MukBEF on the march: taking over chromosome organization in bacteria?

Smc-ScpAB complexes, also called bacterial condensin, are comprised of highly conserved proteins present in a wide variety of bacteria and archaea. They facilitate bacterial chromosome segregation probably by organizing and/or compacting nascent sister chromosomes. In some γ-proteobacteria however the Smc-ScpAB complex appears to have been replaced by a different SMC complex, called MukBEF. Petrushenko et al., (2011) now show in this issue of Molecular Microbiology that distantly related homologues of MukBEF, called MksBEF, can be found in the genomes of many more species of bacteria (such as Pseudomonas aeruginosa and Bacillus cereus) that seem to be scattered over the phylogenetic tree. The newly identified MksBEF proteins are organized in operons like MukBEF and occur in combination with Smc-ScpAB and/or MukBEF. Mutations in mksBEF in P. aeruginosa result in the formation of anucleate cells whereas its mild overexpression can suppress the accumulation of anucleate cells in smc null mutants. These findings suggest that MksBEF and Smc-ScpAB share their duties in chromosome partitioning in P. aeruginosa and possibly in other bacteria too. Thus, chromosome organization and segregation in bacteria appear to be governed by up to three distinct condensin complexes, indicating that this process could be much more sophisticated than previously appreciated.