THE VIBRIO-CHOLERAE GENOME CONTAINS 2 UNIQUE CIRCULAR CHROMOSOMES

Vibrio cholerae, the etiologic agent of the diarrheal disease cholera, is a Gram-negative bacterium that belongs to the gamma subdivision of the family Proteobacteriaceae. The physical map of the genome has bee n reported, and the genome has been described as a single 3.2-Mb chrom osome [Majumder, R., et al. (1996) J. Bacteriol. 178, 1105-1112]. By u sing pulsed-field gel electrophoresis of genomic DNA immobilized in ag arose plugs and digested with the restriction enzymes I-CeuI, SfiI, an d NotI, we have also constructed the physical map of V. cholerae. Our analysis estimates the size of the genome at 4.0 Mb, 25% larger than t he physical map reported by others. Our most notable finding is, howev er, that the V. cholerae chromosome appears to be not the single chrom osome reported but two unique and separate circular megareplicons.