Molecular mechanism by which the nucleoid occlusion factor, SImA, keeps cytokinesis in check

In Escherichia coli, cytokinesis is orchestrated by FtsZ, which forms a Z-ring to drive septation. Spatial and temporal control of Z-ring formation is achieved by the Min and nucleoid occlusion (NO) systems. Unlike the well-studied Min system, less is known about the anti-DNA guillotining NO process. Here, we describe studies addressing the molecular mechanism of SImA (synthetic lethal with a defective Min system)-mediated NO. SImA contains a TetR-like DNA-binding fold, and chromatin immunoprecipitation analyses show that SImA-binding sites are dispersed on the chromosome except the Ter region, which segregates immediately before septation. SImA binds DNA and FtsZ simultaneously, and the SImA-FtsZ structure reveals that two FtsZ molecules sandwich a SImA dimer. In this complex, FtsZ can still bind GTP and form protofilaments, but the separated protofilaments are forced into an anti-parallel arrangement. This suggests that SImA may alter FtsZ polymer assembly. Indeed, electron microscopy data, showing that SImA-DNA disrupts the formation of normal FtsZ polymers and induces distinct spiral structures, supports this. Thus, the combined data reveal how SImA derails Z-ring formation at the correct place and time to effect NO. The EMBO Journal (2011) 30, 154-164. doi:10.1038/emboj.2010.288; Published online 26 November 2010