Journal of General Virology (1999), 80, 1311–1316. Printed in Great Britain .......................................................................................................................................................................................................... SHORT COMMUNICATION The gene product encoded by ORF 57 of herpesvirus saimiri regulates the redistribution of the splicing factor SC-35 Matthew Cooper, Delyth J. Goodwin, Kersten T. Hall, Alex J. Stevenson, David M. Meredith, Alexander F. Markham and Adrian Whitehouse Molecular Medicine Unit, University of Leeds, St James’s University Hospital, Leeds LS9 7TF, UK The herpesvirus saimiri (HVS) gene product encoded by ORF 57 shares limited C-terminal similarity with herpes simplex virus 1 ICP27, a protein that has been demonstrated to be involved in the inhibition of host-cell splicing and is re- sponsible for the redistribution of components of the spliceosome. It has previously been shown that ORF 57 can either activate or repress viral gene expression by a post-transcriptional mechanism. Furthermore, repression of gene expression by ORF 57 is dependent on the presence of an intron within the target gene coding region. In this report, it is shown that HVS infection results in the re- distribution of the SC-35 splicing factor in the infected cell nucleus. Furthermore, the redis- tributed SC-35 colocalized with the ORF 57 protein product and expression of the protein alone was sufficient to cause the redistribution of the spliceosome components. These results suggest that the mechanism by which ORF 57 down- regulates expression of intron-containing genes involves the redistribution of the spliceosome complex. Herpesvirus saimiri (HVS) is a lymphotropic rhadinovirus (γ  herpesvirus) of squirrel monkeys (Saimiri sciureus) that persistently infects its natural host without causing any obvious disease. However, HVS infection of other species of New World primates results in fulminant, polyclonal T-cell lymphomas and lymphoproliferative diseases (Fleckenstein & Desrosiers, 1982). HVS is also capable of transforming simian and human T lymphocytes to continuous growth in vitro (Beisinger et al., 1992). Analysis of the genome of HVS (strain A11) indicates that it shares significant similarity with the herpesviruses Epstein–Barr virus (EBV), bovine herpesvirus-4, Kaposi’s sarcoma-associated herpesvirus (human herpesvirus- Author for correspondence : Adrian Whitehouse. Fax 44 113 244 4475. e-mail A.Whitehouseleeds.ac.uk 8) and murine gammaherpesvirus-68 (Albrecht et al., 1992; Albrecht & Fleckenstein, 1990 ; Bublot et al.,1992 ; Gompels et al., 1988 a, b ; Neipel et al., 1997 ; Russo et al., 1996 ; Virgin et al., 1997). Gene expression during lytic replication of HVS is regulated by the products of two major transcription- regulating genes encoded by ORFs 50 and 57 (Whitehouse et al., 1998 a). ORF 50 produces two gene products, which are homologous to the EBV BRLF1 protein (Nicholas et al., 1991) and function as sequence-specific transactivators (Whitehouse et al., 1997). The ORF 57 protein is homologous to genes identified in all classes of herpesviruses, including the EBV transactivator encoded by BMLF1, ICP27 of herpes simplex virus (HSV), ORF 4 encoded by varicella-zoster virus and UL69 in human cytomegalovirus (Fleckenstein & Desrosiers, 1982 ; Kenney et al., 1989 ; Nicholas et al., 1988; Perera et al., 1994 ; Winkler et al., 1994). The ORF 57 gene product has trans-regulatory functions ; transactivation of late viral genes occurs independently of target gene promoter sequences and appears to be mediated at the post-transcriptional level, whereas repression of gene expression appears to correlate with the presence of introns, suggesting ORF 57 is functionally homologous to ICP27 (Whitehouse et al., 1998 a, b). ICP27 has been shown to contribute to the shut-off of host-cell protein synthesis and contributes to a decrease in cellular mRNA levels during infection (Hardwicke & Sandri-Goldin, 1994 ; Hardy & Sandri- Goldin, 1994 ; Hibbard & Sandri-Goldin, 1995). In addition, it has been shown that HSV-1 infection causes the redistribution of the small nuclear ribonucleoproteins (snRNPs), which are essential for the formation of the spliceosome complex (Martin et al., 1987). The spliceosome removes introns from pre-mRNA and is a complex of many proteins, of which snRNPs are a major component (Green, 1991). Other non-snRNP proteins, such as spliceosome assembly factor SC-35, have been shown to be essential for spliceosome complex formation (Fu & Maniatis, 1990). It has been demonstrated that ICP27 is responsible for the redistribution of snRNPs (Phelan et al., 1993), which is believed to be mediated through an interaction with its C terminus (Sandri-Goldin et al., 1995 ; Sandri-Goldin & Hibbard, 1996). 0001-6006  1999 SGM BDBB