[Abstract unavailable

Most studies of eukaryotic gene regulation have been done looking at mature mRNA levels. Nevertheless, the steady-state mRNA level is the result of two opposing factors: transcription rate (TR) and mRNA degradation. Both can be important points to regulate gene expression. Here we show a new method that combines the use of nylon macroarrays and in vivo radioactive labeling of nascent RNA to quantify TRs, mRNA levels, and mRNA stabilities for all the S. cerevisiae genes. We found that during the shift from glucose to galactose, most genes undergo drastic changes in TR and mRNA stability. However, changes in mRNA levels are less pronounced. Some genes, such as those encoding mitochondrial proteins, are coordinately regulated in mRNA stability behaving as decay regulons. These results indicate that, although TR is the main determinant of mRNA abundance in yeast, modulation of mRNA stability is a key factor for gene regulation.

Chromatin remodeling can facilitate the recruitment of RNA polymerase II (Pol II) to targeted promoters, as well as enhancing the level of transcription. Here, we describe a further key role for chromatin remodeling in transcriptional termination. Using a genetic screen in S. pombe, we identified the CHD-Mi2 class chromatin remodeling ATPase, Hrp1, as a termination factor. In S. cerevisiae, we show that transcriptional termination and chromatin structure at the 3' ends of three genes all depend on the activity of the Hrp1 homolog, Chd1p, either alone or redundantly with the ISWI ATPases, Isw1p, and Isw2p. We suggest that chromatin remodeling of termination regions is a necessary prelude to efficient Pol II termination.

The nucleolus is the site of ribosome synthesis in the nucleus, whose integrity is essential. Epigenetic mechanisms are thought to regulate the activity of the ribosomal RNA (rRNA) gene copies, which are part of the nucleolus. Here we show that human cells lacking DNA methyltransferase 1 (Dnmt1), but not Dnmt33b, have a loss of DNA methylation and an increase in the acetylation level of lysine 16 histone H4 at the rRNA genes. Interestingly, we observed that SirT1, a NAD+-dependent histone deacetylase with a preference for lysine 16 H4, interacts with Dnmt1; and SirT1 recruitment to the rRNA genes is abrogated in Dnmt1 knockout cells. The DNA methylation and chromatin changes at ribosomal DNA observed are associated with a structurally disorganized nucleolus, which is fragmented into small nuclear masses. Prominent nucleolar proteins, such as Fibrillarin and Ki-67, and the rRNA genes are scattered throughout the nucleus in Dnmt1 deficient cells. These findings suggest a role for Dnmt1 as an epigenetic caretaker for the maintenance of nucleolar structure.