A naturally occurring point mutation in the 13-mer R repeat affects the oriC function of the large chromosome of Vibrio cholerae 01 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 (oriCI(VC)) of classical and El Tor biotypes of the pathogen is reported. Extensive nucleotide sequence analyses revealed that the oriCI(VC) region has six DnaA boxes instead of the five found in Escherichia coli oriC. The additional DnaA box, designated R-V, was unique in V. cholerae as well as in other members of the family Vibrionaceae. However, R-V was not found to be essential for the autonomous replication function of the 307-bp oriCI(VC) minimal region. In contrast to El Tor and the recently evolved V. cholerae O139 strains, the oriCI(VC) 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 oriCI(VC) of the classical biotype has a significant effect on its replication initiation.