(p)ppGpp buffers cell division when membrane fluidity decreases in Escherichia coli.

Maintenance of fluidity an inherent property of biological membranes by homeoviscous adaptation is important for optimal functioning of membrane-associated processes. Homeoviscous adaptation in E. coli involves an increase in the concentration of unsaturated fatty acid, cis-vaccenic acid (18:1) with decrease in temperature and vice versa. Lowering unsaturated fatty acid synthesis by inactivation of FadR reduced the proportion of unsaturated fatty acids in the membrane. In this study we show that when the proportion of unsaturated fatty acids in the membrane was reduced, cell division was dependent on the guanine nucleotide analogous (p)ppGpp. Combined expression of cell division genes ftsQ, ftsA and ftsZ from plasmid rescued the growth defect that was associated with cell filamentation followed by lysis. To our knowledge, this is the first report of (p)ppGpp mediated regulation needed for the adaptation to membrane fluidity loss in bacteria.