SUBCELLULAR-LOCALIZATION OF BACILLUS-SUBTILIS SMC, A PROTEIN INVOLVEDIN CHROMOSOME CONDENSATION AND SEGREGATION

We have investigated the subcellular localization of the SMC protein i n the gram-positive bacterium Bacillus subtilis. Recent work has shown that SMC is required for chromosome condensation and faithful chromos ome segregation during the B. subtilis cell cycle. Using antibodies ag ainst SMC and fluorescence microscopy, we have shown that SMC is assoc iated with the chromosome but is also present in discrete foci near th e poles of the cell. DNase treatment of permeabilized cells disrupted the association of SMC with the chromosome but not with the polar foci . The use of a truncated sme gene demonstrated that the C-terminal dom ain of the protein is required for chromosomal binding but not for the formation of polar foci. Regular arrays of SMC-containing foci were s till present between nucleoids along the length of aseptate filaments generated by depleting cells of the cell division protein FtsZ, indica ting that the formation of polar foci does not require the formation o f septal structures. In slowly growing cells, which have only one or t wo chromosomes, SMC foci were principally observed early in the cell c ycle, prior to or coincident with chromosome segregation. Cell cycle-d ependent release of stored SMC from polar foci may mediate segregation by condensation of chromosomes.