Temporal regulation of human cytomegalovirus transcription at immediate early and early times after infection.

The immediate early transcripts of human cytomegalovirus originated from restricted regions of the viral genome. In contrast, transcription at early times was complementary to all regions of the viral genome that were fractionated by restriction endonuclease treatment followed by agarose gel electrophoresis. The viral genome was also extensively transcribed when 2 h of protein synthesis or longer was permitted after infection in permissive cells treated with an inhibitor of viral DNA replication or in nonpermissive cells of animal origin that permit little or no viral DNA replication. The size and in vitro translation products of the cytomegalovirus-specified mRNA's at immediate early and early times after infection were determined. Discrete size classes of virus-specified polyadenylated RNA accumulated on the polyribosomes of cells infected in the presence of an inhibitor of protein synthesis. When 2 or 24 h of protein synthesis occurred after infection, there were changes in the relative abundance of the virus-specified RNAs that accumulated on polyribosomes. Treatment of nonpermissive cells had little effect on the size classes of viral RNA found associated with the polyribosomes at early times after infection. These viral mRNA's were assumed to represent early viral gene expression. In vitro translation of the viral mRNA isolated from polyribosomes at immediate early and early times after infection identified many of the virus-specified gene products and demonstrated (i) a switch from immediate early to early viral gene expression and (ii) a prolonged phase of early viral gene expression. The data also indicated that the initiation of viral RNA synthesis does not depend on the formation of viral protein, but that de novo viral protein synthesis may influence the extent of transcription of the viral genome.