Sir2 is involved in the transcriptional modulation of NHP6A in Saccharomyces cerevisiae

The Sir proteins, namely Sir2, 3 and 4, have roles related to heterochromatin, but genome-wide studies have revealed their presence at many euchromatic loci, although the functional meaning of this is still not clear. Nhp6a is an abundant HMG-like protein in yeast, which has a role in transcription by modulating chromatin structure and nucleosome number. Although much is known about its structure and function, information regarding its regulation is scarce. NHP6A, among other genes, emerges in ChIP-on chip studies of global Sir proteins binding, suggesting it could be regulated by SIR. We have investigated NHP6A expression in sir deletion mutants as well as in SIR2 overexpressing conditions. In addition, we have asked if the Sir2 deacetylation activity is involved by using conditions that either inhibit (treatment with nicotinamide) or enhance (calorie restriction conditions) Sir2 activity. We have found that, consistent with previous microarray studies, NHP6A expression undergoes a slight increase in sir mutant strains, but is strongly repressed when SIR2 is overexpressed. In a sir3 mutant strain the gene continues to be transcribed, even in SIR2 overexpressing conditions. In addition, treating the cells with nicotinamide counteracts the SIR2 overexpressing effect. Finally, conditions that are known to potentiate Sir2 deacetylation activity seem to mimic the effect of SIR2 overexpression on NHP6A. Our results suggest that Sir2 is involved in the regulation of NHP6A promoter, acting more as a specific repressor, rather than a long-range silencer. This effect is specific, and the Sir2 deacetylase activity is required for the Sir2 mediated repression of NHP6A. Moreover, the presence of the SIR complex seems required for Sir2 to silence NHP6A.