Synchronous termination of replication of the two chromosomes is an evolutionary selected feature in Vibrionaceae

Vibrio cholerae, the causative agent of the cholera disease, is commonly used as a model organism for the study of bacteria with multipartite genomes. Its two chromosomes of different sizes initiate their DNA replication at distinct time points in the cell cycle and terminate in synchrony. In this study, the time-delayed start of Chr2 was verified in a synchronized cell population. This replication pattern suggests two possible regulation mechanisms for other Vibrio species with different sized secondary chromosomes: Either all Chr2 start DNA replication with a fixed delay after Chr1 initiation, or the timepoint at which Chr2 initiates varies such that termination of chromosomal replication occurs in synchrony. We investigated these two models and revealed that the two chromosomes of various Vibrionaceae species terminate in synchrony while Chr2-initiation timing relative to Chr1 is variable. Moreover, the sequence and function of the Chr2-triggering crtS site recently discovered in V. cholerae were found to be conserved, explaining the observed timing mechanism. Our results suggest that it is beneficial for bacterial cells with multiple chromosomes to synchronize their replication termination, potentially to optimize chromosome related processes as dimer resolution or segregation.
Author summary Most bacteria encode their genetic information on a single chromosome. The pathogenic bacterium Vibrio cholerae is an exception to this rule and carries two chromosomes of different sizes, each having one origin of replication. A very basic research question is how the replication of the two chromosomes is timed starting from their replication origins. If they start simultaneously, the smaller chromosome would finish replication earlier than the larger chromosome. Interestingly, the timing in V. cholerae is such that the smaller chromosome starts replication after a time delay, resulting in synchronous replication termination of the two chromosomes. Here we answer the question whether it is the termination synchrony which is under evolutionary pressure, or whether a certain duration of the delay between the two chromosomes to start replication is of biological importance. To this end, we analyzed replication in different species of the Vibrionaceae phylogenetic group with differently sized chromosome pairs. Our results indicate that a synchronous termination of the two chromosomes in this group of bacterial species is under evolutionary selection, suggesting it to be potentially important for the process of cell division.