Antisense-mediated repression of SAGA-dependent genes involves the HIR histone chaperone

RNAseq analysis of mutants in coding and non-coding transcription of phosphate genes in the yeast Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae phosphate starvation induces the expression of PHO genes, including PHO84, encoding an high-affinity phosphate transporter, and SPL2, encoding a regulatory protein. PHO84 is down-regulated by antisense transcription. Here, using strand-specific RNAseq the effect is studied of mutations related to sense and antisense transcription of phosphate genes. Replacement of the transcriptional terminator of PHO84 by that of CYC1 resulted, unexpectedly, in an increased antisense transcription and a strongly reduced sense transcription of PHO84 and a strongly reduced SPL2 expression. The expression of unrelated genes was altered as well. The data suggest that antisense transcription of PHO84 and not the Pho84 transporter affects the expression of SPL2. Deletion of the two putative binding sites for Ume6 in the SPL2 promoter or deletion of UME6 differently affected SPL2 expression, suggesting that Ume6 regulates SPL2 by a mechanism different from a simple binding to the putative Ume6 binding sites.

The genomic region of the 3' untranslated region (3'UTR) of PHO84, rather than the antisense RNA, promotes gene repression

PHO84 is a budding yeast gene reported to be negatively regulated by its cognate antisense transcripts both in cis and in trans. In this study, we performed Transient-transcriptome sequencing (TT-seq) to investigate the correlation of sense/antisense pairs in a dbp2Δ strain and found over 700 sense/antisense pairs, including PHO84, to be positively correlated, contrasting the prevailing model. To define what mechanism regulates the PHO84 gene and how this regulation could have been originally attributed to repression by the antisense transcript, we conducted a series of molecular biology and genetics experiments. We now report that the 3' untranslated region (3'UTR) of PHO84 plays a repressive role in sense expression, an activity not linked to the antisense transcripts. Moreover, we provide results of a genetic screen for 3'UTR-dependent repression of PHO84 and show that the vast majority of identified factors are linked to negative regulation. Finally, we show that the PHO84 promoter and terminator form gene loops which correlate with transcriptional repression, and that the RNA-binding protein, Tho1, increases this looping and the 3'UTR-dependent repression. Our results negate the current model for antisense non-coding transcripts of PHO84 and suggest that many of these transcripts are byproducts of open chromatin.

Antisense non-coding transcription represses the PHO5 model gene at the level of promoter chromatin structure

Pervasive transcription of eukaryotic genomes generates non-coding transcripts with regulatory potential. We examined the effects of non-coding antisense transcription on the regulation of expression of the yeast PHO5 gene, a paradigmatic case for gene regulation through promoter chromatin remodeling. A negative role for antisense transcription at the PHO5 gene locus was demonstrated by leveraging the level of overlapping antisense transcription through specific mutant backgrounds, expression from a strong promoter in cis, and use of the CRISPRi system. Furthermore, we showed that enhanced elongation of PHO5 antisense leads to a more repressive chromatin conformation at the PHO5 gene promoter, which is more slowly remodeled upon gene induction. The negative effect of antisense transcription on PHO5 gene transcription is mitigated upon inactivation of the histone deacetylase Rpd3, showing that PHO5 antisense RNA acts via histone deacetylation. This regulatory pathway leads to Rpd3-dependent decreased recruitment of the RSC chromatin remodeling complex to the PHO5 gene promoter upon induction of antisense transcription. Overall, the data in this work reveal an additional level in the complex regulatory mechanism of PHO5 gene expression by showing antisense transcription-mediated repression at the level of promoter chromatin structure remodeling.