Exception to the rule: Bacterial Chromosomes with Two Origins of Replication- a Genomic Approach to Determine Origin Functionality

The genetic endowment of most bacteria is encoded in a single chromosome and is inherited by the daughter cells through bi directional replication of the parental DNA from a single origin of replication. However, some bacteria... [ view full abstract ]

The genetic endowment of most bacteria is encoded in a single chromosome and is inherited by the daughter cells through bi directional replication of the parental DNA from a single origin of replication. However, some bacteria have more than one chromosome with independent origins of replication in each chromosome. Among these, Vibrio cholerae, with two chromosomes has served as a model system to study various aspects of chromosome replication, maintenance and faithful partitioning of multipartite genomes. Here, we describe the genomic characterization of V. cholerae strains that are exception to the two-chromosome rule: Naturally occurring Single Chromosome V. cholerae (NSCV) in which the two chromosomes are fused. Whole genome sequence analyses of NSCV1 and NSCV2, revealed that the origins of replication of Chr 1 and Chr 2 and associated genes are intact in the fused chromosome raising the question on how the two origins function without impeding each other’s replication fork. We have examined the activity of the two origins using next generation sequencing based marker frequency analyses.  The implications of our findings with respect to mechanistic aspects of chromosome replication, partitioning and maintenance of single chromosome with multiple origins and the biological, functional and evolutionary significance of multipartite genome architecture in general will be discussed.