When food gets scarce, cannibalism is always an option. In times of famine, soil-dwelling Bacillus subtilis bacteria (pictured) shut down, forming tough spores that can lie dormant for years. But before that, they stock up on nutrients by releasing a toxin that kills off their neighbouring siblings and then feed off the remains of the dead cells. But how do the toxin-producing bugs shield themselves from the poison's deadly effects? Craig Ellermeier et al. (Cell 124, 549–559; 2006) have unravelled the surprisingly simple mechanism that protects the toxin-producers.
When nutrients are in short supply, a protein called SpoOA is activated to coordinate the cells' response. This regulator seems to be controlled by a bistable genetic switch such that it is turned on in only about half of the cells in a bacterial population. The activated SpoOA triggers manufacture of the toxin (SdpC) and an ‘immunity’ protein.
With some nifty genetics, Ellermeier et al. have identified the immunity protein and worked out how it is regulated. The key to the process is the fact that the immunity protein is also a signal-transduction protein that controls its own synthesis. As toxin accumulates outside the cell, the immunity protein — which lies in the cell membrane — mops it up, preventing the cell from being killed.
At the same time, the toxin works with the immunity protein to grab hold of a repressor protein in the cell and tether it to the cell membrane. The repressor normally inhibits the gene that encodes the immunity protein, but it cannot function when bound to the membrane. So the cell keeps pumping out more immunity protein. When all the toxin is mopped up, free repressor starts to accumulate, shutting down further synthesis of the immunity protein. Thus, the cell produces only as much immunity protein as it needs.