Regulation by the yeast mating-type locus of STE12, a gene required for cell-type-specific expression

Flip/flop mating-type switching in the methylotrophic yeast Ogataea polymorpha is regulated by an Efg1-Rme1-Ste12 pathway

In haploid cells of Ogataea (Hansenula) polymorpha an environmental signal, nitrogen starvation, induces a reversible change in the structure of a chromosome. This process, mating-type switching, inverts a 19-kb DNA region to place either MATa or MATα genes under centromeric repression of transcription, depending on the orientation of the region. Here, we investigated the genetic pathway that controls switching. We characterized the transcriptomes of haploid and diploid O. polymorpha by RNAseq in rich and nitrogen-deficient media, and found that there are no constitutively a-specific or α-specific genes other than the MAT genes themselves. We mapped a switching defect in a sibling species (O. parapolymorpha strain DL-1) by interspecies bulk segregant analysis to a frameshift in the transcription factor EFG1, which in Candida albicans regulates filamentous growth and white-opaque switching. Gene knockout, overexpression and ChIPseq experiments show that EFG1 regulates RME1, which in turn regulates STE12, to achieve mating-type switching. All three genes are necessary both for switching and for mating. Overexpression of RME1 or STE12 is sufficient to induce switching without a nitrogen depletion signal. The homologous recombination genes RAD51 and RAD17 are also necessary for switching. The pathway controlling switching in O. polymorpha shares no components with the regulation of HO in S. cerevisiae, which does not involve any environmental signal, but it shares some components with mating-type switching in Kluyveromyces lactis and with white-opaque phenotypic switching in C. albicans.

The molecular mechanisms of self-fertility (homothallism) vary enormously among fungal species. We previously found that in the yeast Ogataea polymorpha, homothallism is achieved by a novel mating-type switching mechanism that exchanges the locations of MATa and MATα genes between expression and repression contexts. Switching in this species is induced by nitrogen depletion, unlike the analogous process in Saccharomyces cerevisiae. Here, we show that the upstream parts of the genetic pathway controlling the environmental induction of switching in O. polymorpha are the same as the environmental pathway that induces competence for mating in this species.

Multiple MAPK cascades regulate the transcription of IME1, the master transcriptional activator of meiosis in Saccharomyces cerevisiae

The choice between alternative developmental pathways is primarily controlled at the level of transcription. Induction of meiosis in budding yeasts in response to nutrient levels provides a system to investigate the molecular basis of cellular decision-making. In Saccharomyces cerevisiae, entry into meiosis depends on multiple signals converging upon IME1, the master transcriptional activator of meiosis. Here we studied the regulation of the cis-acting regulatory element Upstream Activation Signal (UAS)ru, which resides within the IME1 promoter. Guided by our previous data acquired using a powerful high-throughput screening system, here we provide evidence that UASru is regulated by multiple stimuli that trigger distinct signal transduction pathways as follows: (i) The glucose signal inhibited UASru activity through the cyclic AMP (cAMP/protein kinase A (PKA) pathway, targeting the transcription factors (TFs), Com2 and Sko1; (ii) high osmolarity activated UASru through the Hog1/mitogen-activated protein kinase (MAPK) pathway and its corresponding TF Sko1; (iii) elevated temperature increased the activity of UASru through the cell wall integrity pathway and the TFs Swi4/Mpk1 and Swi4/Mlp1; (iv) the nitrogen source repressed UASru activity through Sum1; and (v) the absence of a nitrogen source was detected and transmitted to UASru by the Kss1 and Fus3 MAPK pathways through their respective downstream TFs, Ste12/Tec1 and Ste12/Ste12 as well as by their regulators Dig1/2. These signaling events were specific to UASru; they did not affect the mating and filamentation response elements that are regulated by MAPK pathways. The complex regulation of UASru through all the known vegetative MAPK pathways is unique to S. cerevisiae and is specific for IME1, likely because it is the master regulator of gametogenesis.

Hypovirus molecular biology: from Koch's postulates to host self-recognition genes that restrict virus transmission

The idea that viruses can be used to control fungal diseases has been a driving force in mycovirus research since the earliest days. Viruses in the family Hypoviridae associated with reduced virulence (hypovirulence) of the chestnut blight fungus, Cryphonectria parasitica, have held a prominent place in this research. This has been due in part to the severity of the chestnut blight epidemics in North America and Europe and early reports of hypovirulence-mediated mitigation of disease in European forests and successful application for control of chestnut blight in chestnut orchards. A more recent contributing factor has been the development of a hypovirus/C. parasitica experimental system that has overcome many of the challenges associated with mycovirus research, stemming primarily from the exclusive intracellular lifestyle shared by all mycoviruses. This chapter will focus on hypovirus molecular biology with an emphasis on the development of the hypovirus/C. parasitica experimental system and its contributions to fundamental and practical advances in mycovirology and the broader understanding of virus-host interactions and fungal pathogenesis.

A Mep2-dependent transcriptional profile links permease function to gene expression during pseudohyphal growth in Saccharomyces cerevisiae

The ammonium permease Mep2 is required for the induction of pseudohyphal growth, a process in Saccharomyces cerevisiae that occurs in response to nutrient limitation. Mep2 has both a transport and a regulatory function, supporting models in which Mep2 acts as a sensor of ammonium availability. Potentially similar ammonium permease-dependent regulatory cascades operate in other fungi, and they may also function in animals via the homologous Rh proteins; however, little is known about the molecular mechanisms that mediate ammonium sensing. We show that Mep2 is localized to the cell surface during pseudohyphal growth, and it is required for both filamentous and invasive growth. Analysis of site-directed Mep2 mutants in residues lining the ammonia-conducting channel reveal separation of function alleles (transport and signaling defective; transport-proficient/signaling defective), indicating transport is necessary but not sufficient to sense ammonia. Furthermore, Mep2 overexpression enhances differentiation under normally repressive conditions and induces a transcriptional profile that is consistent with activation of the mitogen-activated protein (MAP) kinase pathway. This finding is supported by epistasis analysis establishing that the known role of the MAP kinase pathway in pseudohyphal growth is linked to Mep2 function. Together, these data strengthen the model that Mep2-like proteins are nutrient sensing transceptors that govern cellular differentiation.

Specificity of MAPK signaling towards FLO11 expression is established by crosstalk from cAMP pathway

In budding yeast, elements of a single MAP Kinase cascade are shared to regulate a wide range of functions such as mating, differentiation and osmotic stress. However, cells have programmed to execute correct event in response to a given input signal without cross activating other responses. Studies have observed that magnitude and duration of MAPK activation encodes specificity. Similarly, the differential regulation of Tec1p, a transcriptional activator of invasive growth gene, FLO11 by MAP kinases has been observed to bring specificity in mating and invasive growth signaling. However, the understanding of interactions between the shared components and other signaling pathways related to the phenotypic response in contributing towards specificity remains unclear. We specifically address the crosstalk of cAMP pathway with MAPK pathway in haploid invasive growth and show the contribution and importance of cAMP pathway towards invasive growth irrespective of the activation status of MAPK pathway. Our analysis shows that crosstalk from cAMP pathway in haploids might offer an advantage in terms of amplifying the observed weak signaling through MAPK pathway. Further, we show that such a crosstalk in haploids leads to higher FLO11 expression than diploids. We also demonstrate the positive and negative role of Tpk1 and Tpk3 in haploid invasive growth. Finally, we observe that a cross-inhibition at gene level brought about by cAMP pathway controlled inhibitor, Sfl1, perhaps help in deamplifying the MAPK signal and also in preventing FLO11 expression in the absence of cAMP pathway activation.

Cell-type-dependent repression of yeast a-specific genes requires Itc1p, a subunit of the Isw2p-Itc1p chromatin remodelling complex

In Saccharomyces cerevisiae MATa haploid cells, the a-specific genes are expressed, whereas in the MATalpha haploid and MATa/alpha diploid cell types their transcription is repressed. It is shown in this report that Itc1p, a component of the ATP-dependent Isw2p-Itc1p chromatin remodelling complex, is required for the repression of a-specific genes. It has previously been reported that disruption of the ITC1 gene leads, in MATalpha cells, to an aberrant cell morphology resembling the polarized mating projection of cells responding to pheromone. The activation of the pheromone signalling pathway in itc1 mutants of both mating types was examined and found to be constitutively active in MATalpha itc1 but not in MATa itc1 cells. Furthermore, unlike the wild-type, MATalpha itc1 and MATa/alpha itc1/itc1 cells secrete a-factor and express significant levels of other a-specific genes. The results indicate that the inappropriate a-factor production in a MATalpha context, due to the derepression of the a-specific genes, produces an autocrine signalling loop that leads to the aberrant morphology displayed by MATalpha itc1 cells. It is suggested that the Isw2p-Itc1p complex contributes to maintain the repressive chromatin structure described for the asg operator present in the promoters of a-specific genes.

Srb10/Cdk8 regulates yeast filamentous growth by phosphorylating the transcription factor Ste12

The budding yeast Saccharomyces cerevisiae differentiates into filamentous invasively growing forms under conditions of nutrient limitation. This response is dependent on the transcription factor Ste12 and on the mating pheromone-response mitogen-activated protein (MAP) kinase cascade, but a mechanism for regulation of Ste12 by nutrient limitation has not been defined. Here we show that Ste12 function in filamentous growth is regulated by the cyclin-dependent kinase Srb10 (also known as Cdk8), which is associated with the RNA polymerase II holoenzyme. Srb10 inhibits filamentous growth in cells growing in rich medium by phosphorylating Ste12 and decreasing its stability. Under conditions of limiting nitrogen, loss of Srb10 protein and kinase activity occurs, with a corresponding loss of Ste12 phosphorylation. Mutation of the Srb10-dependent phosphorylation sites increases pseudohyphal development but has no effect on the pheromone response of haploid yeast. Srb10 kinase activity is also regulated independently of the mating pheromone-response pathway. This indicates that Srb10 controls Ste12 activity for filamentous growth in response to nitrogen limitation and is consistent with the hypothesis that Srb10 regulates gene-specific activators in response to physiological signals to coordinate gene expression with growth potential.

The Cryptococcus neoformans STE11alpha gene is similar to other fungal mitogen-activated protein kinase kinase kinase (MAPKKK) genes but is mating type specific

Partial sequence analysis of the Cryptococcus neoformans MATalpha mating type locus revealed the presence of a gene with substantial sequence similarity to other fungal mitogen-activated protein (MAP) kinase kinase kinase (MAPKKK) genes. The C. neoformans gene, designated STE11alpha, showed the highest degree of similarity to the Neurospora crassa nrc-1, Schizosaccharomyces pombe byr2 and Saccharomyces cerevisiae STE11 genes. A polymerase chain reaction-mediated sib-selection technique was successfully adapted for the purpose of disrupting STE11alpha. C. neoformans ste11alphaDelta mutants were found to be sterile, consistent with the phenotypes of ste11 and byr2 mutants in S. cerevisiae and S. pombe respectively. Haploid ste11alphaDelta mutants were also found to be unable to produce hyphae, suggesting that the C. neoformans gene is functionally conserved when compared with its S. cerevisiae MAPKKK counterpart. Comparison of the wild-type STE11alpha strain with a ste11alphaDelta disruptant for virulence using the mouse model showed that the ste11alphaDelta strain was less virulent, but the difference was only minor. In spite of some of the conserved functions of STE11alpha, linkage analysis showed that STE11alpha is only found in mating type alpha strains. These results demonstrate that, although functionally conserved, the mating pathway in C. neoformans has a unique organization.

An STE12 homolog from the asexual, dimorphic fungus Penicillium marneffei complements the defect in sexual development of an Aspergillus nidulans steA mutant

Penicillium marneffei is an opportunistic fungal pathogen of humans and the only dimorphic species identified in its genus. At 25 degrees P. marneffei exhibits true filamentous growth, while at 37 degrees P. marneffei undergoes a dimorphic transition to produce uninucleate yeast cells that divide by fission. Members of the STE12 family of regulators are involved in controlling mating and yeast-hyphal transitions in a number of fungi. We have cloned a homolog of the S. cerevisiae STE12 gene from P. marneffei, stlA, which is highly conserved. The stlA gene, along with the A. nidulans steA and Cryptococcus neoformans STE12alpha genes, form a distinct subclass of STE12 homologs that have a C2H2 zinc-finger motif in addition to the homeobox domain that defines STE12 genes. To examine the function of stlA in P. marneffei, we isolated a number of mutants in the P. marneffei-type strain and, in combination with selectable markers, developed a highly efficient DNA-mediated transformation procedure and gene deletion strategy. Deletion of the stlA gene had no detectable effect on vegetative growth, asexual development, or dimorphic switching in P. marneffei. Despite the lack of a detectable function, the P. marneffei stlA gene complemented the sexual defect of an A. nidulans steA mutant. In addition, substitution rate estimates indicate that there is a significant bias against nonsynonymous substitutions. These data suggest that P. marneffei may have a previously unidentified cryptic sexual cycle.

Defects in protein glycosylation cause SHO1-dependent activation of a STE12 signaling pathway in yeast

In haploid Saccharomyces cerevisiae, mating occurs by activation of the pheromone response pathway. A genetic selection for mutants that activate this pathway uncovered a class of mutants defective in cell wall integrity. Partial loss-of-function alleles of PGI1, PMI40, PSA1, DPM1, ALG1, MNN10, SPT14, and OCH1, genes required for mannose utilization and protein glycosylation, activated a pheromone-response-pathway-dependent reporter (FUS1) in cells lacking a basal signal (ste4). Pathway activation was suppressed by the addition of mannose to hexose isomerase mutants pgi1-101 and pmi40-101, which bypassed the requirement for mannose biosynthesis in these mutants. Pathway activation was also suppressed in dpm1-101 mutants by plasmids that contained RER2 or PSA1, which produce the substrates for Dpm1. Activation of FUS1 transcription in the mannose utilization/protein glycosylation mutants required some but not all proteins from three different signaling pathways: the pheromone response, invasive growth, and HOG pathways. We specifically suggest that a Sho1 --> Ste20/Ste50 --> Ste11 --> Ste7 --> Kss1 --> Ste12 pathway is responsible for activation of FUS1 transcription in these mutants. Because loss of pheromone response pathway components leads to a synthetic growth defect in mannose utilization/protein glycosylation mutants, we suggest that the Sho1 --> Ste12 pathway contributes to maintenance of cell wall integrity in vegetative cells.

Cryptococcus neoformans STE12alpha regulates virulence but is not essential for mating

The Cryptococcus neoformans STE12alpha gene, a homologue of Saccharomyces cerevisiae STE12, exists only in mating type (MAT)alpha cells. In S. cerevisiae, STE12 was required for mating and filament formation. In C. neoformans, haploid fruiting on filament agar required STE12alpha. The ability to form hyphae, however, was not affected by deletion of STE12alpha when convergently growing MATa strains were present. Furthermore, ste12alpha disruptants were fertile when mated with MATa strains, albeit with reduced mating frequency. Most importantly, the virulence of a ste12alpha disruptant of serotype D strain was significantly reduced in a mouse model. When the ste12alpha locus was reconstituted with the wild-type allele by cotransformation, virulence was restored. Histopathological analysis demonstrated a reduction in capsular size of yeast cells, less severe cystic lesions, and stronger immune responses in meninges of mice infected with ste12alpha cells than those of mice infected with STE12alpha cells. Using reporter gene constructs, we found that STE12alpha controls the expression of several phenotypes known to be involved in virulence, such as capsule and melanin production. These results demonstrate a clear molecular link between mating type and virulence in C. neoformans.

Potential regulation of Ste20 function by the Cln1-Cdc28 and Cln2-Cdc28 cyclin-dependent protein kinases

The activity of the Saccharomyces cerevisiae pheromone signal transduction pathway is regulated by Cln1/2-Cdc28 cyclin-dependent kinase. High level expression of CLN2 can repress activation of the pathway by mating factor or by deletion of the alpha-subunit of the heterotrimeric G-protein. We now show that CLN2 overexpression can also repress FUS1 induction if the signaling pathway is activated at the level of the beta-subunit of the G-protein (STE4) but not when activated at the level of downstream kinases (STE20 and STE11) or at the level of the transcription factor STE12. This epistatic analysis indicates that repression of pheromone signaling pathway by Cln2-Cdc28 kinase takes place at a level around STE20. In agreement with this, a marked reduction in the electrophoretic mobility of the Ste20 protein is observed at the time in the cell cycle of maximal expression of CLN2. This mobility change is constitutive in cells overexpressing CLN2 and absent in cells lacking CLN1 and CLN2. These changes in electrophoretic mobility correlate with repression of pheromone signaling and suggest Ste20 as a target for repression of signaling by G1 cyclins. Two morphogenic pathways for which Ste20 is essential, pseudohyphal differentiation and haploid-invasive growth, also require CLN1 and CLN2. Together with the previous observation that Cln1 and Cln2 are required for the function of Ste20 in cytokinesis, this suggests that Cln1 and Cln2 regulate the biological activity of Ste20 by promoting morphogenic functions, while inhibiting the mating factor signal transduction function.

The mating factor response pathway regulates transcription of TEC1, a gene involved in pseudohyphal differentiation of Saccharomyces cerevisiae

The transcription factor Tec1 is involved in pseudohyphal differentiation and agar-invasive growth of Saccharomyces cerevisiae cells. The sole element in the TEC1 promoter that has thus far been shown to control Tec1 function is the filament response element. We find that the TEC1 promoter also contains several pheromone response element sequences which are likely to be functional: TEC1 transcription is induced by mating factor, cell cycle regulated and dependent on the Ste4, Ste18 and Ste5 components of the mating factor signal transduction pathway. Using alleles of the transcription factor Ste12 that are defective in DNA binding, transcriptional induction or cooperativity with other transcription factors, we find little correlation between TEC1 transcript levels and agar-invasive growth.

The cell surface flocculin Flo11 is required for pseudohyphae formation and invasion by Saccharomyces cerevisiae

Diploid yeast develop pseudohyphae in response to nitrogen starvation, while haploid yeast produce invasive filaments which penetrate the agar in rich medium. We have identified a gene, FLO11, that encodes a cell wall protein which is critically required for both invasion and pseudohyphae formation in response to nitrogen starvation. FLO11 encodes a cell surface flocculin with a structure similar to the class of yeast serine/threonine-rich GPI-anchored cell wall proteins. Cells of the Saccharomyces cerevisiae strain Sigma1278b with deletions of FLO11 do not form pseudohyphae as diploids nor invade agar as haploids. In rich media, FLO11 is regulated by mating type; it is expressed in haploid cells but not in diploids. Upon transfer to nitrogen starvation media, however, FLO11 transcripts accumulate in diploid cells, but not in haploids. Overexpression of FLO11 in diploid cells, which are otherwise not invasive, enables them to invade agar. Thus, the mating type repression of FLO11 in diploids grown in rich media suffices to explain the inability of these cells to invade. The promoter of FLO11 contains a consensus binding sequence for Ste12p and Tec1p, proteins known to cooperatively activate transcription of Ty1 elements and the TEC1 gene during development of pseudohyphae. Yeast with a deletion of STE12 does not express FLO11 transcripts, indicating that STE12 is required for FLO11 expression. These ste12-deletion strains also do not invade agar. However, the ability to invade can be restored by overexpressing FLO11. Activation of FLO11 may thus be the primary means by which Ste12p and Tec1p cause invasive growth.

Genetic relationships between the G protein beta gamma complex, Ste5p, Ste20p and Cdc42p: investigation of effector roles in the yeast pheromone response pathway

The Saccharomyces cerevisiae G protein beta gamma dimer, Ste4p/Ste18p, acts downstream of the alpha subunit, Gpa1p, to activate the pheromone response pathway and therefore must interact with a downstream effector. Synthetic sterile mutants that exacerbate the phenotype of ste4-ts mutations were isolated to identify proteins that functionally interact with Ste4p. The identification of a ste18 mutant indicated that this screen could identify proteins that interact directly with Ste4p. The other mutations were in STE5 and the STE20 kinase gene, which act near Ste4p in the pathway, and a new gene called STE21. ste20 null mutants showed residual mating, suggesting that another kinase may provide some function. Overexpression of Ste5p under galactose control activated the pheromone response pathway. This activation was dependent on Ste4p and Ste18p and partially dependent on Ste20p. These results cannot be explained by the linear pathway of Ste4p-->Ste20p-->Ste5p. Overexpression of Cdc42p resulted in a slight increase in pheromone induction of a reporter gene, and overexpression of activated forms of Cdc42p resulted in a further twofold increase. Mutations in pheromone response pathway components did not suppress the lethality associated with the activated CDC42 mutations, suggesting that this effect is independent of the pheromone response pathway.

A ste12 allele having a differential effect on a versus alpha cells

The transcriptional activator Ste12p is a key component of the yeast pheromone response pathway: phosphorylated as a consequence of signal transduction, it activates transcription of genes that promote mating and the subsequent fusion of the two cell types a and alpha. Activation by Ste12p requires three types of protein-protein interaction between DNA-binding activator proteins: (1) Ste12p by itself can induce non-cell-type-specific genes involved in mating; (2) cooperation of the transactivator Mcm1p with Ste12p induces a-specific genes; and (3) formation of a complex of the activator proteins Mcm1p and alpha 1 (a transcriptional activator of alpha-specific genes) with Ste12p is believed to induce alpha-specific genes. We isolated and characterized a partially functional ste12 allele (ste12-T50), that is defective only in the activation of alpha-specific genes. ste12-T50 was isolated as a second-site mutation conferring the a mating phenotype on mat alpha 2 mutant cells. In mat alpha 2 cells, where due to the lack of repressor, alpha 2, both sets of cell-type-specific genes are expressed, ste12-T50 apparently tips the balance in favor of a-specific gene expression. Thus, mat alpha 2 ste12-T50 cells mate like a cells. Additional ste12 mutants that confer the a mating phenotype on mat alpha 2 cells have also been isolated.

Genetic interactions between SIN3 mutations and the Saccharomyces cerevisiae transcriptional activators encoded by MCM1, STE12, and SWI1

SIN3 was first identified by a mutation which suppresses the effects of an swi5 mutation on expression of the HO gene in Saccharomyces cerevisiae. We now show that a sin3 mutation also partially suppresses the effects of swi1 on HO transcription, and partially suppresses the growth defect and inositol requirement observed in swi1 mutants. This suggests that SIN3 and SWI1 may play opposite regulatory roles in controlling expression of many yeast genes. Yeast SIN3 has been shown to function as a negative transcriptional regulator of a number of yeast genes. However, expression of the yeast STE6 gene is reduced in a sin3 mutant strain. This suggests that SIN3 functions as a positive regulator for STE6 transcription, although this apparent activation function could be indirect. In order to understand how SIN3 functions in STE6 regulation, we have performed a genetic analysis. It has been previously demonstrated that MCM1 and STE12 are transcriptional activators of a-specific genes such as STE6, and we now show that SWI1 is also required for STE6 expression. Our data suggest that STE12 and SWI1 function in different pathways of activation, and that STE12 is epistatic to SIN3 and SWI1. We show that the activities of the Mcm1p and Ste12p activators are modestly reduced in a sin3 mutant strain, and that phosphorylation of the Ste12p activator is decreased in a sin3 mutant. Thus, it is possible that the decreased transcription of STE6 in sin3 mutants is due to the combined effect of the diminished activities of Mcm1p and Ste12p.

Mutational activation of the STE5 gene product bypasses the requirement for G protein beta and gamma subunits in the yeast pheromone response pathway

The STE5 gene encodes an essential element of the pheromone response pathway which is known to act either after the G subunit encoded by the STE4 gene or at the same step. Mutations in STE5, designated STE5Hyp, that partially activate the pathway in the absence of pheromone were isolated. One allele (STE5Hyp-2) was shown to cause a single amino acid substitution near the N terminus of the predicted STE5 protein. Immunoblotting with anti-Ste5 antibodies indicated that the phenotype was not due to an increased level of the mutant STE5 protein. A multicopy episomal plasmid containing a STE5Hyp allele partially suppressed both the block in pheromone-inducible transcription and the sterility phenotype caused by null alleles of the STE2, STE4, or STE18 gene, indicating that the STE5 product acts after the receptor (STE2 product) and after the G protein beta and gamma subunits (STE4 and STE18 products, respectively). However, the phenotypes of the STE5Hyp mutations were less pronounced in ste4 and ste18 mutants, suggesting that the STE5Hyp-generated signal partially depends on the proposed G beta gamma complex. The STE5Hyp alleles did not suppress ste7, ste11, ste12, or fus3 kss1 null mutants, consistent with previous findings that the STE5 product acts before the protein kinases encoded by STE7, STE11, FUS3, and KSS1 and the transcription factor encoded by STE12. The mating defects of the ste2 deletion mutant and the temperature-sensitive ste4-3 mutant were also suppressed by overexpression of wild-type STE5. The slow-growth phenotype manifested by cells carrying STE5Hyp alleles was enhanced by the sst2-1 mutation; this effect was eliminated in ste4 mutants. These results provide the first evidence that the STE5 gene product performs its function after the G protein subunits.

Positive and negative regulatory elements control expression of the yeast retrotransposon Ty3

We report the results of an analysis of Ty3 transcription and identification of Ty3 regions that mediate pheromone and mating-type regulation to coordinate its expression with the yeast life cycle. A set of strains was constructed which was isogenic except for the number of Ty3 elements, which varied from zero to three. Analysis of Ty3 expression in these strains showed that each of the three elements was transcribed and that each element was regulated. Dissection of the long terminal repeat regulatory region by Northern blot analysis of deletion mutants and reporter gene analysis showed that the upstream junction of Ty3 with flanking chromosomal sequences contained a negative control region. A 19-bp fragment (positions 56-74) containing one consensus copy and one 7 of 8-bp match to the pheromone response element (PRE) consensus was sufficient to mediate pheromone induction in either haploid cell type. Deletion of this region, however, did not abolish expression, indicating that other sequences also activate transcription. A 24-bp block immediately downstream of the PRE region contained a sequence similar to the a1-alpha 2 consensus that conferred mating-type control. A single base pair mutation in the region separating the PRE and a1-alpha 2 sequences blocked pheromone induction, but not mating-type control. Thus, the long terminal repeat of Ty3 is a compact, highly regulated, mobile promoter which is responsive to cell type and mating.

The yeast SFL2 gene may be necessary for mating-type control

We previously reported the isolation of the yeast suppressor gene for flocculation, SFL2 (TUP1). SFL2 gene disruption results in pleiotropic phenotypes; the sfl2 null mutation also causes a morphological change similar to shmoo in both the MAT alpha and MATa/alpha cells. The MAT alpha and MATa/alpha sfl2 null mutant cells incorporate chitin into the new growth zone in the same way as the alpha-factor-treated MATa cells. In order to clarify the molecular basis of this morphological change, we examined the effect of the sfl2 null mutation on the mRNA production of various genes involved in mating-type control. The transcripts of both the STE2 (an a-specific gene) and STE3 (an alpha specific gene) genes are detected in the MAT alpha and MATa/alpha cells carrying the sfl2 null mutation. In addition, mRNA of the GPA1 gene (haploid-cell-specific gene) is also detected in the MATa/alpha sfl2 cells. However, there is no significant difference in the levels of the MAT alpha 2 and MATa1 transcripts. These results suggest that the SFL2 gene product may be necessary for alpha 2 and a1-alpha 2 repression.

Pheromone response elements are necessary and sufficient for basal and pheromone-induced transcription of the FUS1 gene of Saccharomyces cerevisiae

The FUS1 gene of Saccharomyces cerevisiae is transcribed in a and alpha cells, not in a/alpha diploids, and its transcription increases dramatically when haploid cells are exposed to the appropriate mating pheromone. In addition, FUS1 transcription is absolutely dependent on STE4, STE5, STE7, STE11, and STE12, genes thought to encode components of the pheromone response pathway. We now have determined that the pheromone response element (PRE), which occurs in four copies within the FUS1 upstream region, functions as the FUS1 upstream activation sequence (UAS) and is responsible for all known aspects of FUS1 regulation. In particular, deletion of 55 bp that includes the PREs abolished all transcription, and a 139-bp fragment that includes the PREs conferred FUS1-like expression to a CYC1-lacZ reporter gene. Moreover, three or four copies of a synthetic PRE closely mimicked the activity conferred by the 139-bp fragment, and even a single copy of PRE conferred a trace of activity that was haploid specific and pheromone inducible. In the FUS1 promoter context, four copies of the synthetic PRE inserted at the site of the 55-bp deletion restored full FUS1 transcription. Sequences upstream and downstream from the PRE cluster were important for maximal PRE-directed expression but, by themselves, did not have UAS activity. Other yeast genes with PREs, e.g., STE2 and BAR1, are more modestly inducible and have additional UAS elements contributing to the overall activity. In the FUS1 promoter, the PREs apparently act alone to confer activity that is highly stimulated by pheromone.

Identification of a gene necessary for cell cycle arrest by a negative growth factor of yeast: FAR1 is an inhibitor of a G1 cyclin, CLN2

alpha factor is a negative growth factor and differentiation factor that induces G1 arrest and increases transcription of mating genes in S. cerevisiae a cells. We have identified a gene, FAR1 (for "factor arrest"), which is necessary for cell cycle arrest but not for other responses to alpha factor: far1- mutants are insensitive to arrest despite having an intact signal transduction pathway. FAR1 is a nonessential gene whose expression is induced 4- to 5-fold in a cells by alpha factor. The sequence of FAR1 indicates no significant similarities to known proteins. A null mutation in the CLN2 gene, which codes for a G1 cyclin, reverses the effect of a far1 null mutation: far1- cln2- strains arrest in response to alpha factor. We thus propose that FAR1 contributes to cell cycle arrest by inhibiting CLN2. The behavior of far1- cln2- strains indicates that products other than FAR1 are responsible for inhibiting the other G1 cyclins, CLN1 and CLN3.

A regulatory hierarchy for cell specialization in yeast

The specialized sets of genes that determine different cell types in yeast are controlled by combinations of DNA-binding proteins some of which are present only in certain cell types whereas others are present in all cell types. Final differentiation requires an inductive signal that triggers both gene transcription and cell-cycle arrest. Synthesis of the proteins coded by the 'master regulatory' mating-type locus is regulated so as to generate a heterogeneous mitotic cell population containing a stem-cell lineage.

The MF alpha 1 gene of Saccharomyces cerevisiae: genetic mapping and mutational analysis of promoter elements

The activity and cell-type specificity of the promoter of the MF alpha 1 gene of Saccharomyces cerevisiae were examined by measuring expression of an MF alpha 1-SUC2 gene fusion in MATa, MAT alpha, and MATa/MAT alpha cells. A high level of invertase activity was observed only in MAT alpha cells. Weak expression occurred in MATa cells when the hybrid gene was carried on a multicopy plasmid or on a centromere-containing plasmid, but not when the hybrid gene was integrated at the normal MF alpha 1 locus. Analysis of a set of 5'-deletions of the promoter region of the MF alpha 1-SUC2 gene on the multicopy plasmid indicated that sequences from -354 to -274 upstream of the translational start site were required for high level expression in MAT alpha cells. Smaller internal deletions and insertions within the promoter region of the MF alpha 1-SUC2 gene were inserted into the genome at the normal MF alpha 1 locus. These mutations further delineated four promoter domains important for expression: (1) two 26 bp elements (-365 to -340 and -312 to -287) with imperfect dyad symmetry; (2) a 40 bp segment (-264 to -226) that lies about 120 bp upstream of the TATA box; and (3) the TATA box itself (-128 to -122). The transcriptional start sites of the normal MF alpha 1 promoter and of a mutant lacking the TATA box were determined. The MF alpha 1 locus was mapped to the left arm of chromosome XVI, about 22 cM centromere-proximal to the PEP4 gene.