THE EPSTEIN-BARR-VIRUS NUCLEAR-PROTEIN SM IS BOTH A POSTTRANSCRIPTIONAL INHIBITOR AND ACTIVATOR OF GENE-EXPRESSION

The Epstein-Barr virus (EBV) nuclear protein BS-MLF1 (SM) is expressed early after entry of EBV into the lytic cycle, SM transactivates repo rter gene constructs driven by a wide variety of promoters, but the me chanism of SM action is poorly understood. In this study, we demonstra te that the SM protein inhibits expression of intron-containing genes and activates expression of intron-less genes. We demonstrate that SM has the predicted inhibitory effect on expression of a spliced EBV gen e but activates an unspliced early EBV gene. SM inhibited gene express ion at the posttranscriptional level by preventing the accumulation of nuclear and cytoplasmic RNA transcripts. Conversely, SM led to increa sed accumulation of nuclear mRNA from intron-less genes without affect ing the rate of transcription, indicating that SM enhances nuclear RNA stability. The ratio of cytoplasmic to nuclear polyadenylated mRNA wa s increased in the presence of SM, suggesting that SM also enhances nu cleocytoplasmic mRNA transport. The degree of transactivation by SM wa s dependent on the sequence of the 3'-untranslated region of the targe t mRNA, Finally, we demonstrate that the amino-terminal portion of SM fused to glutathione-S-transferase binds radioactively labeled RNA irt vitro, indicating that SM is a single-stranded RNA binding protein. I mportantly, the latent and immediate-early genes of EBV contain intron s whereas many early and late genes do not. Thus, SM may down-regulate synthesis of host cell proteins and latent EBV proteins while simulta neously enhancing expression of specific lytic EBV genes by binding to mRNA and modulating its stability and transport.