Effect of ppGpp on Escherichia coli cyclopropane fatty acid synthesis is mediated through the RpoS sigma factor (sigma(S))

Strains of Escherichia coli carrying mutations at the relA locus are defici ent in cyclopropane fatty acid (CFA) synthesis, a phospholipid modification that occurs as cultures enter stationary phase. RelA protein catalyzes the synthesis of guanosine-3',5'-bisdiphosphate (ppGDD); therefore, pPGpp was a putative direct regulator of CFA synthesis. The nucleotide could act by i ncreasing either the activity or the amount of CFA synthase, the enzyme cat alyzing the lipid modification. We report that the effect of RelA on CFA sy nthesis is indirect. In vitro and in vivo experiments show no direct intera ction between ppGpp and CFA synthase activity. The relA effect is due to Dp Gpp-engendered stimulation of the synthesis of the alternative sigma factor , RpoS, which is required for function of one of the two promoters responsi ble for expression of CFA synthase.