Yarrowia lipolytica SRP receptor alpha-subunit.

The Yarrowia lipolytica SRP101 homologue encoding the signal recognition particle (SRP) receptor alpha-subunit (SRalphap) was cloned using degenerate primers designed for conserved GTP-binding domains. Sequencing of 2814 nucleotides revealed an open reading frame of 1671 base pairs encoding a putative protein of 557 amino acids with a predicted molecular mass of 61 kDa. Like other SRP101 homologues, Y. lipolytica SRP101 contains a highly conserved C-terminal GTP binding site. It has 44%, 34% and 22% sequence identity with S. cerevisiae, mammalian and Escherichia coli homologues, respectively. As found for SRP protein subunits of Y. lipolytica, SRP101 is important but not essential for cell growth. A conditional mutation in SRP101 affected synthesis/translocation of alkaline extracellular protease and Kar2p consistent with Srp101p functioning as an SRP receptor subunit. The SRP101 sequence has been deposited in GenBank under Accession No. AF132597.

Current awareness on yeast.

In order to keep subscribers up-to-date with the latest developments in their field, this current awareness service is provided by John Wiley & Sons and contains newly-published material on yeasts. Each bibliography is divided into 10 sections. 1 Books, Reviews & Symposia; 2 General; 3 Biochemistry; 4 Biotechnology; 5 Cell Biology; 6 Gene Expression; 7 Genetics; 8 Physiology; 9 Medical Mycology; 10 Recombinant DNA Technology. Within each section, articles are listed in alphabetical order with respect to author. If, in the preceding period, no publications are located relevant to any one of these headings, that section will be omitted. (3 weeks journals - search completed 21st June 2000)

Another factor besides hydrophobicity can affect signal peptide interaction with signal recognition particle.

Translocation of alkaline extracellular protease (AEP) into the endoplasmic reticulum of Yarrowia lipolytica is cotranslational and signal recognition particle (SRP)-dependent, whereas translocation of P17M AEP (proline to methionine at position 17, second amino acid in the pro-region) is posttranslational and SRP-independent. P17M signal peptide mutations that resulted in more rapid SRP-dependent translocation of AEP precursor were isolated. Most of these mutations significantly increased hydrophobicity, but the A12P/P17M mutation did not. The switch from SRP-dependent to SRP-independent translocation without a decrease in hydrophobicity (wild type to P17M) and restoration of SRP-dependent translocation without an increase in hydrophobicity (P17M to A12P/P17M) indicate that some factor(s) in addition to hydrophobicity determines selection of targeting pathway. Models of extended forms of wild type and A12P/P17M signal peptides are kinked, whereas the P17M signal peptide is relatively straight. Possibly the conformation/orientation of signal peptides at the ribosomal surface affects SRP binding and consequently the targeting route to the endoplasmic reticulum. Kinked signal peptides might approach SRP more closely more often. Most likely, these effects were only detectable because of the short length and low average hydrophobicity of the AEP signal peptide.

Recombinant bioprocess optimization for heterologous protein production using two-stage, cyclic fed-batch culture.

A two-stage, cyclic fed-batch bioprocess was designed, and its performance evaluated to improve rice alpha-amylase productivity by the yeast Yarrowia lipolytica SMY2 (MatA, ade1, ura3, xpr2), ATCC 201847, containing a replicative plasmid coding for a rice alpha-amlyase. Transcription of the recombinant gene is controlled by the XPR2 promoter. The first stage (or growth stage) was operated in the fed-batch mode, and the growth medium, designed to maintain a constant high cell density (i.e., 60 g/l), was fed according to a predetermined and preprogrammed optimal feed rate which, in turn, maintained the specific cell growth rate at an optimal value (i.e., 0.1 h-1). Typically, when the volume in the first stage reached a preset value, a portion of culture broth (i.e., 55%) was transferred to the second stage (or production stage). The remaining cells in the growth stage were then fed with fresh growth medium according to the bioprocess control strategy developed, while induction of alpha-amylase expression and its production was taking place in the second stage. The second stage was also operated in the fed-batch mode, and the production medium designed to maintain a constant high cell density and high productivity of heterologous protein was fed at a predetermined and preprogrammed rate, which maintained the specific cell growth rate at an optimal level. The volumetric alpha-amylase productivity achieved (1835 units l-1 h-1) from the two-stage, cyclic fed-batch culture process was twofold higher than that of the fed-batch culture process. The genetic stability of the recombinant strain and the design of optimal media for growth and production stages are also critically important to a successful implementation of the two-stage, cyclic fed-batch process for production of heterologous protein.

Four distinct secretory pathways serve protein secretion, cell surface growth, and peroxisome biogenesis in the yeast Yarrowia lipolytica.

We have identified and characterized mutants of the yeast Yarrowia lipolytica that are deficient in protein secretion, in the ability to undergo dimorphic transition from the yeast to the mycelial form, and in peroxisome biogenesis. Mutations in the SEC238, SRP54, PEX1, PEX2, PEX6, and PEX9 genes affect protein secretion, prevent the exit of the precursor form of alkaline extracellular protease from the endoplasmic reticulum, and compromise peroxisome biogenesis. The mutants sec238A, srp54KO, pex2KO, pex6KO, and pex9KO are also deficient in the dimorphic transition from the yeast to the mycelial form and are affected in the export of only plasma membrane and cell wall-associated proteins specific for the mycelial form. Mutations in the SEC238, SRP54, PEX1, and PEX6 genes prevent or significantly delay the exit of two peroxisomal membrane proteins, Pex2p and Pex16p, from the endoplasmic reticulum en route to the peroxisomal membrane. Mutations in the PEX5, PEX16, and PEX17 genes, which have previously been shown to be essential for peroxisome biogenesis, affect the export of plasma membrane and cell wall-associated proteins specific for the mycelial form but do not impair exit from the endoplasmic reticulum of either Pex2p and Pex16p or of proteins destined for secretion. Biochemical analyses of these mutants provide evidence for the existence of four distinct secretory pathways that serve to deliver proteins for secretion, plasma membrane and cell wall synthesis during yeast and mycelial modes of growth, and peroxisome biogenesis. At least two of these secretory pathways, which are involved in the export of proteins to the external medium and in the delivery of proteins for assembly of the peroxisomal membrane, diverge at the level of the endoplasmic reticulum.

Yarrowia lipolytica SRP54 homolog and translocation of Kar2p.

To investigate the role of Srp54p in protein translocation, the Yarrowia lipolytica SRP54 homolog was cloned. Sequencing revealed an open reading frame of 536 amino acids coding for a 57.2 kilodalton polypeptide with 55 to 57% sequence identity to Srp54ps of Saccharomyces cerevisiae, Schizosaccharomyces pombe, and mouse. Like these Srp54ps, Y. lipolytica Srp54p has an N-terminal domain with a highly conserved GTP-binding site and a methionine-rich C-terminal domain. Differing results regarding the essentiality of SRP subunits were obtained. SRP54 is important but not essential for growth, but it was reconfirmed that at least one SRP RNA gene is essential. Cells with SRP54 deleted grow about six times more slowly than wild type; faster-growing colonies, still growing much slower than wild type, appeared quite frequently. In srp54 delta cells, no untranslocated alkaline extracellular protease precursor was detected. Therefore, to develop another reporter molecule the Y. lipolytica KAR2 homolog was cloned and Kar2p antibodies were produced. For Kar2p an untranslocated precursor was detected in srp54 delta but not in wild-type cells, suggesting that its translocation was defective in the srp54 delta cells. These results confirm an in vivo rule for SRP in protein translocation in Y. lipolytica, suggest that SRP RNA or an SRP core-particle has functions not shared by Srp54p, and show that, as in S. cerevisiae and Sz. pombe, reporter molecules differ in their dependency on SRP for translocation.

Expression, secretion, and processing of rice alpha-amylase in the yeast Yarrowia lipolytica.

The gene encoding rice alpha-amylase in Oryza sativa was expressed in the yeast Yarrowia lipolytica, which is a potential host system for heterologous protein expression. For efficient secretion, the strong and inducible XPR2 promoter was used in the construction of four kinds of expression vectors with the following configurations between the XPR2 promoter and terminator: 1) XPR2 prepro-region-rice alpha-amylase coding sequence, 2) rice alpha-amylase signal peptide-rice alpha-amylase coding sequence, 3) XPR2 signal peptide-rice alpha-amylase coding sequence, and 4) XPR2 signal peptide-dipeptide stretch-rice alpha-amylase coding sequence. Secretion of active recombinant rice alpha-amylase into the culture medium was achieved only in the first two cases, demonstrating that the XPR2 signal peptide is not sufficient to direct the secretion of heterologous protein. Furthermore, our study shows that the XPR2 prepro-region causes imprecise processing (after Pro150-Ala151 or Val135-Leu136 instead of Lys156-Arg157) and leads to N-terminal amino acid sequences that differ from that of native rice alpha-amylase. Secondary structure analysis proposed that the structural form in the vicinity of the KEX2-like endopeptidase processing site in the XPR2 pro-region might play a critical role in the processing of heterologous proteins. These results suggest that the XPR2 pro-region is dispensable for obtaining the precise N-terminal amino acid in heterologous protein secretion. In contrast, utilizing the rice alpha-amylase signal peptide was sufficient in directing secretion of recombinant protein with the expected N-terminal sequence, indicating that the signal peptide of rice alpha-amylase was effectively recognized and processed by the Y. lipolytica secretory pathway.

Cloning, nucleotide sequence and functions of XPR6, which codes for a dibasic processing endoprotease from the yeast Yarrowia lipolytica.

Yarrowia lipolytica DO613, carrying the xpr6-13 mutation, secretes an inactive precursor of alkaline extracellular protease that has not been cleaved after the Lys-Arg at the end of the pro-region. Compared to wild type, DO613 membrane preparations had significantly reduced ability to cleave after Lys-Arg of an artificial substrate. The XPR6 gene was cloned by complementation by screening for restoration of production of alkaline protease activity. Sequencing of a 3735 base pair SalI-SphI XPR6 fragment revealed a large open reading frame with a coding capacity of 976 amino acids (molecular weight, 110,016). The deduced amino acid sequence had significant homology to Saccharomyces cerevisiae Kex2p, a processing endoprotease that cleaves after pairs of basic amino acids. Disruption of the XPR6 gene was not lethal, but it resulted in several phenotypic changes. First, essentially no mature alkaline extracellular protease was produced indicating that the low levels produced by strains carrying previously isolated xpr6 alleles were due to leaky mutations. Second, mating type B strains carrying the disrupted XPR6 gene did not mate, but mating type A strains did. Third, the XPR6 disruption strains grew poorly on rich media at pH 5.5 and above. Cells remained physically attached after budding and continued to bud forming large dog balloon-like structures. In addition, these structures aggregated forming visible clumps in liquid culture. These growth aberrations were largely eliminated by growing cells in medium at pH 4. Fourth, no mycelial forms were observed regardless of the pH.

Colocalization of centromeric and replicative functions on autonomously replicating sequences isolated from the yeast Yarrowia lipolytica.

Two sequences (ARS18 and ARS68) displaying autonomous replication activity were previously cloned in the yeast Yarrowia lipolytica. The smallest fragment (1-1.3 kb) required for extrachromosomal replication of a plasmid is significantly larger in Y. lipolytica than in Saccharomyces cerevisiae. Neither autonomously replicating sequence (ARS) is homologous with known ARS or centromere (CEN) consensus sequences. They share short regions of sequence similarity with each other. These ARS fragments also contain Y. lipolytica centromeres: (i) integration of marker genes at the ARS loci results in a CEN-linked segregation of the markers, (ii) an ARS on a plasmid largely maintains sister chromatid attachment in meiosis I, and (iii) integration of these sequences at the LEU2 locus leads to chromosome breakage. Deletions performed on ARS18 show that CEN and ARS functions can be physically separated, but both are needed to establish a replicating plasmid.

Yeast extracellular proteases.

Many species of yeast secrete significant amounts of protease(s). In this article, results of numerous surveys of yeast extracellular protease production have been compiled and inconsistencies in the data and limitations of the methodology have been examined. Regulation, purification, characterization, and processing of yeast extracellular proteases are reviewed. Results obtained from the sequences of cloned genes, especially the Saccharomyces cerevisiae Bar protease, the Candida albicans acid protease, and the Yarrowia lipolytica alkaline protease, have been emphasized. Biotechnological applications and the medical relevance of yeast extracellular proteases are covered. Yeast extracellular proteases have potential in beer and wine stabilization, and they probably contribute to pathogenicity of Candida spp. Yeast extracellular protease genes also provide secretion and processing signals for yeast expression systems designed for secretion of heterologous proteins. Coverage of the secretion of foreign proteases such as prochymosin, urokinase, and tissue plasminogen activator by yeast in included.

A mutation in the signal recognition particle 7S RNA of the yeast Yarrowia lipolytica preferentially affects synthesis of the alkaline extracellular protease: in vivo evidence for translational arrest.

Replacement of the signal recognition particle (SRP) 7S gene (SCR1) on a replicating plasmid with scr1-1 (G to A at 129 and A to T at 131 in the consensus sequence -GNAR- in the loop of domain III) resulted in temperature sensitivity for growth of cells in which both chromosomal SRP 7S RNA genes were deleted. Pulse-chase immunoprecipitation experiments were done after a shift to non-permissive temperature using the major secreted protein the alkaline extracellular protease (AEP) as a reporter molecule. No untranslocated AEP precursor was detected in a strain with scr1-1 on a plasmid, but the amount of the largest AEP precursor (55 kD) immunoprecipitated as a percentage of total protein synthesized was reduced 68% compared to an isogenic strain with SCR1 on the plasmid. The possibility that an untranslocated precursor was synthesized but not detected because of instability was largely eliminated by detection of a 53-kD untranslocated precursor of a mutated AEP (P17M; methionine replaced proline in the second position of the pro-peptide) which chased to the 55-kD translocated AEP precursor. Thus, SRP has a role in the biosynthesis of AEP. Possibly, the scr1-1 mutation does not affect signal recognition or translational arrest but instead results in maintenance of translational arrest of AEP synthesis. The results also suggest that AEP can be translocated in vivo either co-translationally in which SRP is at least involved in biosynthesis or posttranslationally without SRP involvement.

The yeast Yarrowia lipolytica has two, functional, signal recognition particle 7S RNA genes.

Cells containing a deletion of either the SCR1 or SCR2 genes, which code for the 7SL RNA component of the signal recognition particle (SRP) homologue, were found to be viable. Two independent approaches demonstrated that cells containing deletions of both genes were inviable. Therefore, Yarrowia lipolytica contains two (and only two) functional 7SL RNA genes.

A novel location for dipeptidyl aminopeptidase processing sites in the alkaline extracellular protease of Yarrowia lipolytica.

A stretch of 10 consecutive dipeptides with the sequence -X-Ala- or -X-Pro-, possible cleavage sites for dipeptidyl aminopeptidase (DPAPase) activity, are located in the prepro-region of the alkaline extracellular protease (AEP) beginning at Leu14. Evidence for DPAPase processing of this dipeptide stretch was obtained by characterizing the polypeptide secreted by a strain carrying a xpr6 mutation. The secreted polypeptide reacted with antibodies specific for AEP and was essentially identical to the 52-kilodalton intracellular AEP precursor based on mobility during sodium dodecyl sulfate-polyacrylamide gel electrophoresis, content of N-linked carbohydrate, and peptide mapping. Amino-terminal amino acid sequencing of this secreted precursor revealed that it consisted of at least three major polypeptides. One began at the end of the stretch of dipeptides, and two of the others began two and four amino acids upstream. These results confirm that DPAPase activity is involved in the formation of the 52-kilodalton AEP precursor. In other reported cases of DPAPase processing, the dipeptides are located directly upstream of the mature polypeptide. For AEP, the dipeptide stretch is located over 120 amino acids upstream from the N terminus of mature AEP. The novel location of the dipeptide stretch may provide a mechanism for preventing premature activation of AEP in the secretory pathway.

Intracellular precursors and secretion of alkaline extracellular protease of Yarrowia lipolytica.

Processing and secretion of the alkaline extracellular protease (AEP) from the yeast Yarrowia lipolytica was studied by pulse-chase and immunoprecipitation experiments. Over half of newly synthesized AEP was secreted by 6 min. Over 99% of AEP activity which was external to the cytoplasmic membrane was located in the supernatant medium. Polypeptides of 55, 52, 44, 36, and 32 kilodaltons (55K, 52K, 44K, 36K, and 32K polypeptides) were immunoprecipitated from [3H]leucine-labeled cell extracts by rabbit antibodies raised against mature, secreted AEP (32K polypeptide). Experiments with tunicamycin and endoglycosidase H indicated that the 55K, 52K, and 44K polypeptides contained about 2 kilodaltons of N-linked oligosaccharide and that the 36K and 32K polypeptides contained none. Results of pulse-chase experiments did not fit a simple precursor-product relationship of 55K----52K----44K----36K----32K. In fact, maximum labeling intensity of the 52K polypeptide occurred later than for the 44K and 36K polypeptides. Secretion of polypeptides of 19 and 20 kilodaltons derived from the proregion of AEP indicated that one major processing pathway was 55K----52K----32K. The gene coding for AEP (XPR2) was cloned and sequenced. The sequence and the immunoprecipitation results suggest that AEP is originally synthesized with an additional preproI-proII-proIII amino-terminal region. Processing definitely involves cleavage(s) after pairs of basic amino acids and the addition of one N-linked oligosaccharide. Signal peptidase cleavage, dipeptidyl aminopeptidase cleavages, and at least one additional proteolytic cleavage may also be involved.

Development of genetic maps of non-conventional yeasts.

Numerous methods based on classical genetics have been developed for the genetic mapping of yeasts. Recombinant DNA technology and technology for electrophoretic separation of chromosomes make new approaches possible. The state-of-the-art in genetic mapping of Saccharomyces cerevisiae will be briefly reviewed. Then the availability and application of genetic mapping methods to non-conventional yeasts will be surveyed. Development of the genetic maps of the asexual diploid Candida albicans and of the heterothallic yeast Yarrowia lipolytica will be discussed in more detail.

Processing and secretion of the Yarrowia lipolytica RNase.

Secretion of the extracellular RNase from the yeast Yarrowia lipolytica was studied in pulse-chase and immunoprecipitation experiments. A polypeptide of 45,000 daltons was immunoprecipitated from [35S]methionine-labeled cell extracts and supernatant medium by rabbit anti-RNase antiserum. The RNase was secreted rapidly; the time between synthesis and appearance in the extracellular medium was about 5 min. In pulse-chase experiments, about 50% of the RNase was still cell associated 30 min after labeling. A polypeptide of 73,000 daltons whose immunoprecipitation was blocked by an excess of purified RNase was also detected. It broke down to a polypeptide with the same mobility and same peptide map as the mature RNase. Peptide maps of the undegraded 73-kilodalton polypeptide and the intracellular mature RNase contained several peptides of identical mobility. Immunoprecipitates from cells labeled in the presence of tunicamycin contained 66- and 45-kilodalton polypeptides. Endoglycosidase H treatment of the 73-kilodalton polypeptide converted it to a 66-kilodalton form, but did not change the apparent molecular weight of the mature form of the RNase. Labeling kinetics from pulse-chase experiments did not clearly support a precursor-product relationship between the 73-kilodalton polypeptide and the intracellular 45-kilodalton form of the RNase, and other relationships between the two polypeptides are possible.

Extracellular RNase produced by Yarrowia lipolytica.

Production of extracellular RNase(s) by Yarrowia lipolytica CX161-1B was examined in media between pHs 5 and 7. RNase production occurred during the exponential growth phase. High-molecular-weight nitrogen compounds supported the highest levels of RNase production. Several RNases were detected in the supernatant medium. Based on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, the RNases had estimated molecular weights of 45,000, 43,000, and 34,000. It was found that Y. lipolytica secretes only one RNase (the 45,000-molecular-weight RNase) and that the 43,000 and 34,000-molecular-weight RNases are degradation products of this RNase. The alkaline extracellular protease secreted by Y. lipolytica was shown to have a major role in the 45,000- to 43,000-molecular-weight conversion, and it was demonstrated that the 45,000-molecular-weight RNase could be purified from a mutant which does not produce the alkaline extracellular protease. Purification of the RNase from a wild-type strain resulted in purification of the 43,000-molecular-weight RNase. This RNase was a glycoprotein with a molecular weight of 44,000 as estimated by gel filtration, an isoelectric point of pH 4.8, and a pH optimum between 6.5 and 7.0.

Residual effect of storage in an elevated carbon dioxide atmosphere on the microbial flora of rock cod (Sebastes spp.).

A residual inhibitory effect on microbial growth due to modified-atmosphere (MA) storage (MA, 80% CO2-20% air) was demonstrated for rock cod fillets stored in MA and transferred to air at 4 degrees C. Results of measurements of CO2 concentrations of the fillets suggested that the residual effect after transfer from MA to air was not due to retention of CO2 at the surface of the fillets but was probably due to the microbial ecology of the system. Lactobacillus spp. and tan Alteromonas spp. (TAN) predominated after 7 and 14 days of storage in MA. During storage in MA, Pseudomonas spp. were inhibited or killed. Following transfer from MA to air, the percentage of the total flora represented by Lactobacillus spp. and TAN bacteria decreased, and 6 days after transfer Pseudomonas spp. were again dominant.

Construction of lactose-assimilating and high-ethanol-producing yeasts by protoplast fusion.

The availability of a yeast strain which is capable of fermenting lactose and at the same time is tolerant to high concentrations of ethanol would be useful for the production of ethanol from lactose. Kluyveromyces fragilis is capable of fermenting lactose, but it is not as tolerant as Saccharomyces cerevisiae to high concentrations of ethanol. In this study, we have used the protoplast fusion technique to construct hybrids between auxotrophic strains of S. cerevisiae having high ethanol tolerance and an auxotrophic strain of lactose-fermenting K. fragilis isolated by ethyl methanesulfonate mutagenesis. The fusants obtained were prototrophic and capable of assimilating lactose and producing ethanol in excess of 13% (vol/vol). The complementation frequency of fusion was about 0.7%. Formation of fusants was confirmed by the increased amount of chromosomal DNA per cell. Fusants contained 8 x 10 to 16 x 10 mug of DNA per cell as compared with about 4 x 10 mug of DNA per cell for the parental strains, suggesting that multiple fusions had taken place.

Genetic Adaptation to Elevated Carbon Dioxide Atmospheres by Pseudomonas-Like Bacteria Isolated from Rock Cod (Sebastes spp.).

The microorganisms on rock cod fillets stored in a modified atmosphere (MA; 80% CO(2)-20% air) at 4 degrees C for 21 days were isolated. Only Lactobacillus sp. (71 to 87%) and tan-colored Pseudomonas sp.-like isolates (TAN isolates) were found. The TAN isolates grew more slowly in MA than in air at 8 degrees C. When TAN isolates were grown in air at 8 degrees C and then transferred to MA at 8 degrees C, there was an initial decline in viable counts for 10 to 30 h followed by exponential growth. During this exponential growth phase in MA, the growth rates of the TAN isolates from MA-stored fish were significantly greater than those of the TAN isolates from fresh fish. When a TAN isolate from fresh fish was grown under MA for 21 days, it then grew as rapidly under MA as isolates from MA-stored fish. These results suggest that the TAN isolates genetically adapt to high levels of CO(2).