Analysis of immediate-early transcripts of equine cytomegalovirus

Equine cytomegalovirus (ECMV) contains a linear, double-stranded DNA genome composed of a 146-kbp unique region flanked by a pair of 18-kbp direct repeat (DR) sequences at the termini. Cycloheximide, actinomycin D, and phosphonoacetic acid were applied to infected cell cultures to divide viral transcription into immediate-early (IE), early, and late phases. Eight IE transcripts were identified and mapped to two regions (I and II) of the viral genome. Two of these IE RNAs (13.0 and 5.5 kb in size) were transcribed from region I, which is located within the DR regions; these IE genes are diploid. The other IE transcripts (17.0, 9.0, 7.2, 6.8, 4.5, and 4.2 kb) originated from region II. IE region II is adjacent to region I and spans both unique and DR sequences at the left terminus of the genome. Region II IE transcripts are spliced and transcribed in the opposite direction from region I IE transcripts. IE transcripts from region I were present throughout the replication cycle, whereas those from region II were more abundant during the IE stage than at the early and late stages of infection. These studies demonstrate that ECMV differs from other herpesviruses in the organization and unusually large transcription units of its IE genes.