Myxococcus xanthusis a non-pathogenic soil bacterium which moves by means of a poorly understood mechanism known as gliding motility. Directed or social gliding allows cells to move towards nutrients and is driven by the extension of pili at the leading end of the cell. In this study published in Science, Mignot and colleagues show that the chemosensory protein FrzS oscillates between opposite cell poles as bacteria switch their direction of growth.

Directed bacterial motility requires cells to frequently reverse direction. Both the frizzy (Frz) chemosensory system, and pili assembly and disassembly at opposite poles have been proposed to regulate these reversals. To gain further insight into this behaviour, the authors followed the localization of FrzS during gliding motility in FrzS-GFP expressing bacteria.

They see that FrzS-GFP forms clusters at opposite cell poles, but these were notably brighter at the pili-extruding pole. Time lapse analyses showed that as cells switch direction, FrzS fluorescence gradually decreases from the leading edge and increases at the opposite pole, destined to become the new leading edge.

FrzS oscillations were not observed in frz mutants that are unable to reverse their direction of movement. And conversely, in frz mutant strains with an increased reversal frequency — possibly due to an enhancement of Frz signalling — the FrzS oscillation frequency was also increased. These findings suggest that Frz signalling could be modulating FrzS turnover or transport.

To shed some light on this, FrzS dynamics were examined in cells treated with an inhibitor of bacterial protein synthesis, chloramphenicol. Under these conditions FrzS oscillations were unaffected indicating that they can not be attributed to local protein synthesis and degradation at opposite poles. Rather, Fluorescence Recovery After Photobleaching (FRAP) experiments suggest that FrzS actually moves between poles. The trajectory and speed of movement raises the tantalizing possibility that FrzS tracks along cytoskeletal filaments. Switching the location of both pili and FrzS could be an efficient mechanism for bacteria to respond rapidly to changes in the environment. Exactly how Frz signaling regulates FrzS movement during cell reversal remains to be discovered.

Author: Monica Hoyos-Flight