SulA does not sequester FtsZ in Escherichia coli cells during the SOS response.

In Escherichia coli, the SulA protein is synthesized during the SOS response to arrest cell division. Two possible models of SulA action were proposed: the sequestration and the capping. In current paper, to clarify which model better reflects the SulA effect on cell division upon the SOS response, the FtsZ/SulA ratio was estimated inside cells based on fusion of both FtsZ and SulA to fluorescent protein mNeonGreen. This allowed to quantify this ratio by fluorescence microscopy as well as western blotting; moreover, the effect of SulA on FtsZ distribution patterns in cells was analyzed based on fluorescence microscopy images. The SulA concentration in cells under the SOS response was shown to be several times (about 10) lower than that of FtsZ. The effect of SulA was unequal to corresponding decrease in FtsZ concentration. These results are supported by uneven FtsZ distribution in cells under the SOS response. Together the results of current work indicate that the division arrest by SulA protein in E. coli cells could not be explained by the sequestration model.