Transcription elongation factor SII

RNA chain elongation by RNA polymerase II (pol II) is a complex and regulat ed process which is coordinated with capping, splicing, and polyadenylation of the primary transcript. Numerous elongation factors that enable pol II to transcribe faster and/or more efficiently have been purified. SII is one such factor. It helps pol II bypass specific blocks to elongation that are encountered during transcript elongation. SII was first identified biochem ically on the basis of its ability to enable pol II to synthesize long tran scripts.((1)) Both the high resolution structure of SII and the details of its novel mechanism of action have been refined through mutagenesis and sop histicated in vitro assays. SII engages transcribing pol II and assists it in bypassing blocks to elongation by stimulating a cryptic, nascent RNA cle avage activity intrinsic to RNA polymerase. The nuclease activity can also result in removal of misincorporated bases from RNA. Molecular genetic expe riments in yeast suggest that SII is generally involved in mRNA synthesis i n vivo and that it is one type of a growing collection of elongation factor s that regulate pol II. In vertebrates, a family of related SII genes has b een identified; some of its members are expressed in a tissue-specific mann er. The principal challenge now is to understand the isoform-specific funct ional differences and the biology of regulation exerted by the SII family o f proteins on target genes, particularly in multicellular organisms. BioEss ays 22:327-336, 2000, (C) 2000 John Wiley & Sons, Inc.