Subtelomeric Silencing of the MTL3 Locus of Candida glabrata Requires yKu70, yKu80, and Rif1 Proteins

Abstract

Candida glabrata is a haploid opportunistic fungal pathogen that is phylogenetically related to Saccharomyces cerevisiae. Even though C. glabrata has no known sexual cycle, it contains, like S. cerevisiae, three mating type-like loci (MTL) called MTL1, MTL2, and MTL3, as well as most of the genes required for mating, meiosis, and sporulation. MTL1 is localized at an internal position on chromosome B and is thought to be the locus corresponding to the MAT locus in S. cerevisiae. MTL2 and MTL3 are localized close to two telomeres on different chromosomes (29.4 kb from Chr E-L and 10.5 kb from Chr B-L, respectively). By using URA3 reporter gene insertions at the three MTL loci, we found that in contrast to the case for S. cerevisiae, only MTL3 is subject to transcriptional silencing while MTL2 is transcriptionally active, and this is in agreement with previously reported data. We found that the silencing of MTL3 is nucleated primarily at the left telomere of chromosome B and spreads over 12 kb to MTL3, rather than nucleating at flanking, closely positioned cis-acting silencers, like those flanking HMR and HML of S. cerevisiae. Interestingly, the silencing of MTL3 absolutely requires the yKu70, yKu80, and Rif1 proteins, in sharp contrast to the silencing of the HM loci of S. cerevisiae. In addition, we found that several cell type-specific genes are expressed in C. glabrata regardless of the presence, or even absence, of mating type information at any of the MTL loci.Candida glabrata is a haploid yeast found as a normal part of the mammalian microflora, but in recent years it also has emerged as a common opportunistic pathogen of humans, and now it accounts for about 12% of all Candida infections worldwide, second only to Candida albicans (29,36,37,39,50). C. glabrata and Saccharomyces cerevisiae are closely related phylogenetically, and both genomes conserve a high degree of synteny, whereas C. albicans is more distantly related (6,14).Sexual reproduction has been documented in many fungal species; however, several human fungal pathogens either do not reproduce sexually or very rarely do so (33). As the genomes of more fungal species are sequenced it has become clear, however, that the vast majority of the species have highly conserved genes involved in sexual reproduction, and yet some species have not been observed to mate (5, 6, 16). In some cases, a cryptic sexual cycle and even same-sex mating have been discovered recently, as is the case for C. albicans and Aspergillus fumigatus (1, 34). C. glabrata has no known sexual cycle even though it contains the vast majority of the genes required for mating (6,14), and some mating type identity is maintained (32).In (21,26). In addition to the MAT locus, the S. cerevisiae chromosome III contains two silent loci, HMR and HML, located near each telomere (ϳ22.7 and ϳ13.0 kb, respectively). The HMR and HML loci usually contain identical copies of MATa and MAT␣, respectively, that are normally maintained efficiently repressed through a chromatin-based mechanism called sil...

Reinventing Heterochromatin in Budding Yeasts: Sir2 and the Origin Recognition Complex Take Center Stage

“…Interestingly, Reb1 and Rap1 are related myb domain-containing proteins, suggesting that this family of proteins may be well-suited to function as silencer binding proteins. In C. glabrata, silencing of the MTL3 locus is apparently not nucleated at a silencer sequence at all but is instead subject to subtelomeric silencing (94). This loss of silencers is consistent with the absence of the silencer- Comparisons of silencers in S. cerevisiae and closely related (sensu stricto) species provide insights into how silencers diverge.…”

“…Orthologs of ScSir2 have been identified in all examined Saccharomycotina species (36,97), and the silencing function of these orthologs is conserved in the few species that have been investigated, namely, Kluyveromyces lactis (5,23,57), Candida glabrata (30,94,96), and Saccharomyces bayanus (118). In fact, Sir2 has a role in silencing beyond the Saccharomycotina.…”

“…In addition to this variability in organization of the mating-type loci, the extra mating-type loci are not always silenced as expected. In C. glabrata, MTL2 and MTL3 are both located near telomeres, but only MTL3 is silenced (94), and in K. lactis HMRa is repressed by the SUM1 complex instead of the SIR complex (57, 58).…”

Heterogeneous Expression of the Virulence-Related Adhesin Epa1 between Individual Cells and Strains of the Pathogen Candida glabrata

“…One possibility is loss of silencing of a repressor of EPA1 expression. We have noted a putative binding site for the C. glabrata ␣2 repressor protein in the EPA1 promoter (at Ϫ101 to 109 bp), and the ␣2-encoding gene is subject to transcriptional silencing (39).…”

Evolution of sexual reproduction: A view from the fungal kingdom supports an evolutionary epoch with sex before sexes

“…Are there any truly asexual fungi? Candida glabrata is one potential candidate, but also a mystery in that all known isolates are haploids of one or the other of two opposite mating types and much of the machinery associated with mating and meiosis are present in the genome (Muller et al, 2008; Ramirez-Zavaleta et al, 2010; Srikantha et al, 2003; Wong et al, 2003). There is even evidence for recombination in the population.…”