Development of giant bacteriophage φKZ is independent of the host transcription apparatus

Pseudomonas aeruginosa bacteriophage φKZ 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 ∼280-kb φKZ genome to single-nucleotide resolution, we combine 369 φKZ genes into 134 operons. Early transcription is initiated from highly conserved AT-rich promoters distributed across the φKZ 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 φKZ is its ability to complete its infection in the absence of bacterial RNA polymerase (RNAP) enzyme activity. We propose that transcription of the φKZ genome is performed by the consecutive action of two φKZ-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.