Molecular mechanism by which the nucleoid occlusion factor, SlmA, 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 SlmA (synthetic lethal with a defective Min system)-mediated NO. SlmA contains a TetR-like DNA-binding fold, and chromatin immunoprecipitation analyses show that SlmA-binding sites are dispersed on the chromosome except the Ter region, which segregates immediately before septation. SlmA binds DNA and FtsZ simultaneously, and the SlmA-FtsZ structure reveals that two FtsZ molecules sandwich a SlmA 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 SlmA may alter FtsZ polymer assembly. Indeed, electron microscopy data, showing that SlmA-DNA disrupts the formation of normal FtsZ polymers and induces distinct spiral structures, supports this. Thus, the combined data reveal how SlmA derails Z-ring formation at the correct place and time to effect NO.     

Studies on freshwater red algae of Malaysia III. development of carposporophytes ofBatrachospermum cayennense Montagne,B. beraense Kumano andB. hypogynum Kumano et Ratnasabapathy

The development of the carposporophytes ofBatrachospermum cayennense, B. beraense andB. hypogynum is observed. The carpogonium bearing branch ofB. cayennense is very long and its cells are differentiated from those of the vegetative fascicles in size and shape. The trichogyne nucleus ofB. cayennense andB. beraense is not recognized. In the above two species it is observed that after contact with the trichogyne the nucleus of the spermatium divides into two daughter nuclei, one of which fuses with the female nucleus, while the other remains in the trichogyne. InB. beraense the fertilized carpogonium is divided transversely and the gonimoblast filaments are produced from the divided carpogonium. The protoplasmic connection between the carpogonium and the hypogynous nutritive cell is especially pronounced inB. hypogynum. Carpospores ofB. hypogynum germinate within the carposporangia and young germlings often throw away original carpospores, hence these young germlings look like gemmae.