Evolution and Maintenance
of Circular Chromosomes

The organisation of the genome is a key element in the coordination between cell division and the replication and segregation of genetic information (See the Boccard team). In most bacteria, genetic information is distributed on a single chromosome, which simplifies the genome organisation. However, the genetic material is divided on several chromosomes in ~10% of sequenced bacteria. This is notably the case for Vibrio cholerae, the agent of the deadly human diarrheal disease cholera.

Several features motivated the team to use V. cholerae as a model organism. First, V. cholerae belongs to the Vibrionales, which are closely related to Enterobacterales in the phylogenetic tree of bacteria. Correspondingly, V. cholerae shares many cell cycle and genome maintenance machineries with E. coli, possibly the most well studied bacterium (Figure 2). In particular, V. cholerae carries homologues of the E. coli cell division regulatory genes (minCDE, slmA) and co-occurrent genes implicated in the control of replication (seqA), DNA repair (dam), chromosome organisation and segregation (mukBEF and matP) (Brézellec et al., 2006). It suggested that the basic genome management processes were common to both species, as we observed for chromosome dimer resolution (Val et al., 2008). However, the genome of V. cholerae is distributed on two chromosomes, chr1 and chr2. Chr1 derives from the mono-chromosomal ancestor of the Vibrionales and the Enterobacterales, whereas Chr2 derives from a horizontally acquired mega-plasmid that subsequently gained essential genes. The same scenario seems to be at the origin of the evolution of other bacteria with a multipartite genome. Second, V. cholerae is an asymmetric rod. During a division event, each of the two daughter cells inherits one of the pre-existing poles of the mother cell, the “old pole”, and one pole originating from the division of the mother cell, the “new pole”: a polar hub, HubP, addresses the flagellum, chemotaxis machineries and the oriC of chr1 (ori1) to the old pole of the cell (See the Yamaichi team). Third, the evolutionary emergence of the 7th pandemic clone of V. cholerae involved molecular interactions between at least 4 different IMEXs, including the Cholera Toxin Phage, CTXΦ, and a Toxin-Linked Cryptic phage satellite, TLCΦ (Das et al., 2013).