ABSTRACT
In most bacteria, cell division begins with the polymerization of the GTPase FtsZ at the mid-cell, which recruits the division machinery to initiate cell constriction. In the filamentous bacterium Streptomyces, cell division is positively controlled by SsgB, which recruits FtsZ to the future septum sites and promotes Z-ring formation. Here we show via site-saturated mutagenesis that various amino acid substitutions in the highly conserved SsgB protein result in the production of ectopically placed septa, that sever spores diagonally or along the long axis, perpendicular to the division plane. Ectopic septa were especially prominent when cells expressed SsgB variants with substitutions in residue E120. Biochemical analysis of SsgB variant E120G revealed that its interaction with - and polymerization of - FtsZ had been maintained. The crystal structure of S. coelicolor SsgB was resolved and the position of residue E120 suggests its requirement for maintaining the proper angle of helix α3, thus providing a likely explanation for the aberrant septa formed in SsgB E120 substitution mutants. Taken together, our work presents the first example of longitudinal division in a free living bacterium, which is explained entirely by changes in the FtsZ-recruiting protein SsgB.
