The Vibrio cholerae O139 Calcutta bacteriophage CTX phi is infectious and encodes a novel repressor

CTX phi is a lysogenic, filamentous bacteriophage. Its genome includes the genes encoding cholera toxin (ctxAB), one of the principal virulence factor s of Vibrio cholerae; consequently, nonpathogenic strains of V. cholerae ca n be converted into toxigenic strains by CTX phi infection. O139 Calcutta s trains of V. cholerae, which were linked to cholera outbreaks in Calcutta, India, in 1996, are novel pathogenic strains that carry two distinct CTX pr ophages integrated in tandem: CTXET, the prophage previously characterized within El Tor strains, and a new CTX Calcutta prophage (CTXcalc). We found that the CTXcalc prophage gives rise to infectious virions; thus, CTX(ET)ph i is no longer the only known vector for transmission of ctxAB. The most fu nctionally significant differences between the nucleotide sequences of CTX( calc)phi and CTX(ET)phi are located within the phages' repressor genes (rst R(calc) and rstR(ET), respectively) and their RstR operators. RstR(calc) is a novel, allele specific repressor that regulates replication of CTX(calc) phi by inhibiting the activity of the rstA(calc) promoter. RstR(calc) has n o inhibitory effect upon the classical and El Tor rstA promoters, which are instead regulated by their cognate RstRs. Consequently, production of RstR (calc) renders a CTXcalc lysogen immune to superinfection by CTX(calc)phi b ut susceptible (heteroimmune) to infection by CTX(ET)phi. Analysis of the p rophage arrays generated by sequentially integrated CTX phages revealed tha t pathogenic V. cholerae O139 Calcutta probably arose via infection of an O 139 CTX(ET)phi lysogen by CTX(calc)phi.