DNA-bound Bas1 recruits Pho2 to activate ADE genes in Saccharomyces cerevisiae

Grf10 regulates the response to copper, iron, and phosphate in Candida albicans

The pathogenic yeast, Candida albicans, and other microbes must be able to handle drastic changes in nutrient availability within the human host. Copper, iron, and phosphate are essential micronutrients for microbes that are sequestered by the human host as nutritional immunity; yet high copper levels are employed by macrophages to induce toxic oxidative stress. Grf10 is a transcription factor important for regulating genes involved in morphogenesis (filamentation, chlamydospore formation) and metabolism (adenylate biosynthesis, 1-carbon metabolism). The grf10Δ mutant exhibited resistance to excess copper in a gene dosage-dependent manner but grew the same as the wild type in response to other metals (calcium, cobalt, iron, manganese, and zinc). Point mutations in the conserved residues D302 and E305, within a protein interaction region, conferred resistance to high copper and induced hyphal formation similar to strains with the null allele. The grf10Δ mutant misregulated genes involved with copper, iron, and phosphate uptake in YPD medium and mounted a normal transcriptional response to high copper. The mutant accumulated lower levels of magnesium and phosphorus, suggesting that copper resistance is linked to phosphate metabolism. Our results highlight new roles for Grf10 in copper and phosphate homeostasis in C. albicans and underscore the fundamental role of Grf10 in connecting these with cell survival.

The Histone Deacetylases Hst1 and Rpd3 Integrate De Novo NAD+ Metabolism with Phosphate Sensing in Saccharomyces cerevisiae

Nicotinamide adenine dinucleotide (NAD⁺) is a critical cofactor essential for various cellular processes. Abnormalities in NAD⁺ metabolism have also been associated with a number of metabolic disorders. The regulation and interconnection of NAD⁺ metabolic pathways are not yet completely understood. By employing an NAD⁺ intermediate-specific genetic system established in the model organism S. cerevisiae, we show that histone deacetylases (HDACs) Hst1 and Rpd3 link the regulation of the de novo NAD⁺ metabolism-mediating BNA genes with certain aspects of the phosphate (Pi)-sensing PHO pathway. Our genetic and gene expression studies suggest that the Bas1-Pho2 and Pho2-Pho4 transcription activator complexes play a role in this co-regulation. Our results suggest a model in which competition for Pho2 usage between the BNA-activating Bas1-Pho2 complex and the PHO-activating Pho2-Pho4 complex helps balance de novo activity with PHO activity in response to NAD⁺ or phosphate depletion. Interestingly, both the Bas1-Pho2 and Pho2-Pho4 complexes appear to also regulate the expression of the salvage-mediating PNC1 gene negatively. These results suggest a mechanism for the inverse regulation between the NAD⁺ salvage pathways and the de novo pathway observed in our genetic models. Our findings help provide a molecular basis for the complex interplay of two different aspects of cellular metabolism.

Selection markers for transformation of the sequenced reference monokaryon Okayama 7/#130 and homokaryon AmutBmut of Coprinopsis cinerea

BACKGROUND

Two reference strains have been sequenced from the mushroom Coprinopsis cinerea, monokaryon Okayama 7/#130 (OK130) and the self-compatible homokaryon AmutBmut. An adenine-auxotrophy in OK130 (ade8-1) and a para-aminobenzoic acid (PABA)-auxotrophy in AmutBmut (pab1-1) offer selection markers for transformations. Of these two strains, homokaryon AmutBmut had been transformed before to PABA-prototrophy and with the bacterial hygromycin resistance marker hph, respectively.

RESULTS

Gene ade8 encodes a bifunctional enzyme with an N-terminal glycinamide ribonucleotide synthase (GARS) and a C-terminal aminoimidazole ribonucleotide synthase (AIRS) domain required for steps 2 and 5 in the de novo biosynthesis of purines, respectively. In OK130, a missense mutation in ade8-1 rendered residue N231 for ribose recognition by the A loop of the GARS domain into D231. The new ade8 + vector pCcAde8 complements the auxotrophy of OK130 in transformations. Transformation rates with pCcAde8 in single-vector and co-transformations with ade8 +-selection were similarly high, unlike for trp1 + plasmids which exhibit suicidal feedback-effects in single-vector transformations with complementation of tryptophan synthase defects. As various other plasmids, unselected pCcAde8 helped in co-transformations of trp1 strains with a trp1 +-selection vector to overcome suicidal effects by transferred trp1 +. Co-transformation rates of pCcAde8 in OK130 under adenine selection with nuclear integration of unselected DNA were as high as 80% of clones. Co-transformation rates of expressed genes reached 26-42% for various laccase genes and up to 67% with lcc9 silencing vectors. The bacterial gene hph can also be used as another, albeit less efficient, selection marker for OK130 transformants, but with similarly high co-transformation rates. We further show that the pab1-1 defect in AmutBmut is due to a missense mutation which changed the conserved PIKGT motif for chorismate binding in the C-terminal PabB domain to PIEGT in the mutated 4-amino-4-deoxychorismate synthase.

CONCLUSIONS

ade8-1 and pab1-1 auxotrophic defects in C. cinerea reference strains OK130 and AmutBmut for complementation in transformation are described. pCcAde8 is a new transformation vector useful for selection in single and co-transformations of the sequenced monokaryon OK130 which was transformed for the first time. The bacterial gene hph can also be used as an additional selection marker in OK130, making in combination with ade8 + successive rounds of transformation possible.

Functional Mapping of Transcription Factor Grf10 That Regulates Adenine-Responsive and Filamentation Genes in Candida albicans

Grf10, a homeodomain-containing transcription factor, regulates adenylate and one-carbon metabolism and morphogenesis in the human fungal pathogen Candida albicans Here, we identified functional domains and key residues involved in transcription factor activity using one-hybrid and mutational analyses. We localized activation domains to the C-terminal half of the Grf10 protein by one-hybrid analysis and identified motifs using bioinformatic analyses; one of the characterized activation domains (AD1) responded to temperature. The LexA-Grf10 fusion protein activated the lexAop-HIS1 reporter in an adenine-dependent fashion, and this activation was independent of Bas1, showing that the adenine limitation signal is transmitted directly to Grf10. Overexpression of LexA-Grf10 led to filamentation, and this required a functioning homeodomain, consistent with Grf10 controlling the expression of key filamentation genes; filamentation induced by LexA-Grf10 overexpression was independent of adenine levels and Bas1. Alanine substitutions were made within the conserved interaction regions (IR) of LexA-Grf10 and Grf10 to investigate roles in transcription. In LexA-Grf10, the D302A mutation activated transcription constitutively, and the E305A mutation was regulated by adenine. When these mutations were introduced into the native gene locus, the D302A mutation was unable to complement the ADE phenotype and did not promote filamentation under hypha-inducing conditions; the E305A mutant behaved as the native gene with respect to the ADE phenotype and was partially defective in inducing hyphae. These results demonstrate allele-specific responses with respect to the different phenotypes, consistent with perturbations in the ability of Grf10 to interact with multiple partner proteins.IMPORTANCE Metabolic adaptation and morphogenesis are essential for Candida albicans, a major human fungal pathogen, to survive and infect diverse body sites in the mammalian host. C. albicans utilizes transcription factors to tightly control the transcription of metabolic genes and morphogenesis genes. Grf10, a critical homeodomain transcription factor, controls purine and one-carbon metabolism in response to adenine limitation, and Grf10 is necessary for the yeast-to-hypha morphological switching, a known virulence factor. Here, we carried out one-hybrid and mutational analyses to identify functional domains of Grf10. Our results show that Grf10 separately regulates metabolic and morphogenesis genes, and it contains a conserved protein domain for protein partner interaction, allowing Grf10 to control the transcription of multiple distinct pathways. Our findings contribute significantly to understanding the role and mechanism of transcription factors that control multiple pathogenic traits in C. albicans.

Grf10 and Bas1 Regulate Transcription of Adenylate and One-Carbon Biosynthesis Genes and Affect Virulence in the Human Fungal Pathogen Candida albicans

Candida albicans is an opportunistic human fungal pathogen that causes superficial fungal infections and lethal systemic infections. To colonize and establish infections, C. albicans coordinates the expression of virulence and metabolic genes. Previous work showed that the homeodomain transcription factor Grf10 is required for formation of hyphae, a virulence factor. Here we report global gene expression analysis of a grf10Δ strain using a DNA microarray and identify genes for de novo adenylate biosynthesis (ADE genes), one-carbon metabolism, and a nucleoside permease (NUP). Upregulation of these genes in response to adenine limitation required both Grf10 and the myb protein Bas1, as shown by quantitative real-time PCR (qRT-PCR). Phenotypic analysis showed that both mutants exhibited growth defects when grown in the absence of adenine, and the doubling time was slower for the bas1Δ mutant. Bas1 is required for basal expression of these genes, whereas NUP expression is more dependent upon Grf10. Disruption of BAS1 led to only modest defects in hypha formation and weak attenuation of virulence in a systemic mouse model of infection, as opposed to the previously reported strong effects found in the grf10Δ mutant. Our data are consistent with a model in which Grf10 coordinates metabolic effects on nucleotide metabolism by interaction with Bas1 and indicate that AMP biosynthesis and its regulation are important for C. albicans growth and virulence. IMPORTANCECandida albicans is a commensal and a common constituent of the human microbiota; however, it can become pathogenic and cause infections in both immunocompetent and immunocompromised people. C. albicans exhibits remarkable metabolic versatility as it can colonize multiple body sites as a commensal or pathogen. Understanding how C. albicans adapts metabolically to each ecological niche is essential for developing novel therapeutic approaches. Purine metabolism has been targeted pharmaceutically in several diseases; however, the regulation of this pathway has not been fully elucidated in C. albicans. Here, we report how C. albicans controls the AMP de novo biosynthesis pathway in response to purine availability. We show that the lack of the transcription factors Grf10 and Bas1 leads to purine metabolic dysfunction, and this dysfunction affects the ability of C. albicans to establish infections.

Eaf1 Links the NuA4 Histone Acetyltransferase Complex to Htz1 Incorporation and Regulation of Purine Biosynthesis

Proper modulation of promoter chromatin architecture is crucial for gene regulation in order to precisely and efficiently orchestrate various cellular activities. Previous studies have identified the stimulatory effect of the histone-modifying complex NuA4 on the incorporation of the histone variant H2A.Z (Htz1) at the PHO5 promoter (A. Auger, L. Galarneau, M. Altaf, A. Nourani, Y. Doyon, R. T. Utley, D. Cronier, S. Allard, and J. Côté, Mol Cell Biol 28:2257-2270, 2008, http://dx.doi.org/10.1128/MCB.01755-07). In vitro studies with a reconstituted system also indicated an intriguing cross talk between NuA4 and the H2A.Z-loading complex, SWR-C (M. Altaf, A. Auger, J. Monnet-Saksouk, J. Brodeur, S. Piquet, M. Cramet, N. Bouchard, N. Lacoste, R. T. Utley, L. Gaudreau, J. Côté, J Biol Chem 285:15966-15977, 2010, http://dx.doi.org/10.1074/jbc.M110.117069). In this work, we investigated the role of the NuA4 scaffold subunit Eaf1 in global gene expression and genome-wide incorporation of Htz1. We found that loss of Eaf1 affects Htz1 levels mostly at the promoters that are normally highly enriched in the histone variant. Analysis of eaf1 mutant cells by expression array unveiled a relationship between NuA4 and the gene network implicated in the purine biosynthesis pathway, as EAF1 deletion cripples induction of several ADE genes. NuA4 directly interacts with Bas1 activation domain, a key transcription factor of adenine genes. Chromatin immunoprecipitation (ChIP) experiments demonstrate that nucleosomes on the inactive ADE17 promoter are acetylated already by NuA4 and enriched in Htz1. Upon derepression, these poised nucleosomes respond rapidly to activate ADE gene expression in a mechanism likely reminiscent of the PHO5 promoter, leading to nucleosome disassembly. These detailed molecular events depict a specific case of cross talk between NuA4-dependent acetylation and incorporation of histone variant Htz1, presetting the chromatin structure over ADE promoters for subsequent chromatin remodeling and activated transcription.

Modeling regulatory cascades using Artificial Neural Networks: the case of transcriptional regulatory networks shaped during the yeast stress response

Over the last decade, numerous computational methods have been developed in order to infer and model biological networks. Transcriptional networks in particular have attracted significant attention due to their critical role in cell survival. The majority of network inference methods use genome-wide experimental data to search for modules of genes with coherent expression profiles and common regulators, often ignoring the multi-layer structure of transcriptional cascades. Modeling methodologies on the other hand assume a given network structure and vary significantly in their algorithmic approach, ranging from over-simplified representations (e.g., Boolean networks) to detailed -but computationally expensive-network simulations (e.g., with differential equations). In this work we use Artificial Neural Networks (ANNs) to model transcriptional regulatory cascades that emerge during the stress response in Saccharomyces cerevisiae and extend in three layers. We confine the structure of the ANNs to match the structure of the biological networks as determined by gene expression, DNA-protein interaction and experimental evidence provided in publicly available databases. Trained ANNs are able to predict the expression profile of 11 target genes across multiple experimental conditions with a correlation coefficient >0.7. When time-dependent interactions between upstream transcription factors (TFs) and their indirect targets are also included in the ANNs, accurate predictions are achieved for 30/34 target genes. Moreover, heterodimer formation is taken into account. We show that ANNs can be used to (1) accurately predict the expression of downstream genes in a 3-layer transcriptional cascade based on the expression of their indirect regulators and (2) infer the condition- and time-dependent activity of various TFs as well as during heterodimer formation. We show that a three-layer regulatory cascade whose structure is determined by co-expressed gene modules and their regulators can successfully be modeled using ANNs with a similar configuration.

Regulation of amino acid, nucleotide, and phosphate metabolism in Saccharomyces cerevisiae

Ever since the beginning of biochemical analysis, yeast has been a pioneering model for studying the regulation of eukaryotic metabolism. During the last three decades, the combination of powerful yeast genetics and genome-wide approaches has led to a more integrated view of metabolic regulation. Multiple layers of regulation, from suprapathway control to individual gene responses, have been discovered. Constitutive and dedicated systems that are critical in sensing of the intra- and extracellular environment have been identified, and there is a growing awareness of their involvement in the highly regulated intracellular compartmentalization of proteins and metabolites. This review focuses on recent developments in the field of amino acid, nucleotide, and phosphate metabolism and provides illustrative examples of how yeast cells combine a variety of mechanisms to achieve coordinated regulation of multiple metabolic pathways. Importantly, common schemes have emerged, which reveal mechanisms conserved among various pathways, such as those involved in metabolite sensing and transcriptional regulation by noncoding RNAs or by metabolic intermediates. Thanks to the remarkable sophistication offered by the yeast experimental system, a picture of the intimate connections between the metabolomic and the transcriptome is becoming clear.

Nucleoporin mediated nuclear positioning and silencing of HMR

The organization of chromatin domains in the nucleus is an important factor in gene regulation. In eukaryotic nuclei, transcriptionally silenced chromatin clusters at the nuclear periphery while transcriptionally poised chromatin resides in the nuclear interior. Recent studies suggest that nuclear pore proteins (NUPs) recruit loci to nuclear pores to aid in insulation of genes from silencing and during gene activation. We investigated the role of NUPs at a native yeast insulator and show that while NUPs localize to the native tDNA insulator adjacent to the silenced HMR domain, loss of pore proteins does not compromise insulation. Surprisingly we find that NUPs contribute to silencing at HMR and are able to restore silencing to a silencing-defective HMR allele when tethered to the locus. We show that the perinuclear positioning of heterochromatin is important for the NUP-mediated silencing effect and find that loss of NUPs result in decreased localization of HMR to the nuclear periphery. We also show that loss of telomeric tethering pathways does not eliminate NUP localization to HMR, suggesting that NUPs may mediate an independent pathway for HMR association with the nuclear periphery. We propose that localization of NUPs to the tDNA insulator at HMR helps maintain the intranuclear position of the silent locus, which in turn contributes to the fidelity of silencing at HMR.

Unraveling condition-dependent networks of transcription factors that control metabolic pathway activity in yeast

While typically many expression levels change in transcription factor mutants, only few of these changes lead to functional changes. The predictive capability of expression and DNA binding data for such functional changes in metabolism is very limited.

Large-scale ¹³C-flux data reveal the condition specificity of transcriptional control of metabolic function.

Transcription control in yeast focuses on the switch between respiration and fermentation.

Follow-up modeling on the basis of transcriptomics and proteomics data suggest the newly discovered Gcn4 control of respiration to be mediated via PKA and Snf1.

Effective control and modulation of cellular behavior is of paramount importance in medicine (Kreeger and Lauffenburger, 2010) and biotechnology (Haynes and Silver, 2009), and requires profound understanding of control mechanisms. In this study, we aim to elucidate the extent to which transcription factors control the operation of yeast metabolism. As a quantitative readout of metabolic function, we monitored the traffic of small molecules through various pathways of central metabolism by ¹³C-flux analysis (Sauer, 2006). The choosen growth conditions represent two different regulatory states of reduced (galactose) and maximal carbon source repression (glucose), as well as a different nitrogen metabolism and two common, permanent stress conditions.

Depending on the growth condition, between 7 and 13% of the deleted transcription factors altered the determined flux ratios (Figure 3). Of the six quantified flux ratios, only the glycolysis/pentose phosphate pathway split, and the convergent ratio of anaplerosis and TCA cycle were controlled by the deleted transcription factors. Thus, we concluded that 23 transcription factors control flux distributions under at least one of the tested growth conditions, leading to 42 condition-dependent interactions of transcription factors with metabolic pathway activity (Figure 4). With two exceptions, all other identified transcription factors interactions controlled the TCA cycle flux. This condition-specific active control of metabolic function could not have been predicted from DNA binding and expression data; that is, 26.1% false negatives, 48.6% true positives.

Of the 23 transcription factors that controlled TCA cycle flux distributions under the tested conditions, only Bas1, Gcn4, Gcr2 and Pho2 exerted control under more than one condition. We identified Cit1, Mdh1 and Idh1/2 with a proteomics approach as the relevant target enzyme that increase the TCA cycle flux. Next, we asked whether Bas1, Gcr2, Gcn4 and Pho2 act directly on the TCA cycle or mediate their effect indirectly. Based on the transcriptomics data, the pattern of differentially activated transcription factors inferred by the differential expression of their target genes suggested reduced glucose repression in all four mutants as the common mechanism.

Starting from the currently largest set of ¹³C-based flux distributions, we identified networks of individual transcription factors that control metabolic pathway activity. These networks of active metabolic control have the following properties. First, they are highly condition dependent, as at most four transcription factors control the same metabolic flux distribution under more than one growth conditions. Second, they focus almost exclusively on the TCA cycle, thereby controlling the switch between respiratory and fermentative metabolism. Third, with four to 14 active transcription factors, they are small compared with gene regulation networks that were obtained from expression and DNA binding data. For the metabolic network studied here, robustness is also apparent from the fact that upregulated TCA cycle fluxes were not sufficient to achieve full respiratory metabolism; that is, absent or low ethanol formation. Several explanations could potentially explain the observed robustness. The most likely explanation is that environmental signals might be transmitted by different signaling pathways to several transcription factors, whose orchestrated action on multiple target genes is necessary to achieve a functional flux response. This hypothesis would explain why several transcription factors exert flux effects on the same pathway, but each flux effect is relatively small, as further, coordinated manipulations would be necessary to further improve the respiratory flux. Our findings demonstrate the importance of identifying and quantifying the extent to which regulatory effectors alter cellular function.

Which transcription factors control the distribution of metabolic fluxes under a given condition? We address this question by systematically quantifying metabolic fluxes in 119 transcription factor deletion mutants of Saccharomyces cerevisiae under five growth conditions. While most knockouts did not affect fluxes, we identified 42 condition-dependent interactions that were mediated by a total of 23 transcription factors that control almost exclusively the cellular decision between respiration and fermentation. This relatively sparse, condition-specific network of active metabolic control contrasts with the much larger gene regulation network inferred from expression and DNA binding data. Based on protein and transcript analyses in key mutants, we identified three enzymes in the tricarboxylic acid cycle as the key targets of this transcriptional control. For the transcription factor Gcn4, we demonstrate that this control is mediated through the PKA and Snf1 signaling cascade. The discrepancy between flux response predictions, based on the known regulatory network architecture and our functional ¹³C-data, demonstrates the importance of identifying and quantifying the extent to which regulatory effectors alter cellular functions.

Metabolic intermediates selectively stimulate transcription factor interaction and modulate phosphate and purine pathways

Cells use strategic metabolites to sense the metabolome and accordingly modulate gene expression. Here, we show that the purine and phosphate pathways are positively regulated by the metabolic intermediate AICAR (5'-phosphoribosyl-5-amino-4-imidazole carboxamide). The transcription factor Pho2p is required for up-regulation of all AICAR-responsive genes. Accordingly, the binding of Pho2p to purine and phosphate pathway gene promoters is enhanced upon AICAR accumulation. In vitro, AICAR binds both Pho2p and Pho4p transcription factors and stimulates the interaction between Pho2p and either Bas1p or Pho4p in vivo. In contrast, SAICAR (succinyl-AICAR) only affects Pho2p-Bas1p interaction and specifically up-regulates purine regulon genes. Together, our data show that Bas1p and Pho4p compete for Pho2p binding, hence leading to the concerted regulation of cellular nucleotide synthesis and phosphate consumption.

Cooperative regulation of ADE3 transcription by Gcn4p and Bas1p in Saccharomyces cerevisiae

The one-carbon response regulon is essential for the biosynthesis of nucleic acids as well as several amino acids. The ADE3 gene is known to encode a crucial one-carbon regulon enzyme, tetrahydrofolate synthase, which is involved in the biosynthesis of purine and the amino acids methionine and glycine. Therefore, the mechanism through which ADE3 transcription is regulated appears to be critical for the cross-talk among these metabolic pathways. Even so, the direct involvement of ADE3 transcription through gene-specific transcription factors has not been shown clearly. In this study, the promoter structure of the ADE3 gene was investigated in detail, and a genuine Gcn4p responsive element (GCRE) was confirmed among three putative GCRE elements in vivo and in vitro. Through gene deletion studies of Gcn4p and Bas1p, it was established that both factors are involved in the transcriptional regulation of the ADE3 gene. Direct binding to this GCRE and the occupancy of the ADE3 promoter by these factors were also confirmed. Taking these results together, we concluded that Gcn4p is responsible for the basal and inducible expression of the ADE3 gene, while Bas1p is required for its basal expression.

Activation of the ADE genes requires the chromatin remodeling complexes SAGA and SWI/SNF

The activation of the ADE regulon genes requires the pair of transcription factors Bas1 and Pho2. In a genome-wide screen for additional regulators of the pathway, strains with mutations in multiple subunits of the chromatin remodeling complexes SAGA and SWI/SNF were uncovered. These mutants exhibited decreased expression of an ADE5,7-lacZ reporter and native ADE compared to the wild-type strains, but the expression of the BAS1 and PHO2 genes was not substantially decreased. An unregulated Bas1-Pho2 fusion protein depended upon SAGA and SWI/SNF activity to promote transcription of a reporter. A significant but low-level association of Gcn5-myc and Snf2-myc with the ADE5,7 promoter was independent of adenine growth conditions and independent of the presence of the activator proteins Bas1 and Pho2. However, the increase in occupancy of Bas1 and Pho2 at ADE5,7 depended on both SAGA and SWI/SNF. The loss of catalytic activity of both SAGA and SWI/SNF complexes in the gcn5Delta snf2Delta double mutant was severely detrimental to ADE-lacZ reporter expression and native ADE gene expression, indicating complementary roles for these complexes. We conclude that Bas1 and Pho2 do not recruit the SAGA and SWI/SNF complexes to the ADE5,7 promoter but that the remodeling complexes are necessary to increase the binding of Bas1 and Pho2 in response to the adenine regulatory signal. Our data support the model that the SAGA and SWI/SNF complexes engage in global surveillance that is necessary for the specific response by Bas1 and Pho2.

Regulation of purine nucleotide biosynthesis: in yeast and beyond

Purine nucleotides are critically important for the normal functioning of cells due to their myriad of activities. It is important for cells to maintain a balance in the pool sizes of the adenine-containing and guanine-containing nucleotides, which occurs by a combination of de novo synthesis and salvage pathways that interconvert the purine nucleotides. This review describes the mechanism for regulation of the biosynthetic genes in the yeast Saccharomyces cerevisiae and compares this mechanism with that described in several microbial species.

Purine biosynthesis, riboflavin production, and trophic-phase span are controlled by a Myb-related transcription factor in the fungus Ashbya gossypii

Ashbya gossypii is a natural riboflavin overproducer used in the industrial production of the vitamin. We have isolated an insertional mutant exhibiting higher levels of riboflavin production than the wild type. DNA analysis of the targeted locus in the mutant strain revealed that a syntenic homolog of the Saccharomyces cerevisiae BAS1 gene, a member of the Myb family of transcription factors, was inactivated. Directed gene disruption of AgBAS1 confirmed the phenotype observed for the insertional mutant, and the Deltabas1 mutant also showed auxotrophy for adenine and several growth defects, such as a delay in the germination of the spores and an abnormally prolonged trophic phase. Additionally, we demonstrate that the DNA-binding domain of AgBas1p is able to bind to the Bas1-binding motifs in the AgADE4 promoter; we also show a clear nuclear localization of a green fluorescent protein-Bas1 fusion protein. Real-time quantitative PCR analyses comparing the wild type and the Deltabas1 mutant revealed that AgBAS1 was responsible for the adenine-mediated regulation of the purine and glycine pathways, since the transcription of the ADE4 and SHM2 genes was virtually abolished in the Deltabas1 mutant. Furthermore, the transcription of ADE4 and SHM2 in the Deltabas1 mutant did not diminish during the transition from the trophic to the productive phase did not diminish, in contrast to what occurred in the wild-type strain. A C-terminal deletion in the AgBAS1 gene, comprising a hypothetical regulatory domain, caused constitutive activation of the purine and glycine pathways, enhanced riboflavin overproduction, and prolonged the trophic phase. Taking these results together, we propose that in A. gossypii, AgBAS1 is an important transcription factor that is involved in the regulation of different physiological processes, such as purine and glycine biosynthesis, riboflavin overproduction, and growth.