Organization and expression of the major genes from the long inverted repeat of the human cytomegalovirus genome.

The long inverted repeat (TRL:IRL) of the human cytomegalovirus (CMV) genome is a major transcription unit in productively infected human fibroblasts. Cloned DNA fragments of the CMV IRL and complementary DNA (cDNA) copies of RNAs transcribed from the TRL:IRL were used as probes in RNA filter hybridization experiments to characterize and map the RNAs transcribed from this region of the virus genome. In human fibroblasts, three poly A+ RNAs of 2.7, 2.0, and 1.2 kb were detected during the early (E) and late (L) phases of virus gene expression. Analysis of cloned cDNAs and RNA mapping studies indicate that the TRL:IRL can be divided into three transcriptionally active regions. The most highly transcribed region lies between 0.805 and 0.816 map units and encodes a major abundant poly A+ RNA of 2.7 kb that is expressed at E and L times postinfection (p.i.). The second region spans map coordinates 0.792-0.797 and encodes a 1.2-kb poly A+ RNA that is relatively low in abundance at E times p.i. but achieves nearly the same abundance as the 2.7-kb transcript at L times p.i. The third region encompasses map units 0.796-0.804 and encodes a less abundant poly A+ RNA of 2.0 kb that attains maximum expression at E times. The 1.2- and 2.7-kb RNAs are transcribed in the same direction, while the 2.0-kb RNA is transcribed in the opposite direction. The 2.7-, 2.0-, and 1.2-kb RNAs, as well as 5.7- and 1.8-kb transcripts were detected at immediate early times p.i. when human fibroblasts were treated with cycloheximide, but not in cells treated with anisiomycin.