The downstream regulatory sequence of the adenovirus type 2 major late promoter is functionally redundant

The structure and function of the adenovirus major late promoter

The adenovirus major late promoter (MLP) has played a pre-eminent role in the analysis of transcription initiation in mammalian cells, and is an outstanding example of the ways in which the study of adenovirus has led to fundamental insights into general cellular processes. The aim of this chapter is to give a comprehensive review of the structure and function of this model mammalian promoter. After a brief description of late transcription in the adenovirus replication cycle, the experimental evidence for the current consensus on the genetic structure of the MLP, including a consideration of non-primate adenovirus MLPs, will be reviewed. Next, the functions of the MLP in the viral life cycle will be examined, and some of the problems that remain to be resolved will be addressed. The review ends with some ideas on how the knowledge of the structure and function of the MLP can be used in designing virus vectors for specific experimental purposes.

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, the E1A, E1B, E2, E3, and E4 genes are maximally expressed, while the major late promoter (MLP) is minimally expressed and transcription is attenuated. After the onset of DNA replication, the IVa2 and pIX genes are expressed at high levels, transcription from the MLP is unattenuated and fully activated, and early gene expression is repressed. Although the cis elements and trans-acting factors responsible for the late-phase activation of the MLP have been identified and characterized and the role of DNA replication in activation has been established, the mechanism(s) underlying the commensurate decrease in early gene expression has yet to be elucidated. The results of this study demonstrate that 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. These increases were observed at the level of transcription initiation, mRNA accumulation, and protein production. In addition, expression from the late gene pIX, which is not contained within the major late transcription unit (MLTU), is also markedly increased. To begin the analysis of the mechanisms underlying these late-phase effects, mixed-infection experiments with mutant and wild-type viruses were performed. The results show that the effects on early gene expression, as measured both 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.

The product of the adenovirus intermediate gene IVa2 is a transcriptional activator of the major late promoter

During the course of lytic infection, the adenovirus major late promoter (MLP) is induced to high levels after replication of viral DNA has started. We had previously shown that sequence elements located downstream of the MLP start site were implicated in this late-specific transcriptional activation (DE1, between +85 and +98; DE2, between +100 and +120). Two positive transcription factors involved in this activation have been detected. DEF-A, which specifically binds to DE1 and also to the 3' portion of DE2 (DE2a), and DEF-B, which interacts with the 5' part of DE2 (DE2b). When present together, these two proteins cooperatively assemble onto the DE2 element. We now report the purification of DEF-B and show that it is identical to the product of the adenovirus IVa2 gene product. This conclusion is based on microsequence analysis of DEF-B as well as on the inhibitory effect of antibodies against IVa2 on the DNA-binding activity of DEF-B and also on DE-dependent in vitro transcription. In addition, we show that bacterially synthesized IVa2 protein binds to the DE sequences with the same specificity as DEF-B. Finally, in transfected cells, a recombinant IVa2 protein stimulates MLP activity in a DE-dependent fashion. The physiological implications of these findings are discussed.