Abstract
To proliferate efficiently, cells must co-ordinate division with chromosome segregation. In Bacillus subtilis, the nucleoid occlusion protein Noc binds to specific DNA sequences (NBSs) scattered around the chromosome and helps to protect genomic integrity by coupling the initiation of division to the progression of chromosome replication and segregation. However, how it inhibits division has remained unclear. Here, we demonstrate that Noc associates with the cell membrane via an N-terminal amphipathic helix, which is necessary for function. Importantly, the membrane-binding affinity of this helix is weak and requires the assembly of nucleoprotein complexes, thus establishing a mechanism for DNA-dependent activation of Noc. Furthermore, division inhibition by Noc requires recruitment of NBS DNA to the cell membrane and is dependent on its ability to bind DNA and membrane simultaneously. Indeed, Noc production in a heterologous system is sufficient for recruitment of chromosomal DNA to the membrane. Our results suggest a simple model in which the formation of large membrane-associated nucleoprotein complexes physically occludes assembly of the division machinery.
Highlights
Division site selection is a widespread biological problem
We show that simultaneous binding to DNA and the membrane is necessary for Noc function, providing evidence that the mechanism by which Noc acts requires recruitment of DNA to the bacterial cell membrane
Despite extensive analysis, we found no evidence to support a direct interaction between Noc and FtsZ or any other known division protein (Wu et al, 2009; Adams et al, 2011; Wu & Errington, 2012)
Summary
Division site selection is a widespread biological problem. Geometry-sensing mechanisms that link division to cellsize or chromosome segregation appear to represent a convenient solution to this problem and are found in single-celled organisms from bacteria to yeast (Moseley & Nurse, 2010). In the rod-shaped bacteria Bacillus subtilis and Escherichia coli, which represent the best-studied Gram-positive and Gram-negative model organisms, division site selection is primarily controlled by two negative regulatory systems, Min and nucleoid occlusion. These systems act by using the cell poles and the nucleoid (bacterial “chromosome”), respectively, as geometric cues. The combined action of these two overlapping systems helps to ensure that Z-ring assembly only occurs efficiently at mid-cell (Rodrigues & Harry, 2012)
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