Non-Coding RNAs

MicroRNAS (21-22 nt) are processed from hairpin RNAs encoded by cellular DNA and regulate gene expression primarily by inhibiting translation and promoting mRNA degradation. Some 250-350 conserved miRNA genes are encoded in the human genome (see Figure 1). siRNAs function through the miRNA-pathway and these RNAs will inhibit the translation of a reporter gene that contains a partially complementary target site. We have recently used crosslinking to mRNA to map the location of bound Argonaute2, component of miRNP complex above. Combining this with quantitation of both copies per cell of miRNAs and mRNA targets reveals sets of mRNAs that can compete for regulation of one another through a common miRNA.

We have recently reported that divergent transcription is common of promoter sites for genes in embryonic stem cells (see Figure 2). These promoters have an RNA polymerase initiated in the sense direction immediately downstream of the transcription start site and a second polymerase initiated in the antisense direction, about 250 base pairs upstream. This research has been done in collaboration with Professor Richard Young. Surprisingly, the anti-sense polymerase is controlled by elongation processes very similar to those of sense polymerase. For example, both require P-TEFb for elongation beyond about 50 nts. The nature of factors or sequences that differentiate the effective elongation of the polymerase in the sense direction as compared to the ineffective elongation in the anti-sense direction was investigated. A major factor is the difference in frequency of recognition by the splicing factor U1 snRNP, which suppresses termination of transcription through cleavage of the nascent RNA by the canonical polyadenylation process. In fact, recognition by splicing factors is probably a genome-wide regulator of elongation of transcription.

Long non-coding RNAs (lncRNAs) have been described from analysis of deep RNA sequencing from many types of mammalian cells. Comparable RNA species have also been reported from sequencing data of yeast and Drosophila. Recent analysis of several large data sets of RNA sequences expressed in embryonic stem cells shows that a majority of long non-coding RNAs originated from initiation sites that are divergent from known protein-encoding genes or sites with chromatin marks indicating enhancer elements. In collaboration with the Jacks laboratory, we have recently found that one of these lncRNAs, p21-linc, activates transcription of the p21 gene in cis through mechanisms yet to be elucidated. Thus, surprisingly, p21-linc RNA acts as an enhancer to the p21 promoter. In fact, it is possible that many enhancers in mammalian cells are dependent upon eRNAs for stimulation of a cis-located promoter.