Abstract
Mating-types allow single-celled eukaryotic organisms to distinguish self from non-self in preparation for sexual reproduction. The components of mating-type loci provide initial self/non-self-recognition through pheromone and receptor interactions that control early cell fusion events. However, they may also provide a second level of scrutiny that requires differences in alleles leading to production of a transcription factor required for successful downstream developmental pathways after initial cell fusion. Interestingly, the protein subunits of these transcription factors have not been thoroughly examined for their roles, if any, in the haploid cells themselves. In Ustilago maydis, the causative agent of galls in maize plants, the b locus, encoding bEast (bE) and bWest (bW), components of the eventual requisite transcription factor, has been extensively studied for its role in formation of the stable dikaryon after mating and subsequent pathogenic program. Little is known, however, about any roles for bE or bW in haploid cells. Since mating in fungi is often induced under conditions of nitrogen starvation, we have explored connections between the b locus and the nitrogen-sensing and response pathways in U. maydis. We previously identified a connection in haploid cells between the b locus and Ump2, the high-affinity transceptor, a protein that both transports ammonium and triggers filamentous growth as a response to nitrogen starvation. Deletion of the entire b locus abrogates the filamentous response to low ammonium, a phenotype that is rescued by overexpression of Ump2. Here we further investigated the individual roles of bE and bW in haploid cells. We show that bE and bW are expressed differentially in haploid cells starved for ammonium. Their respective deletion elicits different effects on transcription of mating and pathogenic-related genes and, importantly, on the degree of pathogenic development in host plants. This is the first demonstration of a role for these mating locus components on haploid development and the first to demonstrate a connection to the ammonium transceptors.
Highlights
Of note, as with human sex determination, S. cerevisiae has a default program that can be over-ridden by the presence of a dominant mating type determination actor produced by the MATα locus
Our first step in understanding how the components of the b mating type locus contribute to the Low-NH4 response in haploid cells was to examine how these haploid strains behaved when plated in the absence of a mating partner
The role of the heterodimer produced by the interaction of proteins from different alleles of the b mating type locus is well characterized
Summary
The existence of transcription factors directing developmental programs within the sex determining chromosomal regions is found as early as single-celled eukaryotes such as the fungus, Saccharomyces cerevisiae, and other yeasts. Sex determination by transcriptional activation finds its evolutionary antecedents in these early eukaryotes, evidenced by the homology between the SRY region of the Y chromosome and the Mc mating-type protein found in the fission yeast, Schizosaccharomyces pombe [2]. Yeast do not possess sex chromosomes, the region of their genome dedicated to determination of mating type, the mating type (mat or MAT) locus, encodes transcription factors that serve to increase mating type specific gene expression and to repress gene expression of the opposite mating type [3,4,5]. Of note, as with human sex determination, S. cerevisiae has a default program (mating type a) that can be over-ridden by the presence of a dominant mating type determination actor produced by the MATα locus
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