Split-operon control of a prophage gene.

Both prophage integration in bacteriophage P2 and the reverse event, prophage excision, are known to require a specific phage gene product, the so-called int function. We find that P2 can integrate efficiently at a free attachment site also in an immune host (i.e., in the presence of phage specific repressor) provided the superinfecting phage is not deficient in int function. Prophage P2, on the other hand, is not excised from the host chromosome even in a derepressed lysogen unless int function is supplied by a superinfecting phage. Thus, the int function of P2 is expressed constitutively by the superinfecting phage, but is not expressed by the prophage even in the absence of phage repressor. It is proposed that the int function of P2 is not controlled by phage repressor, but belongs instead to a constitutive operon that is physically disrupted by prophage integration.