNAs encodes an (SP)x-rich polypeptide that also displays a repeated YGG motif. Ascorbate, a peroxidase inhibitor, and tyrosine derivatives were shown to inhibit insolubilization of both the vegetative and zygotic cell walls of Chlamydomonas, suggesting that oxidative cross-linking of tyrosines is occurring. Moreover, insolubilization of both walls was concomitant with a burst in H2O2 production and in extracellular peroxidase activity. Finally, both isodityrosine and dityrosine were found in hydrolysates of the insolubilized vegetative wall layer. We propose that the formation of tyrosine cross-links is essential to Chlamydomonas HRGP insolubilization.

Activation of adenylyl cyclase in Chlamydomonas reinhardtii by adhesion and by heat.

Adhesion between Chlamydomonas reinhardtii gametes generates a rapid rise in cAMP levels which stimulates mating responses and zygotic cell fusion (Pasquale and Goodenough, 1987). We show here that sexual adhesion in vivo results in a twofold stimulation of flagellar adenylyl cyclase activity when the enzyme is subsequently assayed in vitro, a stimulation that is specifically blocked by Cd2+. A twofold stimulation is also elicited by the in vitro presentation of soluble cross-linking reagents (antisera and concanavalin A). In contrast, the 10-30-fold stimulation of the flagellar cyclase by in vitro exposure to 40 degrees C, first described by Zhang et al. (1991), is insensitive to Cd2+ but sensitive to such drugs as trifluoperizine and dibucaine. The capacity for twofold stimulation is displayed by the vegetative and gametic enzymes but is lost when gametes fuse to form zygotes; in contrast, the 10-fold stimulation is displayed by the gametic and zygotic enzymes but not the vegetative enzyme. The signal-defective mutant imp-3 fails to generate the normal mating-triggered cAMP production and can be rescued by exogenous dibutyryl cAMP. It displays normal basal rates of flagellar cyclase activity and a normal twofold stimulation by sexual adhesion and by soluble cross-linkers, but it is defective in 40 degrees C activation. The gametic cell-body adenylyl cyclase is stimulated when wild-type flagella, but not imp-3 flagella, undergo adhesive interactions in vivo, and it can be directly stimulated in vitro by cAMP presentation. We propose that the two levels of flagellar cyclase stimulation reflect either sequential steps in the activation of a single cyclase enzyme, with imp-3 blocked in the second step, or else the sequential activation of two different flagellar enzymes, with imp-3 defective in the second enzyme. We further propose that the cell-body enzyme is activated by the cAMP that is generated when flagellar cyclase activity is fully stimulated.

The role of calcium in the Chlamydomonas reinhardtii mating reaction.

The mating reaction of Chlamydomonas reinhardtii entails a rapid series of cell-cell interactions leading to cell fusion. We have demonstrated (Pasquale, S. M., and U. Goodenough. 1987. J. Cell Biol. 105:2279-2293) that cAMP plays a key role in this process: gametic flagellar adhesion elicits a sharp increase in intracellular cAMP, and presentation of dibutyryl-cAMP to unmated gametes elicits all known mating responses. The present study evaluates the role of Ca2+ in this system. We document that the mating-induced increase in cAMP, and hence the mating responses themselves, are blocked by a variety of drugs known to interfere with Ca(2+)-sensitive processes. These data suggest that Ca(2+)-mediated events may couple adhesion to the generation of cAMP. Such events, however, appear to be localized to the flagellar membrane; we find no evidence for the mating-related increase in cytosolic free Ca2+ that has been postulated by others. Indeed, by monitoring the length of the Ca(2+)-sensitive centrin-containing nucleus-basal body connector, we show that cytosolic free Ca2+ levels, if anything, decrease in response to cAMP signaling. We confirm a previous report that Ca2+ levels increase in the mating medium, but document that this represents a response to augmented cAMP levels and not a prelude. Finally, we show that IP3 levels remain constant throughout the mating reaction. These results are discussed in terms of the various signal transduction systems that have now been identified in Chlamydomonas.

Tipping of flagellar agglutinins by gametes of Chlamydomonas reinhardtii.

The migration of cross-linked agglutinins to the gametic flagellar tips (tipping) is a hallmark of the Chlamydomonas mating reaction. In this study, an assay was developed to analyze the kinetics and biological requirements for the tipping response: isolated flagella from mt- gametes of C. reinhardtii were allowed to agglutinate to the immotile flagella of pf-18 mt+ gametes, and their migration to the tips was monitored by phase microscopy. The tipping process is shown to require both adhesion and elevated levels of cAMP. The cAMP may activate tipping motors directly. In addition, cAMP stimulates the recruitment of agglutinins to the flagellar surface to replace those inactivated by adhesion. These results are compared with previous studies on the tipping of flagellar surface proteins cross-linked by soluble ligands, and an integrated model is presented.

A gene/pseudogene tandem duplication encodes a cysteine-rich protein expressed during zygote development in Chlamydomonas reinhardtii.

The Class V zygote-specific gene from Chlamydomonas reinhardtii has been cloned and sequenced. This gene encodes a polypeptide of 86 amino acids, which contains a signal peptide and 6 cysteine residues arranged in an inverted symmetrical repeat. The Class V gene product is postulated to be a component of the zygote cell wall. Southern analysis revealed two tandemly oriented and closely linked copies of the Class V gene, designated A and B. The A gene appears to be a pseudogene, based on analysis of Class V cDNAs, primer extension with gene-specific primers, and Northern analysis which failed to detect an A gene transcript. Genetic analysis using a related Chlamydomonas species that lacks the A gene, but which produces normal zygotic progeny, further indicates that the A gene is not required for zygote development.

Molecular characterization of a zygote wall protein: an extensin-like molecule in Chlamydomonas reinhardtii.

The green alga Chlamydomonas reinhardtii elaborates two biochemically and morphologically distinct cell walls during its life cycle: one surrounds the vegetative and gametic cell and the other encompasses the zygote. Hydroxyproline-rich glycoproteins (HRGPs) constitute a major component of both walls. We describe the isolation and characterization of a zygote-specific gene encoding a wall HRGP. The derived amino acid sequence of this algal HRGP is similar to those of higher plant extensins, rich in proline and serine residues and possessing repeating amino acid motifs, notably X(Pro)3 and (Ser-Pro)n. Antiserum against this zygote wall protein detected common epitopes in several other zygote polypeptides, at least one of which is also encoded by a zygote-specific gene. We conclude that there is one set of HRGP wall genes expressed only in zygotes and another set that is specific to vegetative and gametic cells.