Random, asynchronous, and asymmetric transcriptional activity of enhancer-flanking major immediate-early genes ie1/3 and ie2 during murine cytomegalovirus latency in the lungs

The lungs are a major organ site of cytomegalovirus (CMV) pathogenesis, lat ency, and recurrence. Previous work on murine CMV latency has documented a high load and an even distribution of viral genomes in the lungs after the resolution of productive infection. Initiation of the productive cycle requ ires expression of the ie1/3 transcription unit, which is driven by the imm ediate-early (IE) promoter p(1/3) and generates IE1 and IE3 transcripts by differential splicing. Latency is molecularly defined by the absence of IE3 transcripts specifying the essential transactivator protein IE3. In contra st, IE1 transcripts were found to be generated focally and randomly, reflec ting sporadic p(1/3) activity. Selective generation of IE1 transcripts impl ies molecular control of latency operating after ie1/3 transcription initia tion. p(1/3) is regulated by an upstream enhancer. It is widely assumed tha t the viral transcriptional program is started by activation of the enhance r through the binding of transcription factors. Accordingly, stochastic tra nscription during latency might reflect episodes of enhancer activation by the "noise" activity of intrinsic transcription factors. In addition to ie1 /3, the enhancer controls gene ie2, which has its own promoter, P-2, and is transcribed in opposite direction. We show here that ie2 is also randomly transcribed during latency. Notably, however, iel and ie2 were found to be expressed independently. We infer from this finding that expression of the major IE genes is regulated asymmetrically and asynchronously via the combi ned control unit p(1/3) -E-P-2. Our data are consistent,vith a stochastic n ature of enhancer action as it is proposed by the "binary" or probability m odel.