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Molecular evidences favouring step-wise evolution of Mozambique Vibrio cholerae O1 El Tor hybrid strain

¹ Indian Institute of Chemical Biology, Kolkata, India;
² National Institute of Cholera and Enteric Diseases, Kolkata, India

The ctxAB operon coding cholera toxin (CT) in Vibrio cholerae, is carried by the genome of a filamentous phage CTX. Usually, specific CTX infects each of the two important biotypes, classical and El Tor, of epidemic V. cholerae strains belonging to serogroup O1, which are called CTX^class and CTX^ET, respectively. However, an unusual hybrid El Tor strain carrying CTX^class caused cholera epidemic in Mozambique in 2004. To understand its evolution, we have further analyzed some representative hybrid El Tor strains isolated in Kolkata, India in 1992 and the results indicate that both the Mozambique and Indian strains are infected with a unique CTX^class having only 4 copies of tandem heptamer repeat sequence, 5'-TTTTGAT-3', present in the ctxAB promoter (P_ctxAB) region, like in CTX^ET. Usually the (P_ctxAB)of classical biotype contains 7-8 copies of the heptamer sequences. However, sequence analyses of the (P_ctxAB)regions of several classical strains indicated that the copy number of heptamer repeat sequences might vary from 4-8 copies, which was previously unknown. Since the hybrid strains analyzed in this study carry 4 copies of the heptamer sequences it may thus serve as a marker to trace the strain in future. Interestingly, while the Mozambique strain is devoid of El Tor specific free RS1 element or pre-CTX prophage, the Indian hybrid strains carried such elements. The free RS1 has been mapped, cloned and sequenced. Like pre-CTX and CTX prophages, multiple copies of free RS1 elements were found to be integrated in the large chromosomal dif site. Since Indian hybrid El Tor strains carry either free RS1 or pre-CTX prophage in their large chromosomes, it may be possible that the Mozambique hybrid El Tor strain has evolved from these progenitor strains by step-wise deletion of CTX genetic elements from their large chromosomes.

³ E-mail: rupakbhadra{at}iicb.res.in