READTHROUGH ACTIVATION OF EARLY ADENOVIRUS E1B GENE-TRANSCRIPTION

In cells productively infected with adenovirus type 5, transcription i s not terminated between the E1a gene and the adjacent downstream E1b gene. Insertion of the mouse beta(maj)-globin transcription terminatio n sequence (GGT) into the E1a coding region dramatically reduces early , but not late, E1b expression (E. Falck-Pedersen, J. Logan, T. Shenk, and J. E. Darnell, Jr., Cell 40:897-905, 1985). In the study describe d herein, we showed that base substitution mutations in the globin DNA that specifically relieved transcription termination also restored ea rly E1b promoter activity in cis, establishing that maximal early E1b expression requires readthrough transcription originating from the adj acent upstream gene, To identify potential targets of readthrough acti vation, a series of recombinant viruses,vith double mutations was cons tructed. Each double-mutant virus strain had the transcription termina tion sequences in the first exon of E1a and a deletion within the tran scription control region of E1b. Early E1b expression from the double- mutant strains was more defective than that from strains containing ei ther mutation alone, indicating that the deleted regions (positions -3 62 to -35) are not the target for readthrough activation, Two findings suggested that a cis-dominant property of early viral templates is im portant for readthrough activation. First, the early E1b defect caused by the GGT insertion was not complemented in tuans by factors present in late-infected cells, Second, restoration of E1b transcription at l ate times occurred concurrently with viral DNA replication. Readthroug h activation may help convert virion DNA into a transcriptionally comp etent template prior to DNA replication and late transcription.