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J. Virol., Nov 1997, 8321-8329, Vol 71, No. 11
LF Maxfield and DJ Spector
In cells productively infected with adenovirus type 5, transcription is not
terminated between the E1a gene and the adjacent downstream E1b gene.
Insertion of the mouse beta(maj)-globin transcription termination 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 described herein, we showed that
base substitution mutations in the globin DNA that specifically relieved
transcription termination also restored early E1b promoter activity in cis,
establishing that maximal early E1b expression requires readthrough
transcription originating from the adjacent upstream gene. To identify
potential targets of readthrough activation, a series of recombinant
viruses with double mutations was constructed. Each double-mutant virus
strain had the transcription termination sequences in the first exon of E1a
and a deletion within the transcription control region of E1b. Early E1b
expression from the double-mutant strains was more defective than that from
strains containing either mutation alone, indicating that the deleted
regions (positions -362 to -35) are not the target for readthrough
activation. Two findings suggested that a cis-dominant property of early
viral templates is important for readthrough activation. First, the early
E1b defect caused by the GGT insertion was not complemented in trans by
factors present in late-infected cells. Second, restoration of E1b
transcription at late times occurred concurrently with viral DNA
replication. Readthrough activation may help convert virion DNA into a
transcriptionally competent template prior to DNA replication and late
transcription.
Copyright © 1997, American Society for Microbiology
Readthrough activation of early adenovirus E1b gene transcription
Department of Microbiology and Immunology, College of Medicine, Pennsylvania State University, Hershey 17033, USA.
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