Abstract
Expression of the immediate-early 1 and 2 (IE-1 and IE-2) gene region of human cytomegalovirus (HCMV) was studied during initial phases of the replicative cycle. Accumulation of RNA from IE-1 and -2 was found to be differential. Transcripts from IE-2 reached peak levels very early in infection between 3 and 5 h, whereas IE-1 RNA peak levels were detected later, between 6 and 8 h. A strong decrease in steady-state levels of a 2.2-kb IE-2 RNA was observed at a time when IE-1 transcripts showed a further increase in abundance. Northern (RNA) blot experiments revealed that expression of both the IE-1 RNA and the 2.2-kb IE-2 transcript is controlled by the IE-1 enhancer-promoter. Nuclear run-on experiments demonstrated equal rates of primary transcription for IE-1 and -2 at a time when different steady-state levels of RNA were observed. Concomitant with down-regulation of IE-2 RNAs, a decrease in the size of the IE-1 transcript was detected. At 2 to 5 h after infection the IE-1 transcript migrated at 1.95 kb, whereas later in the replicative cycle the RNA was found at 1.8 kb. RNase H blot analysis revealed that this size discrepancy is due to a shorter poly(A) tail of the IE-1 RNA at early times after infection. These experiments suggest that in addition to transcriptional regulation, specific posttranscriptional mechanisms are involved in controlling expression from the IE-1 and -2 gene region of HCMV.