Descripción: Promoters and enhancers are cis-acting elements that control gene transcription via complex networks of protein-DNA and proteinprotein interactions. Whereas promoters deal with putting in place the RNA polymerase, both enhancers and promoters can control transcriptional initiation and elongation. We have previously shown that promoter structure modulates alternative splicing, strengthening the concept of a physical and functional coupling between transcription and splicing. Here we report that the promoter effect is due to the control of RNA pol II elongation. We found that the simian virus 40 (SV40) transcriptional enhancer, inserted in fibronectin (FN) minigene constructs transfected into mammalian cells, controls alternative splicing by inhibiting inclusion of the FN extra domain I (EDI) exon into mature mRNA. Deletion analysis of enhancer subdomains and competitions in vivo with excess of specific enhancer DNA subfragments demonstrate that the "minimal" enhancer, consisting of two 72-bp repeats, is responsible for the splicing effect. The 72-bp repeat region has been reported to promote RNA pol II elongation. When transcription is driven by the α-globin promoter linked to the SV40 enhancer, basal EDI inclusion and activation by the SR (Ser-Arg-rich) protein SF2/ASF are much lower than with other promoters. Deletion of only one of the two 72-bp repeats not only provokes higher EDI inclusion levels but allows responsiveness to SF2/ASF. These effects are the consequence of a decrease in RNA pol II elongation evidenced both by an increase in the proportions of shorter proximal over full length transcripts and by higher pol II densities upstream of the alternative exon detected by chromatin immunoprecipitation.

Descripción: A large body of work has proved that transcription by RNA polymerase II and pre-mRNA processing are coordinated events within the cell nucleus. Capping, splicing and polyadenylation occur while transcription proceeds, suggesting that RNA polymerase II plays a role in the regulation of these events. The presence and degree of phosphorylation of the carboxy-terminal domain of RNA polymerase II large subunit is important for functioning of the capping enzymes, the assembly of spliceosomes and the binding of the cleavage/polyadenylation complex. Nuclear architecture and gene promoter structure have also been shown to play key roles in coupling between transcription and splicing. © 2001 Published by Elsevier Science B.V. on behalf of the Federation of European Biochemical Societies.

Descripción: The realization that the mammalian proteomic complexity is achieved with a limited number of genes demands a better understanding of alternative splicing regulation. Promoter control of alternative splicing was originally described by our group in studies performed on the fibronectin gene. Recently, other labs extended our findings to the cystic fibrosis, CD44 and CGRP genes strongly supporting a coupling between transcription and pre-mRNA splicing. A possible mechanism that would fit in these results is that the promoter itself is responsible for recruiting splicing factors, such as SR proteins, to the site of transcription, possibly through transcription factors that bind the promoter or the transcriptional enhancers. An alternative model, discussed more extensively in this review, involves modulation of RNA pol II (pol II) elongation rate. The model is supported by findings that cis- and trans-acting factors that modulate pol II elongation on a particular template also provoke changes in the alternative splicing balance of the encoded mRNAs.

Descripción: Changes in promoter structure and occupation have been shown to modify the splicing pattern of several genes, evidencing a coupling between transcription and alternative splicing. It has been proposed that the promoter effect involves modulation of RNA pol II elongation rates. The C4 point mutation of the Drosophila pol II largest subunit confers on the enzyme a lower elongation rate. Here we show that expression of a human equivalent to Drosophila's C4 pol II in human cultured cells affects alternative splicing of the fibronectin EDI exon and adenovirus E1a pre-mRNA. Most importantly, resplicing of the Hox gene Ultrabithorax is stimulated in Drosophila embryos mutant for C4, which demonstrates the transcriptional control of alternative splicing on an endogenous gene. These results provide a direct proof for the elongation control of alternative splicing in vivo.

Descripción: Alternative splicing plays a key role in generating protein diversity. Transfections with minigenes revealed coordination between two distant, alternatively spliced exons in the same gene. Mutations that either inhibit or stimulate inclusion of the upstream alternative exon deeply affect inclusion of the downstream one. However, similar mutations at the downstream alternative exon have little effect on the upstream one. This polar effect is promoter specific and is enhanced by inhibition of transcriptional elongation. Consistently, cells from mutant mice with either constitutive or null inclusion of a fibronectin alternative exon revealed coordination with a second alternative splicing region, located far downstream. Using allele-specific RT-PCR, we demonstrate that this coordination occurs in cis and is also affected by transcriptional elongation rates. Bioinformatics supports the generality of these findings, indicating that 25% of human genes contain multiple alternative splicing regions and identifying several genes with nonrandom distribution of mRNA isoforms at two alternative regions. Copyright ©2005 by Elsevier Inc.