MAT alpha 1 can mediate gene activation by a-mating factor

Fungal natural products galaxy: Biochemistry and molecular genetics toward blockbuster drugs discovery

Secondary metabolites synthesized by fungi have become a precious source of inspiration for the design of novel drugs. Indeed, fungi are prolific producers of fascinating, diverse, structurally complex, and low-molecular-mass natural products with high therapeutic leads, such as novel antimicrobial compounds, anticancer compounds, immunosuppressive agents, among others. Given that these microorganisms possess the extraordinary capacity to secrete diverse chemical scaffolds, they have been highly exploited by the giant pharma companies to generate small molecules. This has been made possible because the isolation of metabolites from fungal natural sources is feasible and surpasses the organic synthesis of compounds, which otherwise remains a significant bottleneck in the drug discovery process. Here in this comprehensive review, we have discussed recent studies on different fungi (pathogenic, non-pathogenic, commensal, and endophytic/symbiotic) from different habitats (terrestrial and marines), the specialized metabolites they biosynthesize, and the drugs derived from these specialized metabolites. Moreover, we have unveiled the logic behind the biosynthesis of vital chemical scaffolds, such as NRPS, PKS, PKS-NRPS hybrid, RiPPS, terpenoids, indole alkaloids, and their genetic mechanisms. Besides, we have provided a glimpse of the concept behind mycotoxins, virulence factor, and host immune response based on fungal infections.

Fungal Sex: The Ascomycota

This article provides an overview of sexual reproduction in the ascomycetes, a phylum of fungi that is named after the specialized sacs or "asci" that hold the sexual spores. They have therefore also been referred to as the Sac Fungi due to these characteristic structures that typically contain four to eight ascospores. Ascomycetes are morphologically diverse and include single-celled yeasts, filamentous fungi, and more complex cup fungi. The sexual cycles of many species, including those of the model yeasts Saccharomyces cerevisiae and Schizosaccharomyces pombe and the filamentous saprobes Neurospora crassa, Aspergillus nidulans, and Podospora anserina, have been examined in depth. In addition, sexual or parasexual cycles have been uncovered in important human pathogens such as Candida albicans and Aspergillus fumigatus, as well as in plant pathogens such as Fusarium graminearum and Cochliobolus heterostrophus. We summarize what is known about sexual fecundity in ascomycetes, examine how structural changes at the mating-type locus dictate sexual behavior, and discuss recent studies that reveal that pheromone signaling pathways can be repurposed to serve cellular roles unrelated to sex.

Molecular and morphological data support the existence of a sexual cycle in species of the genus Paracoccidioides

The genus Paracoccidioides includes the thermodimorphic species Paracoccidioides brasiliensis and P. lutzii, both of which are etiologic agents of paracoccidioidomycosis, a systemic mycosis that affects humans in Latin America. Despite the common occurrence of a sexual stage among closely related fungi, this has not been observed with Paracoccidioides species, which have thus been considered asexual. Molecular evolutionary studies revealed recombination events within isolated populations of the genus Paracoccidioides, suggesting the possible existence of a sexual cycle. Comparative genomic analysis of all dimorphic fungi and Saccharomyces cerevisiae demonstrated the presence of conserved genes involved in sexual reproduction, including those encoding mating regulators such as MAT, pheromone receptors, pheromone-processing enzymes, and mating signaling regulators. The expression of sex-related genes in the yeast and mycelial phases of both Paracoccidioides species was also detected by real-time PCR, with nearly all of these genes being expressed preferentially in the filamentous form of the pathogens. In addition, the expression of sex-related genes was responsive to the putative presence of pheromone in the supernatants obtained from previous cocultures of strains of two different mating types. In vitro crossing of isolates of different mating types, discriminated by phylogenetic analysis of the α-box (MAT1-1) and the high-mobility-group (HMG) domain (MAT1-2), led to the identification of the formation of young ascocarps with constricted coiled hyphae related to the initial stage of mating. These genomic and morphological analyses strongly support the existence of a sexual cycle in species of the genus Paracoccidioides.

Organizational constraints on Ste12 cis-elements for a pheromone response in Saccharomyces cerevisiae

Ste12 of Saccharomyces cerevisiae binds to pheromone-response cis-elements (PREs) to regulate several classes of genes. Genes induced by pheromones require multimerization of Ste12 for binding of at least two PREs on responsive promoters. We have systematically examined nucleotides of the consensus PRE for binding of wild-type Ste12 to DNA in vitro, as well as the organizational requirements of PREs to produce a pheromone response in vivo. Ste12 binds as a monomer to a single PRE in vitro, and two PREs upstream of a minimal core promoter cause induction that is proportional to their relative affinity for Ste12 in vitro. Although consensus PREs are arranged in a variety of configurations in the promoters of responsive genes, we find that there are severe constraints with respect to how they can be positioned in an artificial promoter to cause induction. Two closely-spaced PREs can induce transcription in a directly-repeated or tail-to-tail orientation, although PREs separated by at least 40 nucleotides are capable of inducing transcription when oriented in a head-to-head or tail-to-tail configuration. We characterize several examples of promoters that bear multiple consensus PREs or a single PRE in combination with a PRE-like sequence that match these requirements. A significant number of responsive genes appear to possess only a single PRE, or PREs in configurations that would not be expected to enable induction, and we suggest that, for many pheromone-responsive genes, Ste12 must activate transcription by binding to cryptic or sub-optimal sites on DNA, or may require interaction with additional uncharacterized DNA bound factors.

Function and regulation in MAPK signaling pathways: lessons learned from the yeast Saccharomyces cerevisiae

Signaling pathways that activate different mitogen-activated protein kinases (MAPKs) elicit many of the responses that are evoked in cells by changes in certain environmental conditions and upon exposure to a variety of hormonal and other stimuli. These pathways were first elucidated in the unicellular eukaryote Saccharomyces cerevisiae (budding yeast). Studies of MAPK pathways in this organism continue to be especially informative in revealing the molecular mechanisms by which MAPK cascades operate, propagate signals, modulate cellular processes, and are controlled by regulatory factors both internal to and external to the pathways. Here we highlight recent advances and new insights about MAPK-based signaling that have been made through studies in yeast, which provide lessons directly applicable to, and that enhance our understanding of, MAPK-mediated signaling in mammalian cells.

The B-box dominates SAP-1-SRF interactions in the structure of the ternary complex

The serum response element (SRE) is found in several immediate-early gene promoters. This DNA sequence is necessary and sufficient for rapid transcriptional induction of the human c-fos proto-oncogene in response to stimuli external to the cell. Full activation of the SRE requires the cooperative binding of a ternary complex factor (TCF) and serum response factor (SRF) to their specific DNA sites. The X-ray structure of the human SAP-1-SRF-SRE DNA ternary complex was determined (Protein Data Bank code 1hbx). It shows SAP-1 TCF bound to SRF through interactions between the SAP-1 B-box and SRF MADS domain in addition to contacts between their respective DNA-binding motifs. The SAP-1 B-box is part of a flexible linker of which 21 amino acids become ordered upon ternary complex formation. Comparison with a similar region from the yeast MATalpha2-MCM1-DNA complex suggests a common binding motif through which MADS-box proteins may interact with additional factors such as Fli-1.

Constitutive activation of the Saccharomyces cerevisiae transcriptional regulator Ste12p by mutations at the amino-terminus

The transcriptional activator Ste12p is required for the expression of genes induced by mating pheromone in the yeast Saccharomyces cerevisiae. We identified mutations in the amino-terminal DNA-binding domain of Ste12p that lead to constitutively high-level transcription of pheromone-induced genes. The behaviour of these mutant proteins is consistent with an enhanced DNA-binding ability. Cells carrying these hyperactive proteins retain their sensitivity to pheromone treatment, and their phenotype is largely dependent on the presence of at least one of the MAP kinases (Fus3p or Kss1p) and the scaffold protein Ste5p. Deletion of either FUS3 or KSS1 leads to a marked increase in Ste12p activity, consistent with a negative regulatory role for Fus3p, similar to that described for Kss1p. The properties of the constitutive mutants support the idea that the pheromone response pathway plays a role in basal as well as pheromone-induced transcription.

Two regulators of Ste12p inhibit pheromone-responsive transcription by separate mechanisms

The yeast Saccharomyces cerevisiae transcription factor Ste12p is responsible for activating genes in response to MAP kinase cascades controlling mating and filamentous growth. Ste12p is negatively regulated by two inhibitor proteins, Dig1p (also called Rst1p) and Dig2p (also called Rst2p). The expression of a C-terminal Ste12p fragment (residues 216 to 688) [Ste12p(216-688)] from a GAL promoter causes FUS1 induction in a strain expressing wild-type STE12, suggesting that this region can cause the activation of endogenous Ste12p. Residues 262 to 594 are sufficient to cause STE12-dependent FUS1 induction when overexpressed, and this region of Ste12p was found to bind Dig1p but not Dig2p in yeast extracts. In contrast, recombinant glutathione S-transferase-Dig2p binds to the Ste12p DNA-binding domain (DBD). Expression of DIG2, but not DIG1, from a GAL promoter inhibits transcriptional activation by an Ste12p DBD-VP16 fusion. Furthermore, disruption of dig1, but not dig2, causes elevated transcriptional activation by a LexA-Ste12p(216-688) fusion. Ste12p has multiple regions within the C terminus (flanking residue 474) that can promote multimerization in vitro, and we demonstrate that these interactions can contribute to the activation of endogenous Ste12p by overproduced C-terminal fragments. These results demonstrate that Dig1p and Dig2p do not function by redundant mechanisms but rather inhibit pheromone-responsive transcription through interactions with separate regions of Ste12p.

Comprehensive identification of cell cycle-regulated genes of the yeast Saccharomyces cerevisiae by microarray hybridization

We sought to create a comprehensive catalog of yeast genes whose transcript levels vary periodically within the cell cycle. To this end, we used DNA microarrays and samples from yeast cultures synchronized by three independent methods: alpha factor arrest, elutriation, and arrest of a cdc15 temperature-sensitive mutant. Using periodicity and correlation algorithms, we identified 800 genes that meet an objective minimum criterion for cell cycle regulation. In separate experiments, designed to examine the effects of inducing either the G1 cyclin Cln3p or the B-type cyclin Clb2p, we found that the mRNA levels of more than half of these 800 genes respond to one or both of these cyclins. Furthermore, we analyzed our set of cell cycle-regulated genes for known and new promoter elements and show that several known elements (or variations thereof) contain information predictive of cell cycle regulation. A full description and complete data sets are available at http://cellcycle-www.stanford.edu

TFII-I enhances activation of the c-fos promoter through interactions with upstream elements

The transcription factor TFII-I was initially isolated as a factor that can bind to initiator elements in core promoters. Recent evidence suggests that TFII-I may also have a role in signal transduction. We have found that overexpression of TFII-I can enhance the response of the wild-type c-fos promoter to a variety of stimuli. This effect depends on the c-fos c-sis-platelet-derived growth factor-inducible factor binding element (SIE) and serum response element (SRE). There is no effect of cotransfected TFII-I on the TATA box containing the c-fos basal promoter. Three TFII-I binding sites can be found in c-fos promoter. Two of these overlap the c-fos SIE and SRE, and another is located just upstream of the TATA box. Mutations that distinguish between serum response factor (SRF), STAT, and TFII-I binding to the c-fos SIE and SRE suggest that the binding of TFII-I to these elements is important for c-fos induction in conjunction with the SRF and STAT transcription factors. Moreover, TFII-I can form in vivo protein-protein complexes with the c-fos upstream activators SRF, STAT1, and STAT3. These results suggest that TFII-I may mediate the functional interdependence of the c-fos SIE and SRE elements. In addition, the ras pathway is required for TFII-I to exert its effects on the c-fos promoter, and growth factor stimulation enhances tyrosine phosphorylation of TFII-I. These results indicate that TFII-I is involved in signal transduction as well as transcriptional activation of the c-fos promoter.

Characterization of the basal and pheromone-stimulated phosphorylation states of Ste12p

The Saccharomyces cerevisiae transcription factor Ste12p is required for basal and activated expression of pheromone-responsive genes, and for invasive growth in haploid cells. In diploid yeast, Ste12p is implicated in pseudohyphal development. The ability of Ste12p to effect these various responses in three different cell types must require stringent regulation of its transcriptional activation function and interaction with additional transcription factors. We have examined the phosphorylation state of Ste12p in untreated and pheromone-treated haploid cells, and found eight constitutively phosphorylated peptides. Phosphorylation at the constitutive sites does not require the protein kinases of the pheromone-response pathway. Treatment of haploid yeast with mating pheromone causes the appearance of novel relatively minor phosphorylations on Ste12p. Brief [35S]methionine labeling reveals novel pheromone-dependent, electrophoretically slower migrating Ste12p species. Similarly, the sole difference we observe in tryptic phosphopeptides generated from Ste12p from pheromone-treated and untreated cells is the transient appearance of two novel minor hydrophobic phosphopeptides. The pheromone-dependent phosphorylation of Ste12p requires an intact pheromone-response pathway and localization of Ste12p to the nucleus, but does not require the Ste12p DNA-binding domain. We conclude from these experiments that the pheromone-response pathway induces the formation of specific hyperphosphorylation on Ste12p, which can only be detected as apparently minor modifications in vivo. We argue that, if Ste12p is regulated by direct pheromone-responsive phosphorylation, then that phosphorylation must be represented by the two novel phosphopeptides. However, we cannot exclude the possibility that pheromone-responsive transcription is controlled by direct phosphorylation of a target other than Ste12p.

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.

Transcription of alpha-specific genes in Saccharomyces cerevisiae: DNA sequence requirements for activity of the coregulator alpha 1

Transcription activation of alpha-specific genes in Saccharomyces cerevisiae is regulated by two proteins, MCM1 and alpha 1, which bind to DNA sequences, called P'Q elements, found upstream of alpha-specific genes. Neither MCM1 nor alpha 1 alone binds efficiently to P'Q elements. Together, however, they bind cooperatively in a manner that requires both the P' sequence, which is a weak binding site for MCM1, and the Q sequence, which has been postulated to be the binding site for alpha 1. We analyzed a collection of point mutations in the P'Q element of the STE3 gene to determine the importance of individual base pairs for alpha-specific gene transcription. Within the 10-bp conserved Q sequence, mutations at only three positions strongly affected transcription activation in vivo. These same mutations did not affect the weak binding to P'Q displayed by MCM1 alone. In vitro DNA binding assays showed a direct correlation between the ability of the mutant sequences to form ternary P'Q-MCM1-alpha 1 complexes and the degree to which transcription was activated in vivo. Thus, the ability of alpha 1 and MCM1 to bind cooperatively to P'Q elements is critical for activation of alpha-specific genes. In all natural alpha-specific genes the Q sequence is adjacent to the degenerate side of P'. To test the significance of this geometry, we created several novel juxtapositions of P, P', and Q sequences. When the Q sequence was opposite the degenerate side, the composite QP' element was inactive as a promoter element in vivo and unable to form stable ternary QP'-MCM1-alpha 1 complexes in vitro. We also found that addition of a Q sequence to a strong MCM1 binding site allows the addition of alpha 1 to the complex. This finding, together with the observation that Q-element point mutations affected ternary complex formation but not the weak binding of MCM1 alone, supports the idea that the Q sequence serves as a binding site for alpha 1.

Transcription of alpha-specific genes in Saccharomyces cerevisiae: DNA sequence requirements for activity of the coregulator alpha 1

Transcription activation of alpha-specific genes in Saccharomyces cerevisiae is regulated by two proteins, MCM1 and alpha 1, which bind to DNA sequences, called P'Q elements, found upstream of alpha-specific genes. Neither MCM1 nor alpha 1 alone binds efficiently to P'Q elements. Together, however, they bind cooperatively in a manner that requires both the P' sequence, which is a weak binding site for MCM1, and the Q sequence, which has been postulated to be the binding site for alpha 1. We analyzed a collection of point mutations in the P'Q element of the STE3 gene to determine the importance of individual base pairs for alpha-specific gene transcription. Within the 10-bp conserved Q sequence, mutations at only three positions strongly affected transcription activation in vivo. These same mutations did not affect the weak binding to P'Q displayed by MCM1 alone. In vitro DNA binding assays showed a direct correlation between the ability of the mutant sequences to form ternary P'Q-MCM1-alpha 1 complexes and the degree to which transcription was activated in vivo. Thus, the ability of alpha 1 and MCM1 to bind cooperatively to P'Q elements is critical for activation of alpha-specific genes. In all natural alpha-specific genes the Q sequence is adjacent to the degenerate side of P'. To test the significance of this geometry, we created several novel juxtapositions of P, P', and Q sequences. When the Q sequence was opposite the degenerate side, the composite QP' element was inactive as a promoter element in vivo and unable to form stable ternary QP'-MCM1-alpha 1 complexes in vitro. We also found that addition of a Q sequence to a strong MCM1 binding site allows the addition of alpha 1 to the complex. This finding, together with the observation that Q-element point mutations affected ternary complex formation but not the weak binding of MCM1 alone, supports the idea that the Q sequence serves as a binding site for alpha 1.

Coupling of cell identity to signal response in yeast: interaction between the alpha 1 and STE12 proteins

In Saccharomyces cerevisiae, the STE12 protein mediates transcriptional induction of cell type-specific genes in response to pheromones. STE12 binds in vitro to the pheromone response elements (PREs) present in the control region of a-specific genes. STE12 is also required for transcription of alpha-specific genes, but there is no evidence that it binds directly to these genes. Instead, the MAT alpha-encoded protein alpha 1 and the MCM1 product bind to the DNA element that is responsible for alpha-specific and a-factor-inducible expression. To explore the role of STE12 in the pheromone induction of alpha-specific genes, we cloned STE12 and MAT alpha 1 homologs from the related yeast Kluyveromyces lactis. The K. lactis STE12 protein did not cooperate with the S. cerevisiae alpha 1 protein to promote the overall mating process or the induction of transcription of an alpha-specific gene. However, introduction of both K. lactis STE12 along with K. lactis alpha 1 did restore mating, suggesting that an interaction between STE12 and alpha 1 is important for alpha-specific gene activation. We also show that bacterially expressed STE12 and alpha 1 are able to form a complex in vitro. Thus, we demonstrate a coupling in alpha cells between a protein functioning in cell identity, alpha 1, with a protein responsive to the pheromone-induced signal STE12.

Functional analysis of a growth factor-responsive transcription factor complex

Serum response factor (SRF) forms a ternary complex at the c-fos serum response element (SRE) with an accessory factor, Elk-1. We constructed altered-binding specificity derivatives of SRF and Elk-1 that form a ternary complex at a mutated, inactive SRE; like Elk-1, the Elk-1 variant only binds its target as part of a ternary complex with SRF. Simultaneous expression of these SRF and Elk-1 derivatives restores serum-regulated activity to the mutated SRE in transfected cells. Efficient transcriptional activation is dependent on the regulated phosphorylation of Elk-1 C-terminal MAP kinase sites and requires the C-terminal sequences of SRF as well as SRF sequences that mediate ternary complex formation. These experiments provide direct evidence that SRF and Elk-1 functionally cooperate in the ternary complex at the SRE to regulate transcription.

The N-terminal 96 residues of MCM1, a regulator of cell type-specific genes in Saccharomyces cerevisiae, are sufficient for DNA binding, transcription activation, and interaction with alpha 1

MCM1 performs several functions necessary for its role in regulating cell type-specific gene expression in the yeast Saccharomyces cerevisiae: DNA binding, transcription activation, and interaction with coregulatory proteins such as alpha 1. We analyzed a set of MCM1 deletion derivatives using in vivo reporter gene assays and in vitro DNA-binding studies to determine which regions of MCM1 are important for its various activities. We also analyzed a set of LexA-MCM1 hybrids to examine the ability of different segments of MCM1 to activate transcription independent of MCM1's DNA-binding function. The first third of MCM1 [MCM1(1-96)], which includes an 80-residue segment homologous to the mammalian serum response factor, was sufficient for high-affinity DNA binding, for activation of reporter gene expression, and for interaction with alpha 1 in vitro and in vivo. However, the ability of MCM1(1-96) to activate transcription and to interact with alpha 1 was somewhat reduced compared with wild-type MCM1 [MCM1(1-286)]. Optimal interaction with alpha 1 required residues 99 to 117, in which 18 of 19 amino acids are acidic in character. Optimal transcription activation required a segment from residues 188 to 286, in which 50% of the amino acids are glutamine. Deletion of this segment from MCM1 reduced expression of reporter genes by about twofold. Moreover, LexA-MCM1 hybrids containing this segment were able to activate expression of reporter genes that rely on LexA binding sites as potential upstream activation sequences. Thus, glutamine-rich regions may contribute to the activation function of yeast transcription activators, as has been suggested for glutamine-rich mammalian proteins such as Sp1.

The N-terminal 96 residues of MCM1, a regulator of cell type-specific genes in Saccharomyces cerevisiae, are sufficient for DNA binding, transcription activation, and interaction with alpha 1

MCM1 performs several functions necessary for its role in regulating cell type-specific gene expression in the yeast Saccharomyces cerevisiae: DNA binding, transcription activation, and interaction with coregulatory proteins such as alpha 1. We analyzed a set of MCM1 deletion derivatives using in vivo reporter gene assays and in vitro DNA-binding studies to determine which regions of MCM1 are important for its various activities. We also analyzed a set of LexA-MCM1 hybrids to examine the ability of different segments of MCM1 to activate transcription independent of MCM1's DNA-binding function. The first third of MCM1 [MCM1(1-96)], which includes an 80-residue segment homologous to the mammalian serum response factor, was sufficient for high-affinity DNA binding, for activation of reporter gene expression, and for interaction with alpha 1 in vitro and in vivo. However, the ability of MCM1(1-96) to activate transcription and to interact with alpha 1 was somewhat reduced compared with wild-type MCM1 [MCM1(1-286)]. Optimal interaction with alpha 1 required residues 99 to 117, in which 18 of 19 amino acids are acidic in character. Optimal transcription activation required a segment from residues 188 to 286, in which 50% of the amino acids are glutamine. Deletion of this segment from MCM1 reduced expression of reporter genes by about twofold. Moreover, LexA-MCM1 hybrids containing this segment were able to activate expression of reporter genes that rely on LexA binding sites as potential upstream activation sequences. Thus, glutamine-rich regions may contribute to the activation function of yeast transcription activators, as has been suggested for glutamine-rich mammalian proteins such as Sp1.

The SRF and MCM1 transcription factors

The mammalian transcription factor SRF (serum-response factor) and the related Saccharomyces cerevisiae transcription factor MCM1 are the prototypes of a new class of dimeric DNA-binding proteins. Their function is regulated in part by the interactions of their DNA-binding domains with accessory proteins. Recent work has advanced the functional characterization of the contributions of SRF and MCM1, and their accessory proteins to transcriptional activation.