Abstract
Bacteriophage N4 regulates the temporal expression of its genome through the activity of three distinct RNA polymerases (RNAP). Expression of the early genes is carried out by a phage-encoded, virion-encapsidated RNAP (vRNAP) that is injected into the host at the onset of infection and transcribes the early genes. These encode the components of new transcriptional machinery (N4 RNAPII and cofactors) responsible for the synthesis of middle RNAs. Both N4 RNAPs belong to the T7-like “single-subunit” family of polymerases. Herein, we describe their mechanisms of promoter recognition, regulation, and roles in the phage life cycle.
Highlights
Regulation at the transcriptional level is the primary means used by bacteriophage to progress through distinct developmental stages during infection
Given the inactivity of virion-encapsidated RNAP (vRNAP) on native N4 DNA and its preference for single-stranded DNA, we proposed that the structure of phage DNA might become modified upon its injection into the host, rendering it competent for transcription by vRNAP [12,14]
Comparison of the T7 RNA polymerases (RNAP) and N4 vRNAP sequences indicated that part of the T7 RNAP N-terminal domain responsible for RNA separation and exit is missing from vRNAP; we have proposed that EcoSSB fulfills this role in vRNAP transcription [32,33]
Summary
Regulation at the transcriptional level is the primary means used by bacteriophage to progress through distinct developmental stages during infection. Gene products expressed immediately after infection are primarily involved in the takeover of essential host processes. Middle transcription commences, focusing largely on the synthesis of proteins involved in phage genome replication. Genome replication is followed by late gene transcription, which includes the production of both morphogenetic proteins, involved in virion assembly and DNA packaging, and of proteins required for host cell lysis. Three classes of putative promoter sequences have been identified upstream of genes transcribed at early, middle, and late times during infection. The role of these polypeptides and sequences in transcription of the phage genome has not yet been confirmed [9].
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