Abstract
This article reviews the current state of understanding of the regulated transcription of the bacteriophage T4 late genes, with a focus on the underlying biochemical mechanisms, which turn out to be unique to the T4-related family of phages or significantly different from other bacterial systems. The activator of T4 late transcription is the gene 45 protein (gp45), the sliding clamp of the T4 replisome. Gp45 becomes topologically linked to DNA through the action of its clamp-loader, but it is not site-specifically DNA-bound, as other transcriptional activators are. Gp45 facilitates RNA polymerase recruitment to late promoters by interacting with two phage-encoded polymerase subunits: gp33, the co-activator of T4 late transcription; and gp55, the T4 late promoter recognition protein. The emphasis of this account is on the sites and mechanisms of actions of these three proteins, and on their roles in the formation of transcription-ready open T4 late promoter complexes.
Highlights
T4 late genes are transcribed from simple promoters consisting of an 8-base pair TATA box placed ~1 helical DNA turn upstream of the transcriptional start site (the location of the bacterial s70-family RNA polymerase (RNAP) promoter -10 site)
A significant AT base pair preponderance characterizes the segment immediately downstream of the TATA box that strand-separates when the late promoter opens for initiation of transcription; there is no sequence conservation at the position corresponding to the bacterial promoter -35 site
Variant T4 late promoters are used for transcription in vitro [4] and a number of variant promoters have been associated with RNA 5” ends in vivo [5,6] (Three cautionary notes: 1) these 50 sites have not all been identified as promoters that are active in vivo; 2) some of the RNA 5” ends that have been mapped to putative promoters were specified by primer extension analysis, which does not distinguish between 5” ends generated by bona fide initiation and endonucleolytic processing; 3) the relative rates of initiation at consensus and
Summary
T4 late genes are transcribed from simple promoters consisting of an 8-base pair TATA box placed ~1 helical DNA turn upstream of the transcriptional start site (the location of the bacterial s70-family RNA polymerase (RNAP) promoter -10 site). The functional consequences of attachment of the sliding clamp to the upstream end of RNAP in the activated late promoter complex through its interactions with hydrophobic and acidic motifs at the C-termini of gp and gp are a greatly increased overall rate of promoter opening. The mechanism that couples transcription of the viral late genes to replication in the T4 multiplication cycle elegantly dispenses with (or, depending on perspective, is too primitive for) symbolic communication, instead directly using universal components of cellular DNA replication, the primer-template junction and the clamploading factors, as generators of activation and the ubiquitous sliding clamp as the activator. Other instances of the use of this direct and simple mechanism for coupling transcriptional regulation to DNA replication in nature have not been found
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