Structure of the Bacteriophage PhiKZ non-Virion RNA Polymerase
Bacteriophage {Phi}KZ is the founding member of a family of massive bacterial viruses. It is considered to have therapeutic potential as its host, Pseudomonas aeruginosa, is an opportunistic, intrinsically antibiotic resistant, pathogen that kills tens of thousands worldwide each year. {Phi}KZ is an incredibly interesting virus, expressing many systems the host already possesses. On infection, it forms a "nucleus", erecting a barrier around its massive genome to exclude host restriction endonucleases and CRISPR-Cas systems. {Phi}KZ infection is independent of the host transcriptional apparatus. It expresses two different multi-subunit RNA polymerases (RNAPs): the virion RNAP (vRNAP) is injected with the viral DNA during infection to transcribe early genes, including those encoding the non-virion RNAP (nvRNAP), which transcribes all further genes. {Phi}KZ nvRNAP is formed by four polypeptides thought to represent homologues of the eubacterial {beta}/{beta}' subunits, and a fifth with unclear homology, but essential for transcription. We have resolved the structure of {Phi}KZ nvRNAP to 3.3 [A], shedding light on its assembly, homology, and the biological role of the fifth subunit: it is an embedded, integral member of the complex, with structural homology and a biochemical role implying that it has evolved from an ancestral homologue to {sigma}-factor.