The dual role of Apl in prophage induction of coliphage 186

In the present study we show that the Apl protein of the temperate coliphage 186 combines, in one protein, the activities of the coliphage lambda proteins Cro and Xis. We have shown previously that Apl represses both the lysogenic promoter, pL, and the major lytic promoter, pR, and is required for excision of the prophage. Apl binds at two locations on the phage chromosome, i.e. between pR and pL and at the phage-attachment site. Using an in vivo recombination assay, we now show that the role of Apl in excision is in the process itself and is not simply a consequence of repression of pR or pL. To study the repressive role of Apl at the switch promoters we isolated Apl-resistant operator mutants and used them to demonstrate a requirement for Apl in the efficient derepression of the lysogenic promoter during prophage induction. We conclude that Apl is both an excisionase and transcriptional repressor.