The herpesvirus saimiri (HVS) immediate-early gene product encoded by open reading frame (ORF) 57 shares limited amino acid homology with HSV-1 ICP27 and Epstein-Barr virus BMLF1, both regulatory proteins. The ORF 57 gene has been proposed to be spliced based on the genome sequence, and here we confirm the intron-exon structure of the gene. We also demonstrate that a cDNA construct of the ORF 57 gene product represses the transactivating capability of the ORF 50a gene product (which is produced from a spliced transcript), but activates that of ORF 50b (an unspliced transcript). Further analyses with cotransfection experiments show that ORF 57 can either activate or repress expression from a range of both early and late HVS promoters, depending on the target gene. These results indicate that repression of gene expression mediated by the ORF 57 gene product is dependent on the presence of an intron within the target gene encoding region. Furthermore, Northern blot analysis demonstrates that the levels of mRNA transcribed from genes not containing an intron are not significantly affected in the presence of the ORF 57 gene product. This suggests that it regulates gene expression through a posttranscriptional mechanism
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