THE DUAL ROLE OF APL IN PROPHAGE INDUCTION OF COLIPHAGE-186

In the present study we show that the Apl protein of the temperate col iphage 186 combines, in one protein, the activities of the coliphage l ambda proteins Cro and Xis, We have shown previously that Apl represse s both the lysogenic promoter, P-L, and the major lytic promoter, P-R, and is required for excision of the prophage. Apl binds at two locati ons on the phage chromosome, i.e. between P-R and P-L and at the phage -attachment site, Using an in vivo recombination assay, we now show th at the role of Apl in excision is in the process itself and is not sim ply a consequence of repression of P-R or F-L. To study the repressive role of Apl at the switch promoters we isolated Apl-resistant operato r mutants and used them to demonstrate a requirement for Apl in the,ef ficient derepression of the lysogenic promoter during prophage inducti on. We conclude that Apl is both an excisionase and transcriptional re pressor.