Structure of the yeast HIS5 gene responsive to general control of amino acid biosynthesis

MIG1 as a positive regulator for the histidine biosynthesis pathway and as a global regulator in thermotolerant yeast Kluyveromyces marxianus

Kmmig1 as a disrupted mutant of MIG1 encoding a regulator for glucose repression in Kluyveromyces marxianus exhibits a histidine-auxotrophic phenotype. Genome-wide expression analysis revealed that only HIS4 in seven HIS genes for histidine biosynthesis was down-regulated in Kmmig1. Consistently, introduction of HIS4 into Kmmig1 suppressed the requirement of histidine. Considering the fact that His4 catalyzes four of ten steps in histidine biosynthesis, K. marxianus has evolved a novel and effective regulation mechanism via Mig1 for the control of histidine biosynthesis. Moreover, RNA-Seq analysis revealed that there were more than 1,000 differentially expressed genes in Kmmig1, suggesting that Mig1 is directly or indirectly involved in the regulation of their expression as a global regulator.

Bidirectional-genetics platform, a dual-purpose mutagenesis strategy for filamentous fungi

Rapidly increasing fungal genome sequences call for efficient ways of generating mutants to translate quickly gene sequences into their functions. A reverse genetic strategy via targeted gene replacement (TGR) has been inefficient for many filamentous fungi due to dominant production of undesirable ectopic transformants. Although large-scale random insertional mutagenesis via transformation (i.e., forward genetics) facilitates high-throughput uncovering of novel genes of interest, generating a huge number of transformants, which is necessary to ensure the likelihood of mutagenizing most genes, is time-consuming. We propose a new strategy, entitled the Bidirectional-Genetics (BiG) platform, which combines both forward and reverse genetic strategies by recycling ectopic transformants derived from TGR as a source for random insertional mutants. The BiG platform was evaluated using the rice blast fungus Magnaporthe oryzae as a model. Over 10% of >1,000 M. oryzae ectopic transformants, generated during disruption of specific genes, displayed abnormality in vegetative growth, pigmentation, and/or asexual reproduction. In this pool of putative mutants, we isolated insertional mutants with mutations in three genes involved in histidine biosynthesis (MoHIS5), vegetative growth (MoVPS74), or conidiophore formation (MoFRQ) (where "Mo" indicates "M. oryzae"), supporting the utility of this platform for systematic gene function studies.

A modular and extensible RNA-based gene-regulatory platform for engineering cellular function

Engineered biological systems hold promise in addressing pressing human needs in chemical processing, energy production, materials construction, and maintenance and enhancement of human health and the environment. However, significant advancements in our ability to engineer biological systems have been limited by the foundational tools available for reporting on, responding to, and controlling intracellular components in living systems. Portable and scalable platforms are needed for the reliable construction of such communication and control systems across diverse organisms. We report an extensible RNA-based framework for engineering ligand-controlled gene-regulatory systems, called ribozyme switches, that exhibits tunable regulation, design modularity, and target specificity. These switch platforms contain a sensor domain, comprised of an aptamer sequence, and an actuator domain, comprised of a hammerhead ribozyme sequence. We examined two modes of standardized information transmission between these domains and demonstrate a mechanism that allows for the reliable and modular assembly of functioning synthetic RNA switches and regulation of ribozyme activity in response to various effectors. In addition to demonstrating examples of small molecule-responsive, in vivo functional, allosteric hammerhead ribozymes, this work describes a general approach for the construction of portable and scalable gene-regulatory systems. We demonstrate the versatility of the platform in implementing application-specific control systems for small molecule-mediated regulation of cell growth and noninvasive in vivo sensing of metabolite production.

Yrb1p interaction with the gsp1p C terminus blocks Mog1p stimulation of GTP release from Gsp1p

Mog1p, a multicopy suppressor of gsp1, the temperature-sensitive mutant of the Saccharomyces cerevisiae Ran homologue, binds to GTP-Gsp1p but not to GDP-Gsp1p. The function of Mog1p in the Ran cycle is as yet unknown. This study found that Mog1p releases a nucleotide from GTP-Gsp1p but not from GDP-Gsp1p. Yrb1p, the S. cerevisiae homologue of RanBP1, which is a strong inhibitor of RCC1-stimulated nucleotide release, also inhibited the Mog1p-stimulated nucleotide release from GTP-Gsp1p. At a concentration corresponding to the molar concentration of GTP-Gsp1p, Yrb1p completely inhibited the Mog1p-stimulated nucleotide release. Consistently, the Yrb1p.GTP-Gsp1p complex was more stable than the Mog1p.GTP-Gsp1p complex. Yrb1p did not inhibit the Mog1p-stimulated nucleotide release from GTP-Gsp1DeltaC. The Gsp1DeltaC protein lacks the final eight amino acids of the C terminus, and for this reason, the interaction between GTP-Gsp1DeltaC and Yrb1p was strongly reduced. On the other hand, Mog1p binds to GTP-Gsp1DeltaC more efficiently than to GTP-Gsp1p.

Analysis of a YAC with human telomeres and oriP from epstein-barr virus in yeast and 293 cells

One approach to the construction and propagation of a mammalian artificial chromosome is to build it up in Saccharomyces cerevisiae, using a yeast artificial chromosome (YAC) base. We have demonstrated that circular YACs carrying the Epstein-Barr virus origin of plasmid replication ( oriP ) are maintained as stable, episomal elements in human cells. We wished to determine whether this technology could be extended, to generate linear extrachromosomal elements. Here, we describe the generation of retrofitting constructs, which permit the addition of human telomeres and the oriP domain to YACs. The constructs contain 0.8 kb of human telomere sequence separated by a unique Not I site from 0.7 kb of Tetrahymena telomere sequence. These constructs seed telomere formation with approximately 40-60% efficiency in human 293-EBNA and HT1080 cells whether or not the Tetrahymena sequence is removed by Not I digestion. A detailed analysis demonstrates that YACs carrying the human telomere cassettes on both arms show instability of the telomere sequences in S.cerevisiae at a frequency of approximately 50%. Introduction of correctly retrofitted, linear oriP YACs into human 293-EBNA cells by lipofection resulted in the generation of circular extrachromosomal elements varying in size from 8 to 300 kb. However, no apparently linear YACs could be detected, suggesting that extrachromosomal maintenance of DNA with the oriP /EBNA-1 system is not compatible with linear molecules capped by telomeres.

Recombination trapping: an in-vivo approach to recover cDNAs encoded in YACs

We have developed an approach to identify and localize cDNAs encoded by YACs. In this scheme, a YAC truncation vector containing a cDNA library is used to interrupt the YAC by homologous recombination in yeast. This approach generates YACs truncated at the site of recombination between the cDNA and the cognate YAC sequence and thus localizes the gene in the YAC. This method results in the production of a large percentage of true recombinants identifying gene encoding regions of the genome. This approach is shown to identify an unique EST sequence from a YAC in Xp22, the recently described transketolase-like gene in a YAC from Xq28 and a putative kinesin-like gene in Xq13. This system should also be useful in the mapping of YACs by targeted integration. We have constructed a new telomere truncation vector, pGR8, which incorporates two selectable markers, HIS5 and LYS2. This vector overcomes problems of previous vectors including: incompatibility with most YAC libraries, vector homology with the YAC arms and high backgrounds resulting from the use of a single selectible marker. A third counterselection with 5-fluoroorotic acid (5FOA) against yeast clones retaining the URA3 gene was also employed to reduce background further. Therefore, this vector and approach should be useful to the transcriptional analysis of YAC maps of any genome.

A new yeast artificial chromosome vector designed for gene transfer into mammalian cells

This report describes the construction of a new yeast artificial chromosome (YAC) vector designed for gene transfer into mammalian cells. For ease of use, the two arms of the vector were cloned separately. The vector harbours the Neo and Hyg genes for dominant selection in mammalian cells, a putative human origin of replication, a synthetic matrix attachment region and two loxP sites (one on each arm). The cloning ability of the vector was demonstrated by successful propagation of the cDNA of the cystic fibrosis gene, CFTR, as a YAC in Saccharomyces cerevisiae. A YAC containing the entire CFTR gene was also constructed by retrofitting the two arms of a pre-existing clone (37AB12) with the two arms of the novel vector. Both the cDNA and entire gene containing YACs were circularized in yeast by inducible expression of the Cre recombinase. Recombination occurred very specifically at the loxP sequences present on the two arms of the YAC. Applications of the vector to gene transfer are discussed.

Paralogous histidine biosynthetic genes: evolutionary analysis of the Saccharomyces cerevisiae HIS6 and HIS7 genes

The HIS6 gene from Saccharomyces cerevisiae strain YNN282 is able to complement both the S. cerevisiae his6 and the Escherichia coli hisA mutations. The cloning and the nucleotide sequence indicated that this gene encodes a putative phosphoribosyl-5-amino-1-phosphoribosyl-4-imidazolecarboxiamide isomerase (5' Pro-FAR isomerase, EC 5.3.1.16) of 261 amino acids, with a molecular weight of 29,554. The HIS6 gene product shares a significant degree of sequence similarity with the prokaryotic HisA proteins and HisF proteins, and with the C-terminal domain of the S. cerevisiae HIS7 protein (homologous to HisF), indicating that the yeast HIS6 and HIS7 genes are paralogous. Moreover, the HIS6 gene is organized into two homologous modules half the size of the entire gene, typical of all the known prokaryotic hisA and hisF genes. The structure of the yeast HIS6 gene supports the two-step evolutionary model suggested by Fani et al. (J. Mol. Evol. 1994; 38: 489-495) to explain the present-day hisA and hisF genes. According to this idea, the hisF gene originated from the duplication of an ancestral hisA gene which, in turn, was the result of an earlier gene elongation event involving an ancestral module half the size of the extant gene. Results reported in this paper also suggest that these two successive paralogous gene duplications took probably place in the early steps of molecular evolution of the histidine pathway, well before the diversification of the three domains, and that this pathway was one of the metabolic activities of the last common ancestor. The molecular evolution of the yeast HIS6 and HIS7 genes is also discussed.

Isolation and characterization of mutations affecting expression of the delta9- fatty acid desaturase gene, OLE1, in Saccharomyces cerevisiae

Expression of the delta9- fatty acid desaturase gene, OLE1, of Saccharomyces cerevisiae is negatively regulated transcriptionally and post-transcriptionally by unsaturated fatty acids. In order to isolate mutants exhibiting irregulation of OLE1 expression, we constructed an OLE1p-PHO5 fusion gene as a reporter consisting of the PHO5 gene encoding repressible acid phosphatase (rAPase) under the control of the OLE1 promoter (OLE1p). By EMS mutagenesis, we isolated three classes of mutants, pfo1, pfo2 and pfo3 positive regulatory factor for OLE1) mutants, which show decreased rAPase activity under derepression conditions (absence of oleic acid). Analysis of the transcription of OLE1 in these pfo mutants revealed that pfo1 and pfo3 mutants have a defect in the regulation of OLE1 expression at the transcriptional level while pfo2 mutants were suggested to have a mutation affecting OLE1 expression at a post-transcriptional step. In addition, four other classes of mutants, nfo1, nfo2, nfo3 and nfo4 (negative factor for OLE1) mutants that have mutations causing strong expression of the OLE1p-PHO5 fusion gene under repression conditions (presence of oleic acid), were isolated. Results of Northern analysis of OLE1 as well as OLE1p-PHO5 transcripts in nfo mutants suggested that these mutations occurred in genes encoding global repressors. We also demonstrated that TUP1 and SSN6 gene products are required for full repression of OLE1 gene expression, by showing that either tup1 or ssn6 mutations greatly increase the level of the OLE1 transcript.

Construction and characterization of a rad51rad52 double mutant as a host for YAC libraries

RAD52 or RAD51 recombination-deficient yeast strains stabilize otherwise unstable YACs containing ribosomal DNA or the human color vision locus (Kohno et al., 1994). Thus the RAD52RAD51 pathways(s) are apparently involved in the instability of YACs containing tandem repeat loci, presumably by promoting recombination-based deletion formation. Some other genomic loci are still unstable or unrecoverable in those strains, but we now find that greater stability is observed in a rad51rad52 double mutant strain that we have newly constructed. YACs containing a highly unstable region around DXS49 or centromeric regions throw off a variety of products in single mutants, but are much more stable in the rad51rad52 strain, which could therefore provide a better host for library construction and maintenance.

Yrb2p, a Nup2p-related yeast protein, has a functional overlap with Rna1p, a yeast Ran-GTPase-activating protein

The Ran-GTPase cycle is important for nucleus-cytosol exchange of macromolecules and other nuclear processes. We employed the two-hybrid method to identify proteins interacting with Ran and the Ran GTP/GDP exchange factor. Using PRP20, encoding the Ran GTP/GDP exchange factor, we identified YRB1, previously identified as a protein able to interact with human Ran GTP/GDP exchange factor RCC1 in the two-hybrid system. Using GSP1, encoding the yeast Ran, as bait, we isolated YRB2. YRB2 encodes a protein containing a Ran-binding motif similar to that found in Yrb1p and Nup2p. Yrb1p is located in the cytosol whereas Nup2p is nuclear. Similar to Yrb1p, Yrb2p bound to GTP-Gsp1p but not to GDP-Gsp1p and enhanced the GTPase-activating activity of Rna1p. However, unlike Yrb1p, Yrb2p did not inhibit the nucleotide-releasing activity of Prp20p. While overproduction of Yrb1p inhibited the growth of a mutant possessing a PRP20 mutation (srm1-1) and suppressed the rna1-1 mutation, overproduction of Yrb2p showed no effect on the growth of these mutants. Disruption of YRB2 made yeast cold sensitive and was synthetically lethal with rna1-1 but not with nup2delta. Nuclear protein import and the mRNA export were normal in strains possessing mutations of YRB2. We propose that Yrb2p is involved in the nuclear processes of the Ran-GTPase cycle which are not related to nucleus-cytosol exchange of macromolecules.

Putative GTPase Gtr1p genetically interacts with the RanGTPase cycle in Saccharomyces cerevisiae

In order to identify a protein interacting with RCC1, a guanine nucleotide-exchange factor for the nuclear GTPase Ran, we isolated a series of cold-sensitive suppressors of mtr1-2, a temperature-sensitive mutant of the Saccharomyces cerevisiae RCC1 homologue. One of the isolated suppressor mutants was mutated in the putative GTPase Gtr1p, being designated as gtr1-11. It also suppressed other alleles of mtr1-2, srm1-1 and prp20-1 in contrast to overexpression of the S. cerevisiae Ran/TC4 homologue Gsp1p, previously reported to suppress prp20-1, but not mtr1-2 or srm1-1. Furthermore, gtr1-11 suppressed the rna1-1, temperature-sensitive mutant of the Gsp1p GTPase-activating protein, but not the srp1-31, temperature-sensitive mutant of the S. cerevisiae importin alpha homologue. mtr1-2, srm1-1 and prp20-1 were also suppressed by overexpression of the mutated Gtr1p, Gtr1-11p. In summary, Gtr1p that was localized in the cytoplasm by immunofluoresence staining was suggested to function as a negative regulator for the Ran/TC4 GTPase cycle.

Mutations causing high basal level transcription that is independent of transcriptional activators but dependent on chromosomal position in Saccharomyces cerevisiae

Two single (bel2 and bel4) and two double (bel3 bel7 and bel5 be16) mutations causing enhanced transcription of a gene fusion, consisting of the open reading frame of PHO5 connected to the HIS5 promoter (HIS5p) integrated at the ura3 or leu2 locus, were isolated from a gcn4-disrupted mutant of Saccharomyces cerevisiae. The PHO5 gene, encoding repressible acid phosphatase, in the HIS5p-PHO5 construct was derepressed under amino acid starved conditions by the action of the transcriptional activator Gcn4p. The bel mutants showed temperature-sensitive cell growth and/or cell aggregation. All the mutants except bel4 also showed high levels of transcription of an intact PHO5 DNA integrated at the URA3 locus in the absence of the cognate transcriptional activator, Pho4p, and in the absence of upstream activating sequences of PHO5. The HIS5 and PHO5 genes at their original chromosomal positions were, however, not affected by the bel2 mutation. The BEL2 gene was found to be identical with SIN4/TSF3, mutations in which cause high levels of transcription of the HO and GAL genes in the absence of their respective transcriptional activators, Swi5p and Gal4p. The effect of the bel2/sin4/tsf3 mutation on PHO5 transcription was additive with the Pho4p function. Thus the effect of the bel2/sin4/tsf3 mutation is dependent on the position of PHO5 in the chromosome and independent of Pho4p and Gen4p activation.

RanBP1, a Ras-like nuclear G protein binding to Ran/TC4, inhibits RCC1 via Ran/TC4

A human protein that is 92% identical and 97% homologous at the amino acid level to RanBP1 from mouse was identified by the two-hybrid method, using two types of target cDNAs fused to sequences encoding the GAL4 DNA-binding domain. The target cDNAs encoded the human Ran/TC4 and human RCC1 proteins, respectively. An in vitro binding experiment showed that RanBP1 binds to RCC1 with the aid of Ran. Partially purified, GST-fused RanBP1 inhibited RCC1-stimulated guanine nucleotide release from Ran in vitro. Consistent with this in vitro finding, overproduction of human RanBP1 was detrimental to growth of tsBN2, a temperature-sensitive BHK21 hamster cell line defective in the RCC1 gene, and inhibited the growth of the Saccharomyces cerevisiae rcc1 mutants prp20, mtr1 and srm1. The specific effect of RanBP1 on rcc1- cells was confirmed by the finding that overproduction of RanBP1 induces significant levels of expression of a FUS1-lacZ gene and an increase in mating efficiencies in a ste3, pheromone receptor-deficient yeast mutant. This phenotype is similar to the srm1, a mutant isolated as a suppressor that restores mating to receptorless mutants. These findings indicate that RanBP1 negatively regulates RCC1.

Molecular cloning and characterization of the Schizosaccharomyces pombe his3 gene for use as a selectable marker

A DNA fragment which carries the his3 gene of Schizosaccharomyces pombe has been isolated and characterized for use as a selectable marker in transformations. The his3 gene encodes the imidazole acetol phosphate transaminase enzyme (E.C.2.6.1.9), which is responsible for converting imidazole acetol-P to histidinol-P in step 8 of histidine biosynthesis. The nucleotide sequences of a 2196 bp gene fragment and a corresponding cDNA clone were determined. Three intron sequences punctuate the 1451 bp coding region which generates a predicted polypeptide of 384 amino acids with a molecular mass of 42736 daltons. Northern analysis of his3 mRNAs indicates that the transcript is approximately 1.6 kb in size. Steady-state levels are down-regulated by nitrogen limitation but are unaffected by histidine starvation. The deduced amino acid sequence was compared to the Saccharomyces cerevisiae HIS5, Escherichia coli HisC, and Salmonella typhimurium HisC proteins, all of which are imidazole acetol phosphate transaminases. The S. pombe his3 protein was 49.5% identical to the S. cerevisiae HIS5 protein and 21.5% identity was found when all four proteins were compared. The shuttle vector pBG1 was constructed by subcloning the smallest functional region of his3 and the S. pombe ars1 sequence into pUC18 for use in transformation of His3--S. pombe strains. New S. pombe strains in which the his3 gene was deleted have also been constructed.

Cloning, primary structure, and regulation of the HIS7 gene encoding a bifunctional glutamine amidotransferase: cyclase from Saccharomyces cerevisiae

The Saccharomyces cerevisiae HIS7 gene was cloned by its location immediately downstream of the previously isolated and characterized ARO4 gene. The two genes have the same orientation with a distance of only 416 bp between the two open reading frames. The yeast HIS7 gene represents the first isolated eukaryotic gene encoding the enzymatic activities which catalyze the fifth and sixth step in histidine biosynthesis. The open reading frame of the HIS7 gene has a length of 1,656 bp resulting in a gene product of 552 amino acids with a calculated molecular weight of 61,082. Two findings implicate a bifunctional nature of the HIS7 gene product. First, the N-terminal and C-terminal segments of the deduced HIS7 amino acid sequence show significant homology to prokaryotic monofunctional glutamine amidotransferases and cyclases, respectively, involved in histidine biosynthesis. Second, the yeast HIS7 gene is able to suppress His auxotrophy of corresponding Escherichia coli hisH and hisF mutants. HIS7 gene expression is regulated by the general control system of amino acid biosynthesis. GCN4-dependent and GCN4-independent (basal) transcription use different initiator elements in the HIS7 promoter.

Aminotransferases: demonstration of homology and division into evolutionary subgroups

A total of 150 amino acid sequences of vitamin B6-dependent enzymes are known to date, the largest contingent being furnished by the aminotransferases with 51 sequences of 14 different enzymes. All aminotransferase sequences were aligned by using algorithms for sequence comparison, hydropathy patterns and secondary structure predictions. The aminotransferases could be divided into four subgroups on the basis of their mutual structural relatedness. Subgroup I comprises aspartate, alanine, tyrosine, histidinol-phosphate, and phenylalanine aminotransferases; subgroup II acetylornithine, ornithine, omega-amino acid, 4-aminobutyrate and diaminopelargonate aminotransferases; subgroup III D-alanine and branched-chain amino acid aminotransferases, and subgroup IV serine and phosphoserine aminotransferases. (N-1) Profile analysis, a more stringent application of profile analysis [Gribskov, M., McLachlan, A. D. and Eisenberg, D. (1987) Proc. Natl Acad. Sci. USA 84, 4355-4358], established the homology among the enzymes of each subgroup as well as among all subgroups except subgroup III. However, similarity of active-site segments and the hydropathy patterns around invariant residues suggest that subgroup III, though most distantly related, might also be homologous with the other aminotransferases. On the basis of the comprehensive alignment, a new numbering of amino acid residues applicable to aminotransferases (AT) in general is proposed. In the multiply aligned sequences, only four out of a total of about 400 amino acid residues proved invariant in all 51 sequences, i.e. Gly(314AT)197, Asp/Glu(340AT)222, Lys(385AT)258 and Arg(562AT)386, the number not in parentheses corresponding to the structure of porcine cytosolic aspartate aminotransferase. Apparently, the aminotransferases constitute a group of homologous proteins which diverged into subgroups and, with some exceptions, into substrate-specific individual enzymes already in the universal ancestor cell.

Promoter analysis of the PHO81 gene encoding a 134 kDa protein bearing ankyrin repeats in the phosphatase regulon of Saccharomyces cerevisiae

The PHO81 gene encoding one of the regulators of the phosphatase regulon in Saccharomyces cerevisiae was mapped 9.8 centimorgans distal from the ser2 locus on the right arm of chromosome VII. Determination of the nucleotide sequence of cloned PHO81 DNA revealed a 3537 bp open reading frame encoding a 134 kDa protein. This protein has six repeats of a 33-amino acid sequence homologous to the ankyrin repeat and an asparagine-rich region. Transcription of PHO81 is activated by Pho4 protein in cooperation with Pho2 (i.e., Bas2/Grf10) protein under the influence of the inorganic phosphate (Pi) concentration in the medium, through the PHO regulatory system. Major transcription initiation sites of PHO81, determined by primer extension analysis, are at nucleotide positions -66 and -65 relative to the ATG codon. Deletion analysis showed that a 95 bp region from nucleotide position -385 to -291 is essential for response to the Pi signals. Purified Pho4 protein protected a 19 bp region (positions -350 to -332) in the 95 bp fragment from DNase I digestion in vitro and the protected region includes the core sequence 5'-CACGTG-3', which is also observed in other genes of phosphate metabolism.

GCR3 encodes an acidic protein that is required for expression of glycolytic genes in Saccharomyces cerevisiae

Screening of a mutagenized strain carrying a multicopy ENO1-'lacZ fusion plasmid revealed a new mutation affecting several glycolytic enzyme activities. The recessive single nuclear gene mutation, named gcr3, caused an extremely defective growth phenotype on fermentable carbon sources such as glucose, while growth on respiratory media was almost normal. The GCR3 gene was obtained by growth complementation from a genomic DNA library, and the complemented strains had normal enzyme levels. GCR3 gene was sequenced, and a 99,537-Da protein was predicted. The predicted GCR3 protein was fairly acidic (net charge, -34). The C-terminal region was highly charged, and an acidic stretch was found in it.

AAR2, a gene for splicing pre-mRNA of the MATa1 cistron in cell type control of Saccharomyces cerevisiae

We have isolated a class of mutants, aar2, showing the alpha mating type due to a defect in a1-alpha 2 repression but with alpha 2 repression activity from a nonmater strain of Saccharomyces cerevisiae expressing both a and alpha mating-type information in duplicate. Cells of the aar2 mutant and the aar2 disruptant also show a growth defect. A DNA fragment complementing the aar2 mutation contains an open reading frame consisting of 355 amino acid codons. Northern hybridization showed that cells of the aar2 mutant and disruptant contained alpha 1 and alpha 2 transcripts of the MAT alpha gene (or HML alpha in sir3 cells), but their a1 transcript of MATa (or HMRa in sir3 cells) migrated more slowly than that of the wild-type cells on gel electrophoresis and gave a diffused band. Primer extension analysis showed that the aar2 mutant and disruptant have a defect in splicing two short introns of the a1 pre-mRNA but not in splicing pre-mRNA of ACT1. The alpha mating type, but not the slow-growing phenotype, of the aar2 mutant was suppressed by introduction of an intronless MATa1 DNA. Thus, the AAR2 gene is involved in splicing pre-mRNA of the a1 cistron and other genes that are important for cell growth. The AAR2 locus was mapped on chromosome II beside the SSA3 locus, with a 276-bp space, but was not allelic to either PRP5 or PRP6, which are both located on chromosome II and function in splicing pre-mRNA of ACT1.

AAR2, a gene for splicing pre-mRNA of the MATa1 cistron in cell type control of Saccharomyces cerevisiae

We have isolated a class of mutants, aar2, showing the alpha mating type due to a defect in a1-alpha 2 repression but with alpha 2 repression activity from a nonmater strain of Saccharomyces cerevisiae expressing both a and alpha mating-type information in duplicate. Cells of the aar2 mutant and the aar2 disruptant also show a growth defect. A DNA fragment complementing the aar2 mutation contains an open reading frame consisting of 355 amino acid codons. Northern hybridization showed that cells of the aar2 mutant and disruptant contained alpha 1 and alpha 2 transcripts of the MAT alpha gene (or HML alpha in sir3 cells), but their a1 transcript of MATa (or HMRa in sir3 cells) migrated more slowly than that of the wild-type cells on gel electrophoresis and gave a diffused band. Primer extension analysis showed that the aar2 mutant and disruptant have a defect in splicing two short introns of the a1 pre-mRNA but not in splicing pre-mRNA of ACT1. The alpha mating type, but not the slow-growing phenotype, of the aar2 mutant was suppressed by introduction of an intronless MATa1 DNA. Thus, the AAR2 gene is involved in splicing pre-mRNA of the a1 cistron and other genes that are important for cell growth. The AAR2 locus was mapped on chromosome II beside the SSA3 locus, with a 276-bp space, but was not allelic to either PRP5 or PRP6, which are both located on chromosome II and function in splicing pre-mRNA of ACT1.

AAR1/TUP1 protein, with a structure similar to that of the beta subunit of G proteins, is required for a1-alpha 2 and alpha 2 repression in cell type control of Saccharomyces cerevisiae

We have cloned a DNA fragment complementing the aar1 mutation defective in the a1-alpha 2 repression of the alpha 1 cistron and haploid-specific genes in Saccharomyces cerevisiae. Nucleotide sequence and mapping data indicated that the AAR1 gene is identical with TUP1, which is allelic to the SFL2, FLK1, CYC9, UMR7, AMM1, and AER2 genes, whose mutations are known to confer a variety of phenotypes, such as thymidine uptake, flocculation, insensitivity to glucose repression, a defect in UV-induced mutagenesis, and a defect in ARS plasmid maintenance. The TUP1/AER2 protein is known to have significant similarity with the beta subunits of G proteins in the C-terminal half, in two glutamine-rich domains in the N-terminal half, and in a central region rich in serine and threonine residues. Disruption of the chromosomal AAR1 gene in alpha and a/alpha cells conferred the nonmating phenotype, and the a/alpha diploids could not sporulate. The AAR1/TUP1 gene is transcribed into a 2.5-kb mRNA independently of the mating-type information of the cell. These observations and mRNA analysis of cell-type-specific genes indicated that the AAR1/TUP1 protein is also indispensable for a1-alpha 2 repression of RME1 and for alpha 2 repression of a-specific genes.

The PHO84 gene of Saccharomyces cerevisiae encodes an inorganic phosphate transporter

The PHO84 gene specifies Pi-transport in Saccharomyces cerevisiae. A DNA fragment bearing the PHO84 gene was cloned by its ability to complement constitutive synthesis of repressible acid phosphatase of pho84 mutant cells. Its nucleotide sequence predicted a protein of 596 amino acids with a sequence homologous to that of a superfamily of sugar transporters. Hydropathy analysis suggested that the secondary structure of the PHO84 protein consists of two blocks of six transmembrane domains separated by 74 amino acid residues. The cloned PH084 DNA restored the Pi transport activity of pho84 mutant cells. The PHO84 transcription was regulated by Pi like those of the PHO5, PHO8, and PHO81 genes. A PHO84-lacZ fusion gene produced beta-galactosidase activity under the regulation of Pi, and the activity was suggested to be bound to a membrane fraction. Gene disruption of PHO84 was not lethal. By comparison of nucleotide sequences and by tetrad analysis with GAL80 as a standard, the PHO84 locus was mapped at a site beside the TUB3 locus on the left arm of chromosome XIII.

AAR1/TUP1 protein, with a structure similar to that of the beta subunit of G proteins, is required for a1-alpha 2 and alpha 2 repression in cell type control of Saccharomyces cerevisiae

We have cloned a DNA fragment complementing the aar1 mutation defective in the a1-alpha 2 repression of the alpha 1 cistron and haploid-specific genes in Saccharomyces cerevisiae. Nucleotide sequence and mapping data indicated that the AAR1 gene is identical with TUP1, which is allelic to the SFL2, FLK1, CYC9, UMR7, AMM1, and AER2 genes, whose mutations are known to confer a variety of phenotypes, such as thymidine uptake, flocculation, insensitivity to glucose repression, a defect in UV-induced mutagenesis, and a defect in ARS plasmid maintenance. The TUP1/AER2 protein is known to have significant similarity with the beta subunits of G proteins in the C-terminal half, in two glutamine-rich domains in the N-terminal half, and in a central region rich in serine and threonine residues. Disruption of the chromosomal AAR1 gene in alpha and a/alpha cells conferred the nonmating phenotype, and the a/alpha diploids could not sporulate. The AAR1/TUP1 gene is transcribed into a 2.5-kb mRNA independently of the mating-type information of the cell. These observations and mRNA analysis of cell-type-specific genes indicated that the AAR1/TUP1 protein is also indispensable for a1-alpha 2 repression of RME1 and for alpha 2 repression of a-specific genes.

The PHO84 gene of Saccharomyces cerevisiae encodes an inorganic phosphate transporter

The PHO84 gene specifies Pi-transport in Saccharomyces cerevisiae. A DNA fragment bearing the PHO84 gene was cloned by its ability to complement constitutive synthesis of repressible acid phosphatase of pho84 mutant cells. Its nucleotide sequence predicted a protein of 596 amino acids with a sequence homologous to that of a superfamily of sugar transporters. Hydropathy analysis suggested that the secondary structure of the PHO84 protein consists of two blocks of six transmembrane domains separated by 74 amino acid residues. The cloned PH084 DNA restored the Pi transport activity of pho84 mutant cells. The PHO84 transcription was regulated by Pi like those of the PHO5, PHO8, and PHO81 genes. A PHO84-lacZ fusion gene produced beta-galactosidase activity under the regulation of Pi, and the activity was suggested to be bound to a membrane fraction. Gene disruption of PHO84 was not lethal. By comparison of nucleotide sequences and by tetrad analysis with GAL80 as a standard, the PHO84 locus was mapped at a site beside the TUB3 locus on the left arm of chromosome XIII.

Specific cis-acting sequence for PHO8 expression interacts with PHO4 protein, a positive regulatory factor, in Saccharomyces cerevisiae

The PHO8 gene of Saccharomyces cerevisiae encodes repressible alkaline phosphatase (rALPase; EC 3.1.3.1). The rALPase activity of the cells is two to three times higher in medium containing a low concentration of Pi than in high-Pi medium due to transcription of PHO8. The Pi signals are conveyed to PHO8 by binding of PHO4 protein, a positive regulatory factor, to a promoter region of PHO8 (PHO8p) under the influence of the PHO regulatory circuit. Deletion analysis of PHO8p DNA revealed two separate regulatory regions required for derepression of rALPase located at nucleotide positions -704 to -661 (distal region) and -548 to -502 (proximal region) and an inhibitory region located at -421 to -289 relative to the translation initiation codon. Gel retardation experiments showed that a beta-galactosidase-PHO4 fusion protein binds to a 132-bp PHO8p fragment bearing the proximal region but not to a 226-bp PHO8 DNA bearing the distal region. The fusion protein also binds to a synthetic oligonucleotide having the same 12-bp nucleotide sequence as the PHO8p DNA from positions -536 to -525. The 132-bp PHO8p fragment, connected at position -281 of the 5' upstream region of a HIS5'-'lacZ fused gene, could sense Pi signals in vivo, but a 20-bp synthetic oligonucleotide having the same sequence from -544 to -525 of the PHO8p DNA could not. Linker insertions in the PHO8p DNA indicated that the 5-bp sequence 5'-CACGT-3' from positions -535 to -531 is essential for binding the beta-galactosidase-PHO4 fusion protein and for derepression of rALPase.

Specific cis-acting sequence for PHO8 expression interacts with PHO4 protein, a positive regulatory factor, in Saccharomyces cerevisiae

The PHO8 gene of Saccharomyces cerevisiae encodes repressible alkaline phosphatase (rALPase; EC 3.1.3.1). The rALPase activity of the cells is two to three times higher in medium containing a low concentration of Pi than in high-Pi medium due to transcription of PHO8. The Pi signals are conveyed to PHO8 by binding of PHO4 protein, a positive regulatory factor, to a promoter region of PHO8 (PHO8p) under the influence of the PHO regulatory circuit. Deletion analysis of PHO8p DNA revealed two separate regulatory regions required for derepression of rALPase located at nucleotide positions -704 to -661 (distal region) and -548 to -502 (proximal region) and an inhibitory region located at -421 to -289 relative to the translation initiation codon. Gel retardation experiments showed that a beta-galactosidase-PHO4 fusion protein binds to a 132-bp PHO8p fragment bearing the proximal region but not to a 226-bp PHO8 DNA bearing the distal region. The fusion protein also binds to a synthetic oligonucleotide having the same 12-bp nucleotide sequence as the PHO8p DNA from positions -536 to -525. The 132-bp PHO8p fragment, connected at position -281 of the 5' upstream region of a HIS5'-'lacZ fused gene, could sense Pi signals in vivo, but a 20-bp synthetic oligonucleotide having the same sequence from -544 to -525 of the PHO8p DNA could not. Linker insertions in the PHO8p DNA indicated that the 5-bp sequence 5'-CACGT-3' from positions -535 to -531 is essential for binding the beta-galactosidase-PHO4 fusion protein and for derepression of rALPase.

gcr2, a new mutation affecting glycolytic gene expression in Saccharomyces cerevisiae

Screening of a mutagenized strain carrying a multicopy ENO1-'lacZ fusion plasmid revealed a new mutation affecting most glycolytic enzyme activities in a pattern resembling that caused by gcr1: levels in the range of 10% of wild-type levels on glycerol plus lactate but somewhat higher on glucose. The recessive single nuclear gene mutation, named gcr2-1, was unlinked to gcr1, and GCR1 in multiple copies did not restore enzyme levels. GCR2 was obtained by complementation from a YCp50 genomic library; the complemented strain had normal enzyme levels, as did a strain with GCR2 in multiple copies. GCR2 in multiple copies did not suppress gcr1. A chromosomal gcr2 null mutant was constructed; its pattern of enzyme activities resembled that of the gcr2-1 mutant and, like the gcr2-1 mutant, its growth defect on glucose was only partial (in contrast to the glucose negativity of the gcr1 mutant). Northern (RNA) analysis showed that gcr2 and gcr1 affect ENO1 mRNA levels.

gcr2, a new mutation affecting glycolytic gene expression in Saccharomyces cerevisiae

Screening of a mutagenized strain carrying a multicopy ENO1-'lacZ fusion plasmid revealed a new mutation affecting most glycolytic enzyme activities in a pattern resembling that caused by gcr1: levels in the range of 10% of wild-type levels on glycerol plus lactate but somewhat higher on glucose. The recessive single nuclear gene mutation, named gcr2-1, was unlinked to gcr1, and GCR1 in multiple copies did not restore enzyme levels. GCR2 was obtained by complementation from a YCp50 genomic library; the complemented strain had normal enzyme levels, as did a strain with GCR2 in multiple copies. GCR2 in multiple copies did not suppress gcr1. A chromosomal gcr2 null mutant was constructed; its pattern of enzyme activities resembled that of the gcr2-1 mutant and, like the gcr2-1 mutant, its growth defect on glucose was only partial (in contrast to the glucose negativity of the gcr1 mutant). Northern (RNA) analysis showed that gcr2 and gcr1 affect ENO1 mRNA levels.

Characterization and sequencing of the active site of 1-aminocyclopropane-1-carboxylate synthase

The pyridoxal phosphate (PLP)-dependent 1-aminocyclopropane-1-carboxylic acid (ACC) synthase (S-adenosyl-L-methionine methylthioadenosine-lyase, EC 4.4.1.14), the key enzyme in ethylene biosynthesis, is inactivated by its substrate S-adenosylmethionine (AdoMet). Apple ACC synthase was purified with an immunoaffinity gel, and its active site was probed with NaB3H4 or Ado[14C]Met. HPLC separation of the trypsin digest yielded a single radioactive peptide. Peptide sequencing of both 3H- and 14C-labeled peptides revealed a common dodecapeptide of Ser-Leu-Ser-Xaa-Asp-Leu-Gly-Leu-Pro-Gly-Phe-Arg, where Xaa was the modified, radioactive residue in each case. Acid hydrolysis of the 3H-labeled enzyme released radioactive N-pyridoxyllysine, indicating that the active-site peptide contained lysine at position 4. Mass spectrometry of the 14C-labeled peptide indicated a protonated molecular ion at m/z 1390.6, from which the mass of Xaa was calculated to be 229, a number that is equivalent to the mass of a lysine residue alkylated by the 2-aminobutyrate portion of AdoMet, as we previously proposed. These results indicate that the same active-site lysine binds the PLP and convalently links to the 2-aminobutyrate portion of AdoMet during inactivation. The active site of tomato ACC synthase was probed in the same manner with Ado[14C]Met. Sequencing of the tomato active-site peptide revealed two highly conserved dodecapeptides; the minor peptide possessed a sequence identical to that of the apple enzyme, whereas the major peptide differed from the minor peptide in that methionine replaced leucine at position 6.

Characterization of a gene involved in histidine biosynthesis in Halobacterium (Haloferax) volcanii: isolation and rapid mapping by transformation of an auxotroph with cosmid DNA

Techniques for the transformation of halophilic archaebacteria have been developed recently and hold much promise for the characterization of these organisms at the molecular level. In order to understand genome organization and gene regulation in halobacteria, we have begun the characterization of genes involved in amino acid biosynthesis in Halobacterium (Haloferax) volcanii. These studies are facilitated by the many auxotrophic mutants of H. volcanii that have been isolated. In this project we demonstrate that cosmid DNA prepared from Escherichia coli can be used to transform an H. volcanii histidine auxotroph to prototrophy. A set of cosmid clones covering most of the genome of H. volcanii was used to isolate the gene which is defective in H. volcanii WR256. Subcloning identified a 1.6-kilobase region responsible for transformation. DNA sequence analysis of this region revealed an open reading frame encoding a putative protein 361 amino acids in length. A search of the DNA and protein data bases revealed that this open reading frame encodes histidinol-phosphate aminotransferase (EC 2.6.1.9), the sequence of which is also known for E. coli, Bacillus subtilis, and Saccharomyces cerevisiae.

Cloning and characterization of the histidine biosynthetic gene cluster of Streptomyces coelicolor A3(2)

Biochemical and genetic data indicate that in Streptomyces coelicolor A3(2) the majority of the genes involved in the biosynthesis of histidine are clustered in a small region of the chromosome [Carere et al., Mol. Gen. Genet. 123 (1973) 219-224; Russi et al., Mol. Gen. Genet. 123 (1973) 225-232]. To investigate the structural organization and the regulation of these genes, we have constructed genomic libraries from S. coelicolor A3(2) in pUC vectors. Recombinant clones were isolated by complementation of an Escherichia coli hisBd auxotroph. A recombinant plasmid containing a 3.4-kb fragment of genomic DNA was further characterized. When cloned in the plasmid vector, pIJ699, this fragment was able to complement S. coelicolor A3(2) hisB mutants. Overlapping clones spanning a 15-kb genomic region were isolated by screening other libraries with labeled DNA fragments obtained from the first clone. Derivative clones were able to complement mutations in four different cistrons of the his cluster of S. coelicolor A3(2). Nucleotide sequence analysis of a 4-kb region allowed the identification of five ORFs which showed significant homology with the his gene products of E. coli. The order of the genes in S. coelicolor A3(2) (5'--hisD-hisC-hisBd-hisH-hisA-3') is the same as in the his operon of E. coli.

Functional domains of a positive regulatory protein, PHO4, for transcriptional control of the phosphatase regulon in Saccharomyces cerevisiae

The PHO4 gene encodes a positive regulatory factor involved in regulating transcription of various genes in the phosphatase regulon of Saccharomyces cerevisiae. Besides its own coding region, the 1.8-kilobase PHO4 transcript contains a coding region for a mitochondrial protein which does not appear to be translated. Four functional domains were found in the PHO4 protein, which consists of 312 amino acid (aa) residues as deduced from the open reading frame of PHO4. A gel retardation assay with beta-galactosidase::PHO4 fused protein revealed that the 85-aa C terminus is the domain responsible for binding to the promoter DNA of PHO5, a gene under the control of PHO4. This region has similarities with the amphipathic helix-loop-helix motif of c-myc protein. Determination of the nucleotide sequences of four PHO4c mutant alleles and insertion and deletion analyses of PHO4 DNA indicated that a region from aa 163 to 202 is involved in interaction with a negative regulatory factor PHO80. Complementation of a pho4 null allele with the modified PHO4 DNAs suggested that the N-terminal region (1 to 109 aa), which is rich in acidic aa, is the transcriptional activation domain. The deleterious effects of various PHO4 mutations on the constitutive transcription of PHO5 in PHO4c mutant cells suggested that the region from aa 203 to 227 is involved in oligomerization of the PHO4 protein.

Functional domains of a positive regulatory protein, PHO4, for transcriptional control of the phosphatase regulon in Saccharomyces cerevisiae

The PHO4 gene encodes a positive regulatory factor involved in regulating transcription of various genes in the phosphatase regulon of Saccharomyces cerevisiae. Besides its own coding region, the 1.8-kilobase PHO4 transcript contains a coding region for a mitochondrial protein which does not appear to be translated. Four functional domains were found in the PHO4 protein, which consists of 312 amino acid (aa) residues as deduced from the open reading frame of PHO4. A gel retardation assay with beta-galactosidase::PHO4 fused protein revealed that the 85-aa C terminus is the domain responsible for binding to the promoter DNA of PHO5, a gene under the control of PHO4. This region has similarities with the amphipathic helix-loop-helix motif of c-myc protein. Determination of the nucleotide sequences of four PHO4c mutant alleles and insertion and deletion analyses of PHO4 DNA indicated that a region from aa 163 to 202 is involved in interaction with a negative regulatory factor PHO80. Complementation of a pho4 null allele with the modified PHO4 DNAs suggested that the N-terminal region (1 to 109 aa), which is rich in acidic aa, is the transcriptional activation domain. The deleterious effects of various PHO4 mutations on the constitutive transcription of PHO5 in PHO4c mutant cells suggested that the region from aa 203 to 227 is involved in oligomerization of the PHO4 protein.

The OGD1 gene, affecting 2-oxoglutarate dehydrogenase in S. cerevisiae, is closely linked to HIS5 on chromosome IX

Ogd1 mutants of Saccharomyces cerevisiae are deficient in mitochondrial 2-oxoglutarate dehydrogenase activity; they cannot grow on glycerol and produce an increased amount of organic acids during growth on glucose as substrate. Using gamma ray-induced rad52-mediated chromosome loss the ogd1 mutation can be assigned to chromosome IX. Tetrad analysis of crosses between ogd1 and other markers on chromosome IX revealed that the OGD1 gene maps on the left arm of this chromosome 1.9 cM from his5.

Mating-type control in Saccharomyces cerevisiae: isolation and characterization of mutants defective in repression by a1-alpha 2

The alpha 2 protein, the product of the MAT alpha 2 cistron, represses various genes specific to the a mating type (alpha 2 repression), and when combined with the MATa1 gene product, it represses MAT alpha 1 and various haploid-specific genes (a1-alpha 2 repression). One target of a1-alpha 2 repression is RME1, which is a negative regulator of a/alpha-specific genes. We have isolated 13 recessive mutants whose a1-alpha 2 repression is defective but which retain alpha 2 repression in a genetic background of ho MATa HML alpha HMRa sir3 or ho MAT alpha HMRa HMRa sir3. These mutations can be divided into three different classes. One class contains a missense mutation, designated hml alpha 2-102, in the alpha 2 cistron of HML, and another class contains two mat alpha 2-202, in the MAT alpha locus. These three mutants each have an amino acid substitution of tyrosine or acid substitution of tyrosine or phenylalanine for cysteine at the 33rd codon from the translation initiation codon in the alpha 2 cistron of HML alpha or MAT alpha. The remaining 10 mutants make up the third class and form a single complementation group, having mutations designated aar1 (a1-alpha 2 repression), at a gene other than MAT, HML, HMR, RME1, or the four SIR genes. Although a diploid cell homozygous for the aarl and sir3 mutations and for the MATa, HML alpha, and HMRa alleles showed alpha mating type, it could sporulate and gave rise to asci containing four alpha mating-type spores. These facts indicate that the domain for alpha2 repression is separable from that for a1-alpha2 protein interaction or complex formation in the alpha2 protein and that an additional regulation gene, AAR1, is associated with the a1-alpha2 repression of the alpha1 cistron and haploid-specific genes.

Evolutionary relationships among aminotransferases. Tyrosine aminotransferase, histidinol-phosphate aminotransferase, and aspartate aminotransferase are homologous proteins

A data base was compiled containing the amino acid sequences of 12 aspartate aminotransferases and 11 other aminotransferases. A comparison of these sequences by a standard alignment method confirmed the previously reported homology of all aspartate aminotransferases and Escherichia coli tyrosine aminotransferase. However, no significant similarity between these proteins and any of the other aminotransferases was detected. A more rigorous analysis, focusing on short sequence segments rather than the total polypeptide chain, revealed that rat tyrosine aminotransferase and Saccharomyces cerevisiae and Escherichia coli histidinol-phosphate aminotransferase share several homologous sequence segments with aspartate aminotransferases. For comparison of the complete sequences, a multiple sequence editor was developed to display the whole set of amino acid sequences in parallel on a single work-sheet. The editor allows gaps in individual sequences or a set of sequences to be introduced and thus facilitates their parallel analysis and alignment. Several clusters of invariant residues at corresponding positions in the amino acid sequences became evident, clearly establishing that the cytosolic and the mitochondrial isoenzyme of vertebrate aspartate aminotransferase, E. coli aspartate aminotransferase, rat and E. coli tyrosine aminotransferase, and S. cerevisiae and E. coli histidinol-phosphate aminotransferase are homologous proteins. Only 12 amino acid residues out of a total of about 400 proved to be invariant in all sequences compared; they are either involved in the binding of pyridoxal 5'-phosphate and the substrate, or appear to be essential for the conformation of the enzymes.

An improved host-vector system for Candida maltosa using a gene isolated from its genome that complements the his5 mutation of Saccharomyces cerevisiae

The host-vector system of an n-alkane-assimilating-yeast, Candida maltosa, which we previously constructed using an autonomously replicating sequence (ARS) region isolated from the genome of this yeast, utilizes C. maltosa J288 (leu2-) as a host. As this host had a serious growth defect on n-alkane, we developed an improved host-vector system using C. maltosa CH1 (his-) as host. The vectors were constructed with the Candida ARS region and a DNA fragment isolated from the genome of C. maltosa. Since this DNA fragment could complement histidine auxotrophy of both C. maltosa CH1 and S. cerevisiae (his5-), we termed the gene contained in this DNA fragment C-HIS5. The vectors were characterized in terms of transformation frequency and stability, and the nucleotide sequence of C-HIS5 was determined. The deduced amino acid sequence (389 residues) shared 51% homology with that of HIS5 of S. cerevisiae (384 residues; Nishiwaki et al. 1987).

Mating-type control in Saccharomyces cerevisiae: isolation and characterization of mutants defective in repression by a1-alpha 2

The alpha 2 protein, the product of the MAT alpha 2 cistron, represses various genes specific to the a mating type (alpha 2 repression), and when combined with the MATa1 gene product, it represses MAT alpha 1 and various haploid-specific genes (a1-alpha 2 repression). One target of a1-alpha 2 repression is RME1, which is a negative regulator of a/alpha-specific genes. We have isolated 13 recessive mutants whose a1-alpha 2 repression is defective but which retain alpha 2 repression in a genetic background of ho MATa HML alpha HMRa sir3 or ho MAT alpha HMRa HMRa sir3. These mutations can be divided into three different classes. One class contains a missense mutation, designated hml alpha 2-102, in the alpha 2 cistron of HML, and another class contains two mat alpha 2-202, in the MAT alpha locus. These three mutants each have an amino acid substitution of tyrosine or acid substitution of tyrosine or phenylalanine for cysteine at the 33rd codon from the translation initiation codon in the alpha 2 cistron of HML alpha or MAT alpha. The remaining 10 mutants make up the third class and form a single complementation group, having mutations designated aar1 (a1-alpha 2 repression), at a gene other than MAT, HML, HMR, RME1, or the four SIR genes. Although a diploid cell homozygous for the aarl and sir3 mutations and for the MATa, HML alpha, and HMRa alleles showed alpha mating type, it could sporulate and gave rise to asci containing four alpha mating-type spores. These facts indicate that the domain for alpha2 repression is separable from that for a1-alpha2 protein interaction or complex formation in the alpha2 protein and that an additional regulation gene, AAR1, is associated with the a1-alpha2 repression of the alpha1 cistron and haploid-specific genes.

Mode of expression of the positive regulatory genes PHO2 and PHO4 of the phosphatase regulon in Saccharomyces cerevisiae

The mode of expression was investigated for two positive regulatory genes, PHO2 and PHO4, whose products are indispensable for the transcriptional control of the structural genes of repressible acid phosphatase and the inorganic phosphate (Pi) transport system in Saccharomyces cerevisiae. Northern analysis of poly(A)+ RNA of the wildtype and the pho regulatory mutants with PHO4 DNA as hybridization probe and expressional analysis of a pho4'-'lacZ fused gene on a YEp plasmid revealed that PHO4 is expressed at a low level, constitutively, and independently of the PHO regulatory system and Pi in the medium. Similar analyses with PHO2 DNA indicated that PHO2 is expressed at an even lower level than PHO4, and is repressed by Pi and by the active PHO2 product, possibly at the translational level, while retaining a substantial level of basal activity.