3 lysine 18 (H3K18), indicating that the CUT is responsible for recruiting the histone deacetylase. Along with antisense TY1 transcripts, PHO84 antisense transcripts can serve a potential regulatory function in S. Cerevisiae. PROMPTs Promoter upstream transcripts (PROMPTs) are found around 1–1.5 kb upstream of human transcription start sites in nongenic regions. Like CUTs, PROMPTs are a form of noncoding RNAs that become detectable in the absence of a degrading exosome enzyme. PROMPTs were first identified in siRNA-silenced hRrp40 human cells, where hRrp40 serves as a core subunit of the human exoribonucleoytic exosome. PROMPT-encoding regions have been found to produce sense and antisense transcripts, both of which are equally targeted by the exosome. In terms of function, ncRNAs with putative regulatory functions have been located to potential PROMPT regions. As a large portion of the human genome has been shown to be transcribed, the existence of PROMPTs helps explain a portion of the non-coding transcripts that are still generated. Function Although an endogenous RNA interference pathway does not exist within S. cerevisiae, CUTs and SUTs may serve a comparable function. There has been an observed similarity between the suppression of the transposable element TY1 in yeast and small interfering RNA activity within plants. In XRN1 mutants, TY1 transcripts decrease in number and TY1 antisense transcripts increase. These antisense TY1 transcripts reduce TY1 transposition activity in a trans manner and mitigates its expression, indicating a potential role for CUTs and SUTs in epigenetics. Similarly, expression of the ncRNA SRG1 in S. Cerevisiae represses the transcriptional activity of the SER3 phosphoglycerate dehydrogenase gene. The rapidly degraded antisense transcripts of the gene PHO84 have also been shown to recruit the histone deacetylase Hda1 to the PHO84 gene, effectively suppressing PHO84 expression. See also Non-coding RNA References RNA