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Genetic organization of pre-CTX and CTX prophages in the genome of an environmental Vibrio cholerae non-O1, non-O139 strain

Arjun Saha^1,

¹ Infectious Diseases Group, Indian Institute of Chemical Biology, 4, Raja S. C. Mullick Road, Kolkata 700 032, India
² National Institute of Cholera and Enteric Diseases, Beliaghata, Kolkata 700 010, India
³ International Centre for Diarrhoeal Disease Research, Bangladesh, Dhaka 1212, Bangladesh

Correspondence
Rupak K. Bhadra
rupakbhadra{at}iicb.res.in

The cholera toxin (CT) is a critical determinant of the virulence of epidemic Vibrio cholerae strains. The ctxAB operon encoding CT is part of the genome of a filamentous bacteriophage CTX, which may integrate as a single copy or as multiple copies in the genome of V. cholerae. The CTX genome is composed of RS2 (2.4 kb) and core (4.5 kb) regions. In the present study extensive genetic mapping analyses indicated that two copies of tandemly arrayed CTX prophages are integrated in the small chromosome of an environmental V. cholerae strain, VCE232, belonging to serogroup O4. Further mapping revealed that the integration of prophages has occurred in the same genetic locus of the small chromosome of VCE232 as that of V. cholerae O1 biotype El Tor strains. Interestingly, a new type of RS2-like element 3.5 kb in size was found in the CTX prophage genome in the small chromosome of VCE232. Cloning followed by sequencing of the new RS2-like element of VCE232 revealed the presence of three ORFs, which probably encode highly divergent types of phage regulatory proteins. Furthermore, the strain VCE232 also harbours two copies of a tandemly arranged CTX prophage devoid of the ctxAB genes, called pre-CTX prophage, in its large chromosome. The presence of multiple copies of diverse CTX prophages in both the chromosomes of VCE232 suggests that toxigenic environmental V. cholerae non-O1, non-O139 strains could play a role in the emergence of new epidemic clones.

The GenBank/EMBL/DDBJ accession number for the RS2-like sequence of V. cholerae strain VCE232 reported in this paper is DQ288668.

Present address: Department of Developmental Genetics, NICHD, National Institutes of Health, Bethesda, MD 20892, USA.

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