A naturally occurring point mutation in the 13-mer R repeat affects the oriC function of the large chromosome of Vibrio cholerae O1 classical biotype.

The genome of Vibrio cholerae consists of two circular chromosomes of different sizes. Here, a comparative analysis of the replication origins of the large chromosomes (oriCIvc) of classical and El Torbio types of the pathogen is reported. Extensive nucleotide sequence analyses revealed that the oriCIvc region has six DnaA boxes instead of the five found in Escherichia coli oriC. The additional DnaA box, designated Rv, was unique in V. cholerae as well as in other members of the family Vibrionaceae. However, Rv was not found to be essential for the autonomous replication function of the 307-bp oriCIvc minimal region. In contrast to El Tor and the recently evolved V. cholerae 0139 strains, the oriCIvc region of the classical biotype showed only a single base transition (T-->G) in a highly conserved AT-rich 13-mer R repeat region. From the minichromosome copy number and its transformational efficiency analyses, it appears that the single base substitution in the oriCIvc of the classical biotype has a significant effect on its replication initiation.