Expression of Subtelomeric lncRNAs Links Telomeres Dynamics to RNA Decay in S. cerevisiae.

Long non-coding RNAs (lncRNAs) have been shown to regulate gene expression, chromatin domains and chromosome stability in eukaryotic cells. Recent observations have reported the existence of telomeric repeats containing long ncRNAs ⁻ TERRA in mammalian and yeast cells. However, their functions remain poorly characterized. Here, we report the existence in S. cerevisiae of several lncRNAs within Y' subtelomeric regions. We have called them subTERRA. These belong to Cryptic Unstable Transcripts (CUTs) and Xrn1p-sensitive Unstable Transcripts (XUTs) family. subTERRA transcription, carried out mainly by RNAPII, is initiated within the subtelomeric Y' element and occurs in both directions, towards telomeres as well as centromeres. We show that subTERRA are distinct from TERRA and are mainly degraded by the general cytoplasmic and nuclear 5'- and 3'- RNA decay pathways in a transcription-dependent manner. subTERRA accumulates preferentially during the G1/S transition and in C-terminal rap1 mutant but independently of Rap1p function in silencing. The accumulation of subTERRA in RNA decay mutants coincides with telomere misregulation: shortening of telomeres, loss of telomeric clustering in mitotic cells and changes in silencing of subtelomeric regions. Our data suggest that subtelomeric RNAs expression links telomere maintenance to RNA degradation pathways.

H3 lysine 4 di- and tri-methylation deposited by cryptic transcription attenuates promoter activation.

Set1-dependent H3K4 di- and tri-methylation (H3K4me2/3) have been associated with active transcription. Recent data indicate that the H3K4me2/3 also plays a poorly characterized RNA-dependent repressive role. Here, we show that GAL1 promoter is attenuated by the H3K4me2/3 deposited by cryptic transcription. The H3K4me2/3 delay the recruitment of RNA polymerase II (RNAPII) and TBP on GAL1 promoter. Inactivation of RNA decay components revealed the existence of the RNAPII-dependent unstable RNAs, initiating upstream of GAL1 (GAL1ucut). GAL1ucut RNAs are synthesized in glucose and require the Reb1 transcription factor. Consistent with a regulatory function of the cryptic transcription, Reb1 depletion leads to a decrease of H3K4me3 on GAL10-GAL1 locus in glucose and to an acceleration of GAL1 induction. A candidate approach shows that the RPD3 histone deacetylase attenuates GAL1 induction and is tethered at the GAL10-GAL1 locus by H3K4me2/3 upon repression. Strikingly, Set1-dependent Rpd3 recruitment represses also the usage of a hidden promoter within SUC2, suggesting a general function for H3K4me2/3 in promoter fidelity. Our data support a model wherein certain promoters are embedded in a repressive chromatin controlled by cryptic transcription.