ToxR-independent expression of cholera toxin from the replicative form of CTXphi

The ctxAB operon, which encodes cholera toxin, resides in the genome of CTXphi, a filamentous bacteriophage. Within Vibrio cholerae cells, the CTXphi genome can exist either as a replicating plasmid or as a prophage integrated into the chromosome. Previous work established that ToxR is required for chromosomal ctxAB expression. We have learned that strains harboring the CTXphi replicative form produce cholera toxin under all conditions tested, independently of ToxR. During passage of CTXphi lysogens through the infant mouse intestine, transduction of CTXphi to a recipient strain can be detected, indicating that phage excision and replication occur in vivo. These results suggest that phage induction might provide a novel mechanism for the regulation of cholera toxin production.