Distinct Replication Requirements for the Two Vibrio cholerae Chromosomes

Studies of prokaryotic chromosome replication have focused almost exclusively on organisms with one chromosome. We defined and characterized the origins of replication of the two Vibrio cholerae chromosomes, oriCI(vc) and oriCII(vc). OriCII(vc) differs from the origin assigned by bioinformatic analysis and is unrelated to oriCI(vc). OriCII(vc)-based replication requires an internal 12 base pair repeat and two hypothetical genes that flank oriCII(vc). One of these genes is conserved among diverse genera of the family Vibrionaceae and encodes an origin binding protein. The other gene codes for an RNA and not a protein. OriCII(vc)- but not oriCI(vc)-based replication is negatively regulated by a DNA sequence adjacent to oriCII(vc). There is an unprecedented requirement for DNA adenine methyltransferase in both oriCI(vc)- and oriCII(vc)-based replication. Our studies of replication in V. cholerae indicate that microorganisms having multiple chromosomes may utilize unique mechanisms for the control of replication.