FIG 6.

Model for the control of sporulation-specific cell division in Streptomyces. When sporulation starts, SsgA localizes dynamically in young aerial hyphae, while SsgB and FtsZ are still diffuse at this stge. At this point, ParA is constrained to the hyphal tip. During early cell division, SsgA and SsgB colocalize temporarily at either side of the aerial hyphae, with ParA extending downward as filaments along the aerial hypha. ParB complexes are then formed over the uncondensed chromosomes, while FtsZ assembles in spiral-like filaments. Subsequently, FtsZ and SsgB colocalize and stay together until FtsZ disperses, whereby SsgB recruits FtsZ and stimulate its polymerization into protofilaments. The way the SsgB-FtsZ complex is tethered to the membrane in the absence of a membrane domain in either protein is unclear, but a likely role is played by the SepG protein (SCO2078 in S. coelicolor) encoded by a gene upstream of divIVA (L. Zhang, J. Willemse, D. Claessen, and G. P. van Wezel, unpublished data). Z-rings are then formed at the sporulation stage, followed by chromosome condensation and segregation and the production of sporulation septa. SsgA eventually marks the future germination sites. The figure was adapted from references 277 and 316.