The essential herpes simplex virus type 1 (HSV-1) immediate-early IE63 (ICP27) is pleiotropic in function, promoting the switch from the early to late phase of virus gene expression, and has effects on the posttranscriptional processes of
mRNA splicing and 3' processing. We have investigated the role of IE63 in the regulation of viral
mRNA 3' processing and of late gene expression. Our in vitro 3' processing studies demonstrated that HSV-1
infection induces an activity, which requires IE63 gene expression, responsible for an observed increase in 3' processing of selected HSV-1
poly(A) sites. Processing efficiencies at the
poly(A) sites of two late genes, UL38 and UL44, shown to be inherently weak processing sites, were increased by the IE63-induced activity. In contrast, 3' processing at the
poly(A) sites of selected immediate-early and early genes, stronger processing sites, was unaffected by IE63 expression. UV cross-linking experiments demonstrated that HSV
infection caused enhanced binding of
protein factors, including the 64-kDa component of
cleavage stimulation factor (CstF), to
poly(A) site RNAs from virus genes of all temporal classes and that this enhanced binding required expression of IE63. By immunofluorescence, the homogeneous pattern of the 64-kDa
CstF protein distribution became slightly clumped with
infection, whereas the splicing
small nuclear ribonucleoprotein particles were recognized into a highly punctate distribution away from the sites of virus transcription. This effect could create an increase in the relative concentration of 3' processing factors available to pre-mRNAs. Western blot (immunoblot) analysis showed that IE63 was required for expression of several true late genes and for the efficient and timely expression of the UL29 and UL42 early genes, integral components on the
viral DNA synthesis machinery. Our data are consistent with two effects of IE63 on late gene regulation: firstly, a stimulation of
pre-mRNA 3' processing and, secondly, as a requirement for expression of functions necessary for
viral DNA synthesis.