CONTROL OF ADENOVIRUS EARLY GENE-EXPRESSION DURING THE LATE-PHASE OF INFECTION

The adenovirus gene regulatory program occurs in two distinct phases, as defined by the onset of DNA replication. During the early phase, th e E1A, E1B, E2, E3, and E4 genes are maximally expressed, while the ma jor late promoter (MLP) is minimally expressed and transcription is at tenuated. After the onset of DNA replication, the IVa2 and pIX genes a re expressed at high levels, transcription from the MLP is unattenuate d and fully activated, and early gene expression is repressed. Althoug h the cis elements and trans-acting factors responsible for the late-p hase activation of the MLP have been identified and characterized and the role of DNA replication in activation has been established, the me chanism(s) underlying the commensurate decrease in early gene expressi on has yet to be elucidated. The results of this study demonstrate tha t this decrease depends on a fully functional MLP. Specifically, virus mutants with severely deficient transcription from the MLP exhibit a marked increase in expression of the E1A, E1B, and E2 early genes. The se increases were observed at the level of transcription initiation, m RNA accumulation, and protein production. In addition, expression from the late gene pIX, which is not contained within the major late trans cription unit (MLTU), is also markedly increased. To begin the analysi s of the mechanisms underlying these late-phase effects, mixed-infecti on experiments with mutant and wild-type viruses were performed. The r esults show that the effects on early gene expression, as measured bot h at the protein and RNA levels, are mediated in trans and not in cis. These observations are consistent either with a model in which one or more late protein products encoded by the MLTU acts as a repressor of early gene expression or with one in which the wild-type MLP competes with early promoters for limiting transcription factors.