SMC proteins in bacteria: condensation motors for chromosome segregation?

SMC proteins are a ubiquitous protein family, present in almost all organisms so far analysed except for a few bacteria. They function in chromosome condensation, segregation, cohesion, and DNA recombination repair in eukaryotes, and can introduce positive writhe into DNA in vitro. SMC proteins and the structurally homologous MukB protein are unusual ATPases that form antiparallel dimers, with long coiled coil segments separating globular ends capable of binding DNA. Recently, SMC proteins have been shown to be essential for chromosome condensation, segregation and cell cycle progression in bacteria. Identification of a suppressor mutation for MukB in topoisomerase I in Escherichia coli suggests that SMC proteins are involved in negative DNA supercoiling in vivo, and by this means organize and compact chromosomes. A model is discussed in which bacterial SMC proteins act after an initial separation of replicated chromosome origins into the future daughter cell, separating sister chromatids by condensing replicated DNA strands within both cell halves. This would be analogous to a pulling of DNA strands into opposite cell halves by a condensation mechanism exerted at two specialised subregions in the cell.