Phenotypic switching through the white to the opaque phase is a necessary step for mating in the pathogenic fungus (hemoglobin response gene 1), was identified based on its specific induction following growth in the presence of exogenous hemoglobin. encode the transcriptional regulators a1, 1, and 2, but control of their mating regulatory circuits differs significantly. Mating in is carried out between diploid mating partners, while in a and a cells are the products of meiosis. Functional a and cells in have been generated only through directed deletion of genes or loss of an entire chromosome containing an locus (21, 35). Thus, genomic rearrangements at the locus are proposed to be the primary mechanism for generating mating-competent cells. This was supported by the deletion of alleles in some isolates from mammals and clinical specimens. Second, possesses a fourth gene in its mating locus, a2 (61). This gene product, as well as the 1 gene product, acts as a positive regulator of some genes required for their respective mating cell-type specificity. Panobinostat inhibitor database Third, a unique morphological change is required for mating. Cells must convert from the typical yeast form to an elongated, opaque cell for high-efficiency mating. Opaque cells mate with a 106-fold-higher frequency than white cells (36). White-opaque switching is one of several known processes that permit reversible changes in cellular morphology without detectable genomic rearrangements (51, 52, 56). In the white-opaque phase transition, cells switch between oval budding cells with smooth cell walls to opaque colonies of elongated cells with surface protrusions known as pimples (3). Opaque cells can be easily distinguished as red colonies on modified Lee’s agar containing phloxine B (3). The opaque phenotype in the prototypical switching strain WO-1 (52) results from allelic loss of (31). Consistent with the model of switching regulated through the a1/2 dimer (22, 36, 55), disruption of either the or the infection (28, 53), indicating that it is a normal component of the fungal life cycle within the host. Considering the vulnerability of opaque cells to host defenses, however, a mechanism to selectively suppress switching might provide a success benefit when enters the blood stream. The homeodomains from the a1 and 2 proteins are conserved in Panobinostat inhibitor database and function much like the MAT a1 and 2 proteins in identifying cell destiny (20). In diploid cells, the a1/2 dimer represses haploid-specific genes (a1/2 dimer represses a subset of these genes repressed by Panobinostat inhibitor database MAT a1/2 in (61). Intriguingly, ortholog from the in can be consistent with its repression by the Mat a1/2 dimer in diploid cells (48). Indeed, a transcriptional reporter using the predicted operator sequences from the promoter demonstrated that has an a1/2 transcriptional repressor activity that requires the gene (20). We have now identified a suppressor of white-opaque switching that was isolated based on its specific induction following exposure of cells to hemoglobin. Hemoglobin is usually a host factor Panobinostat inhibitor database that regulates expression of cell surface receptors for fibronectin, laminin, and fibrinogen (64, 65) through a low-affinity, multivalent hemoglobin receptor (41). Hemoglobin induces increased adhesion to endothelial cells (64), and responsiveness to hemoglobin is usually conserved in other pathogenic species of the genus (45). Therefore, hemoglobin may be an important environmental signal DP2 for pathogenesis of in a mammalian host. To define molecular mechanisms for these phenotypic alterations, we identified genes that are transcriptionally regulated in response to hemoglobin (40, 42). We show here that modulating the expression of one of these genes, background. We further show that Hbr1p suppresses phenotypic switching through stimulation of strains were routinely cultured in yeast nitrogen base (YNB) with ammonium sulfate, 2% glucose, low methionine, and appropriate supplements (50) with shaking at 250 rpm at 30C. Bovine methemoglobin was added to cell cultures at 0.5 mg/ml and was prepared as previously described (41). Modified Lee’s medium (8) made up of 5 g of phloxine B (Sigma,.