Development of Giant Bacteriophage φKZ Is Independent of the Host Transcription Apparatus

Pseudomonas aeruginosa bacteriophage phi KZ is the type representative of the giant phage genus, which is characterized by unusually large virions and genomes. By unraveling the transcriptional map of the similar to 280-kb phi KZ genome to single-nucleotide resolution, we combine 369 phi KZ genes into 134 operons. Early transcription is initiated from highly conserved AT-rich promoters distributed across the phi KZ genome and located on the same strand of the genome. Early transcription does not require phage or host protein synthesis. Transcription of middle and late genes is dependent on protein synthesis and mediated by poorly conserved middle and late promoters. Unique to phi KZ is its ability to complete its infection in the absence of bacterial RNA polymerase (RNAP) enzyme activity. We propose that transcription of the phi KZ genome is performed by the consecutive action of two phi KZ-encoded, noncanonical multisubunit RNAPs, one of which is packed within the virion, another being the product of early genes. This unique, rifampin-resistant transcriptional machinery is conserved within the diverse giant phage genus. IMPORTANCE The data presented in this paper offer, for the first time, insight into the complex transcriptional scheme of giant bacteriophages. We show that Pseudomonas aeruginosa giant phage phi KZ is able to infect and lyse its host cell and produce phage progeny in the absence of functional bacterial transcriptional machinery. This unique property can be attributed to two phage-encoded putative RNAP enzymes, which contain very distant homologues of bacterial beta and beta'-like RNAP subunits.