Identification of a Cellular Repressor of Transcription of the Adenoviral Late IVa2 Gene That Is Unaltered in Activity in Infected Cells

Abstract

The gene encoding the adenovirus type 2 IVa2 protein, a sequence-specific activator of transcription from the viral major late promoter, is itself transcribed only during the late phase of infection. We previously identified a cellular protein (IVa2-RF) that binds specifically to an intragenic sequence of the IVa2 transcription unit. We now report that precise substitutions within the IVa2-RF-binding site that decreased binding affinity increased the efficiency of IVa2 transcription in in vitro reactions containing IVa2-RF. Consistent with the conclusion that this cellular protein represses IVa2 transcription, mutations that led to more efficient transcription in the presence of IVa2-RF were without effect in reactions lacking this cellular protein. No change in the concentration or activity of IVa2-RF could be detected in adenovirus-infected cells during the period in which the IVa2 gene is transcribed. We therefore propose that restriction of IVa2 transcription to the late phase is the result of titration of this cellular repressor as the number of copies of the IVa2 promoter increases upon replication of the viral genome.