IDENTIFICATION OF AN HP1 PHAGE PROTEIN REQUIRED FOR SITE-SPECIFIC EXCISION

Transposon insertion mutagenesis and transformation were used to locat e genes responsible for excision in the temperate phage HP1 of Haemoph ilus influenzae. A 6.5 kb segment of DNA near the left end of the phag e genome was sequenced, and 11 new open reading frames were identified . Two face-to-face overlapping promoter sequences organized these open reading frames into two operons transcribed in opposite directions. I nterruption of the first open reading frame in the rightward operon cr eated lysogens unable to produce phages. Provision of the uninterrupte d open reading frame in trans restored phage production. The gene iden tified by this procedure, cox, was cloned and the protein product was expressed at high levels in Escherichia coli. The Cox protein is a 79- residue basic protein with a predicted strong helix-turn-helix DNA-bin ding motif. Extracts induced to express high levels of Cox contained a 9 kDa protein. These extracts inhibited integrative recombination and were required for excisive recombination mediated by HP1 integrase. T he HP1 cox gene location is similar to that of the homologous excisive and regulatory genes from coliphages P2 and 186. These phages appear tb share a distinctive organization of recombination proteins and tran scriptional domains differing markedly from phage lambda and its relat ives.