Adenovirus Late Gene Expression
Julie Boyer and Gary Ketner
The late phase of an adenovirus infection begins with the onset of viral DNA replication. During the late phase, protein synthesis in adenovirus-infected cells is dominated by the production of large quantities of the adenovirus capsid proteins and of a few non-structural proteins required for capsid assembly, about 13 in all. Almost all of the proteins produced late in infection are the products of translation of ‘late mRNAs’ derived from the major late transcriptional unit (MLTU; see below for exceptions). The MLTU extends rightward from the major late promoter (MLP), located at genome position 17, almost to the end of the genome. The nearly exclusive expression of MLTU products late in infection is the result of profound changes in patterns of both viral and host gene expression that coincide with the beginning of viral DNA replication. On the viral genome, the rate of transcription from the MLP is dramatically upregulated from the low level observed earlier in infection, and transcriptional termination within the major late transcriptional unit is abolished. Transcription of most of the viral early genes is gradually reduced. Posttranscriptionally, splicing and polyadenylation site utilization in the primary transcriptional product of the MLTU changes, as do rates of transport and stability of viral RNAs. Transport and translation of most host mRNAs are concomitantly inhibited. As these events in gene expression are occurring, host cell DNA synthesis is inhibited in favor of synthesis of viral DNA. Cells thus become efficient machines for the production of virus particles for subsequent release.