Serine/Arginine-Rich Proteins Contribute to Negative Regulator of Splicing Element-Stimulated Polyadenylation in Rous Sarcoma Virus

Abstract

Rous sarcoma virus (RSV) requires large amounts of unspliced RNA for replication. Splicing and polyadenylation are coupled in the cells they infect, which raises the question of how viral RNA is efficiently polyadenylated in the absence of splicing. Optimal RSV polyadenylation requires a far-upstream splicing control element, the negative regulator of splicing (NRS), that binds SR proteins and U1/U11 snRNPs and functions as a pseudo-5 splice site that interacts with and sequesters 3 splice sites. We investigated a link between NRS-mediated splicing inhibition and efficient polyadenylation. In vitro, the NRS alone activated a model RSV polyadenylation substrate, and while the effect did not require the snRNP-binding sites or a downstream 3 splice site, SR proteins were sufficient to stimulate polyadenylation. Consistent with this, SELEX-binding sites for the SR proteins ASF/SF2, 9G8, and SRp20 were able to stimulate polyadenylation when placed upstream of the RSV poly(A) site. In vivo, however, the SELEX sites improved polyadenylation in proviral clones only when the NRS-3 splice site complex could form. Deletions that positioned the SR protein-binding sites closer to the poly(A) site eliminated the requirement for the NRS-3 splice site interaction. This indicates a novel role for SR proteins in promoting RSV polyadenylation in the context of the NRS-3 splice site complex, which is thought to bridge the long distance between the NRS and poly(A) site. The results further suggest a more general role for SR proteins in polyadenylation of cellular mRNAs.Generation of mature mRNA in eukaryotes generally requires multiple processing steps, including capping, splicing, and polyadenylation, that are coupled to ensure proper processing (reviewed in reference 31). Retroviruses utilize the host transcription/RNA processing machinery to generate viral RNA, but due to peculiarities of their replication scheme, they often utilize the RNA processing machinery in unique ways. In the simple avian retrovirus Rous sarcoma virus (RSV), the env and src mRNAs are generated by RNA splicing from a common 5Ј splice site (ss) to one of two alternative 3Ј ss (10). However, unlike most host genes, retrovirus replication requires that a substantial portion of the primary viral transcripts remain completely unspliced to serve as gag-pol mRNA and as genomic RNA for progeny virions. RSV employs several mechanisms to preserve the pool of unspliced RNA, including the maintenance of suboptimal 3Ј ss (25, 59) and the action of splicing repressor elements within the gag gene (the negative regulator of splicing, or NRS) (3, 39, 49) and upstream of the src 3Ј ss (the suppressor of src splicing) (1, 39, 50). Generation of functional unspliced viral mRNA poses problems for the coupling of the splicing and polyadenylation reactions.The NRS has been well characterized and is thought to act as a pseudo-5Ј ss that sequesters viral 3Ј ss in a nonproductive splicing complex (reviewed in reference 9). An upstream region of the ϳ230-nucleotide (nt) element bind...

Nuclear pre-mRNA 3′-end processing regulates synapse and axon development in C. elegans

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Genes involved in pre-mRNA 3′-end formation and transcription termination revealed by a lin-15 operon Muv suppressor screen

“…Finally, another allele-specific suppressor, rsp-6, encodes SRp20, an SR protein that affects alternative splicing (25). This protein has no known role in transcription termination, although it has been implicated in polyadenylation of Rous sarcoma virus (26), an alternative polyadenylation site choice (27).…”

“…However, there are two reports of SRp20 at the 3Ј ends of genes. SRp20 has been shown to co-immunoprecipitate with premessenger RNA cleavage factor 1 (CFIm), a factor involved in 3Ј-end formation in mammals (36), and to affect polyadenylation in Rous sarcoma virus (26). RNAi against the C. elegans homologue to CFIm did not suppress the Muv phenotype, so interaction with CFlm is unlikely to be the mechanism by which SRp20 functions.…”

Control of the Papillomavirus Early-to-Late Switch by Differentially Expressed SRp20

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