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.