Abstract
Control of transcription is crucial for correct gene expression and orderly development. For many years, bacteriophage T4 has provided a simple model system to investigate mechanisms that regulate this process. Development of T4 requires the transcription of early, middle and late RNAs. Because T4 does not encode its own RNA polymerase, it must redirect the polymerase of its host, E. coli, to the correct class of genes at the correct time. T4 accomplishes this through the action of phage-encoded factors. Here I review recent studies investigating the transcription of T4 prereplicative genes, which are expressed as early and middle transcripts. Early RNAs are generated immediately after infection from T4 promoters that contain excellent recognition sequences for host polymerase. Consequently, the early promoters compete extremely well with host promoters for the available polymerase. T4 early promoter activity is further enhanced by the action of the T4 Alt protein, a component of the phage head that is injected into E. coli along with the phage DNA. Alt modifies Arg265 on one of the two α subunits of RNA polymerase. Although work with host promoters predicts that this modification should decrease promoter activity, transcription from some T4 early promoters increases when RNA polymerase is modified by Alt. Transcription of T4 middle genes begins about 1 minute after infection and proceeds by two pathways: 1) extension of early transcripts into downstream middle genes and 2) activation of T4 middle promoters through a process called sigma appropriation. In this activation, the T4 co-activator AsiA binds to Region 4 of σ70, the specificity subunit of RNA polymerase. This binding dramatically remodels this portion of σ70, which then allows the T4 activator MotA to also interact with σ70. In addition, AsiA restructuring of σ70 prevents Region 4 from forming its normal contacts with the -35 region of promoter DNA, which in turn allows MotA to interact with its DNA binding site, a MotA box, centered at the -30 region of middle promoter DNA. T4 sigma appropriation reveals how a specific domain within RNA polymerase can be remolded and then exploited to alter promoter specificity.
Highlights
Expression of the T4 genome is a highly regulated and elegant process that begins immediately after infection of the host
This review focuses on investigations of T4 early and middle transcription since those detailed in the last T4 book [1,5]
Work with host promoters argued that contact between the a-CTDs of RNA polymerase (RNAP) and promoter UP elements or certain activators increases transcription; in particular, a residue Arg265 was crucial for this interaction
Summary
Expression of the T4 genome is a highly regulated and elegant process that begins immediately after infection of the host. The C-terminal half of the AsiA ortholog of the vibrio phage KVP40 (discussed below) has little or no sequence homology with its T4 counterpart yet in the presence of T4 MotA and E. coli RNAP, it effectively co-activates transcription from PuvsX in vitro [119], and NMR analyses indicate that the addition of MotA to the AsiA/s70 Region 4 complex does not significantly perturb chemical shifts of AsiA residues [104]. The presence of the b G1249D mutation impairs transcription from T4 middle promoters in vivo, but whether the substitution directly or indirectly affects protein-protein interactions is not yet known [120] Taken together, these results suggest that MotA/AsiA activation employs multiple contacts, some of which are essential under all circumstances (AsiA with s70 Regions 4.1 and 4.2, MotA with s70 H5) and some of which may provide additional contacts perhaps under certain circumstances to strengthen the complex. A thorough investigation of these processes in the simple T4 system could provide a powerful tool to understanding this mode of gene regulation
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