Predominant immediate-early transcripts of human cytomegalovirus AD 169.

Transcription of the human cytomegalovirus genome (strain AD 169) was investigated at the immediate-early (IE) time after infection, by using cycloheximide to suppress virus-specific protein synthesis. In total cell RNA, four predominant IE transcripts were found which were encoded by one contiguous region of the long unique segment between map units 0.06 and 0.16 in prototype arrangement of human cytomegalovirus AD 169 DNA. Analysis by Northern blot hybridizations demonstrated that the transcripts possessed a size of 1.9, 2.2, 2.3, and 5.0 kilobases, respectively. Coding sequences and directions of transcriptions were mapped by Northern blots and hybridizations with oligodeoxythymidylic acid-primed and randomly primed cDNA. The 1.9-, 2.2-, and 2.3-kilobase RNAs were found in the polyadenylated fraction of IE RNA exclusively; in contrast, a part of the 5.0-kilobase RNA appeared polyadenylated, although the majority of the same transcript was found in the nonpolyadenylated pool. Also, different from the other IE genes, the DNA coding for the 5.0-kilobase IE RNA was transcribed in high quantities during the late phase of virus replication, suggesting an exemption from the temporal regulation of herpesvirus transcription.