Gene responsible for superinfection exclusion of heteroimmune corynebacteriophage

Wild-type beta and gamma corynebacteriophages are heteroimmune and infect lysogens of each other productively. Unlike their wild-type counterparts, the bin mutants of each phage are excluded in lysogens carrying the heteroimmune phage. The wild-type phages overcome exclusion by means of the bin gene product which appears to act as an antirepressor. When repression is lifted, exclusion of bin mutants is abolished (N. Groman and M. Rabin, J. Virol. 28:28-33, 1978; J. Virol. 36:526-532, 1980). It has not been clear whether the excluding compound is the immune repressor itself or one whose synthesis is positively regulated by repressor. We have isolated beta exclusion mutants (xcl) that as prophage exhibited normal immune repression but no longer excluded gamma-bin mutants. Furthermore, we have shown that an xcl phage with an active immune repressor acted in trans to continue the positive regulation of exclusion by a second xcl+ prophage whose immune repressor was inactivated. From these results it was concluded that there is a gene distinct from the imm gene which is directly or indirectly responsible for exclusion. The xcl gene, mapped in prophage crosses, was located between imm and bin, that is, in the regulatory region of the phage genome. The simplest hypothesis compatible with the established observations is that beta immune repressor regulates the expression of the xcl and bin genes, the former positively and the latter negatively. It is likely that an analogous regulatory model applies to gamma phage since it has already been shown that both beta and gamma have bin alleles.