Selective inhibition of adenovirus type 2 early region II and III transcription by an anisomycin block of protein synthesis

The transcription of adenovirus type 2 genes proceeds through a broad three-phase program. From 1 to 4 h postinfection six early transcription units (EIa, EIb, EII, EIII, EIV, and the promoter-proximal segment of the late transcription unit) are activated. From 4 to 6 h postinfection transcription of the early genes is depressed. After the onset of viral DNA replication at approximately 6 h postinfection, the transcript from the late promoter is antiterminated, and this transcript dominates viral RNA synthesis. The early activation period also proceeds through a series of stages; early regions EIa and EIV are activated first, followed by early region EII. We show that in the presence of anisomycin, a stringent inhibitor of protein synthesis, nuclear RNA and cytoplasmic polyadenylated RNA from regions EIa and EIV accumulate at normal rates, whereas RNAs from regions EII and EIII do not accumulate. We also show that failure to accumulate RNAs from regions EII and EIII is due to reduction of the rate of transcription by greater than 90%. We conclude that the regulation of the promoters for early regions EII and EIII is distinct from the mechanism that operates on the other early transcription units. The promoters for early regions EII and EIII diverge and lie approximately 500 nucleotides apart. We examined the structure of viral chromatin in this region early during infection by mild DNase I digestion in isolated nuclei and indirect end labeling with a DNA probe near these promoters. In control, drug-free cells where EII and EIII are transcribed and in anisomycin-treated cells where EII and EIII are not transcribed we detect the same regular DNase I pattern. We suggest that regulation of EII and EIII is not mediated through a change in gross chromatin structure, but rather by a viral effector, possibly a product of region EIa.