Gene activation and repression regulated by acetylation and deacetylation represent a paradigm for the function of histone modifications. We provide evidence that, in contrast, histone H2B monoubiquitylation and its deubiquitylation are both involved in gene activation. Substitution of the H2B ubiquitylation site at Lys 123 (K123) lowered transcription of certain genes regulated by the acetylation complex SAGA. Gene-associated H2B ubiquitylation was transient, increasing early during activation, and then decreasing coincident with significant RNA accumulation. We show that Ubp8, a component of the SAGA acetylation complex, is required for SAGA-mediated deubiquitylation of histone H2B in vitro. Loss of Ubp8 in vivo increased both gene-associated and overall cellular levels of ubiquitylated H2B. Deletion of Ubp8 lowered transcription of SAGA-regulated genes, and the severity of this defect was exacerbated by codeletion of the Gcn5 acetyltransferase within SAGA. In addition, disruption of either ubiquitylation or Ubp8-mediated deubiquitylation of H2B resulted in altered levels of gene-associated H3 Lys 4 methylation and Lys 36 methylation, which have both been linked to transcription. These results suggest that the histone H2B ubiquitylation state is dynamic during transcription, and that the sequence of histone modifications helps to control transcription.

Covalent modifications of the histone N tails play important roles in eukaryotic gene expression. Histone acetylation, in particular, is required for the activation of a subset of eukaryotic genes through the targeted recruitment of histone acetyltransferases. We have reported that a histone C tail modification, ubiquitylation of H2B, is required for optimal expression of several inducible yeast genes, consistent with a role in transcriptional activation. H2B was shown to be ubiquitylated and then deubiquitylated at the GAL1 core promoter following galactose induction. We now show that the Rad6 protein, which catalyzes monoubiquitylation of H2B, is transiently associated with the GAL1 promoter upon gene activation, and that the period of its association temporally overlaps with the period of H2B ubiquitylation. Rad6 promoter association depends on the Gal4 activator and the Rad6-associated E3 ligase, Bre1, but is independent of the histone acetyltransferase, Gcn5. The SAGA complex, which contains a ubiquitin protease that targets H2B for deubiquitylation, is recruited to the GAL1 promoter in the absence of H2B ubiquitylation. The data suggest that Rad6 and SAGA function independently during galactose induction, and that the staged recruitment of these two factors to the GAL1 promoter regulates the ubiquitylation and deubiquitylation of H2B. We additionally show that both Rad6 and ubiquitylated H2B are absent from two regions of transcriptionally silent chromatin but present at genes that are actively transcribed. Thus, like histone H3 lysine 4 and lysine 79 methylation, two modifications that it regulates, Rad6-directed H2B ubiquitylation defines regions of active chromatin.

Low levels of histone covalent modifications are associated with gene silencing at telomeres and other regions in the yeast S. cerevisiae. Although the histone deacetylase Sir2 maintains low acetylation, mechanisms responsible for low H2B ubiquitylation and low H3 methylation are unknown. Here, we show that the ubiquitin protease Ubp10 targets H2B for deubiquitylation, helping to localize Sir2 to the telomere. Ubp10 exhibits reciprocal Sir2-dependent preferential localization proximal to telomeres, where Ubp10 serves to maintain low H2B Lys123 ubiquitylation in this region and, through previously characterized crosstalk, maintains low H3 Lys4 and Lys79 methylation in a slightly broader region. Ubp10 is also localized to the rDNA locus, a second silenced domain, where it similarly maintains low histone methylation. We compare Ubp10 to Ubp8, the SAGA-associated H2B deubiquitylase involved in gene activation, and show that telomeric and gene-silencing functions are specific to Ubp10. Our results suggest that these H2B-deubiquitylating enzymes have distinct genomic functions.

Infections due to Candida albicans are usually treated with azole antifungals such as fluconazole, but treatment failure is not uncommon especially in immunocompromised individuals. Relatedly, in vitro studies demonstrate that azoles are nonfungicidal, with continued growth at strain-dependent rates even at high azole concentrations. We hypothesized that upregulation of ERG11, which encodes the azole target enzyme lanosterol demethylase, contributes to this azole tolerance in Candida species. RNA analysis revealed that ERG11 expression in C. albicans is maximal during logarithmic-phase growth and decreases as the cells approach stationary phase. Incubation with fluconazole, however, resulted in a two-to fivefold increase in ERG11 RNA levels within 2 to 3 h, and this increase was followed by resumption of culture growth. ERG11 upregulation also occurred following treatment with other azoles (itraconazole, ketoconazole, clotrimazole, and miconazole) and was not dependent on the specific medium or pH. Within 1 h of drug removal ERG11 upregulation was reversed. Azole-dependent upregulation was not limited to ERG11: five of five ERG genes tested whose products function upstream and downstream of lanosterol demethylase in the sterol biosynthetic pathway were also upregulated. Similarly, ERG11 upregulation occurred following treatment of C. albicans cultures with terbinafine and fenpropimorph, which target other enzymes in the pathway. These data suggest a common mechanism for global ERG upregulation, e.g., in response to ergosterol depletion. Finally, azole-dependent ERG11 upregulation was demonstrated in three additional Candida species (C. tropicalis, C. glabrata, and C. krusei), indicating a conserved response to sterol biosynthesis inhibitors in opportunistic yeasts.

SummaryThe most important group of antifungals is the azoles (e.g. miconazole), which act by inhibiting lanosterol demethylase in the sterol biosynthesis pathway. Azole activity can be modulated through structural changes in lanosterol demethylase, altered expression of its gene ERG11 , alterations in other sterol biosynthesis enzymes or altered expression of multidrug transporters. We present evidence that azole activity versus Saccharomyces cerevisiae is also modulated by Ca 2 + + + + -regulated signalling.