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Journal of Virology, October 2007, p. 11208-11217, Vol. 81, No. 20
0022-538X/07/$08.00+0     doi:10.1128/JVI.00919-07
Copyright © 2007, American Society for Microbiology. All Rights Reserved.

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

Department of Microbiology and Molecular Genetics, Medical College of Wisconsin, 8701 Watertown Plank Road, Milwaukee, Wisconsin 53226

Received 30 April 2007/ Accepted 21 July 2007

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.

Published ahead of print on 1 August 2007.