Stringent control of fatty acid synthesis in Escherichia coli. Possible regulation of acetyl coenzyme A carboxylase by ppGpp.

The effect of amino acid starvation on the rate of fatty acid synthesis was examined in stringent (CP 78, ref+) and relaxed (CP 79, rel-) isogenic strains of Escherichia coli (leu-, his-, arg-, fhr-, fhi-). Rates of incorporation of [UJ4C]glucose, [lJ4C]acetate, and 3H,0 into chloroform-methanol-soluble lipids ( >95% phospholipid) were instantly reduced 50 to 60 % by leucine starvation of stringent, but not relaxed, cells. That this depressed rate of lipid labeling was not due to leucine starvation-induced diversion of labeled fatty acyls into chloroform-methanol-insoluble form (e.g. lipopolysaccharide, etc.) is indicated by the fact that the rate of labeling from [lJ4C]acetate of the lipid extract of saponified cells was decreased to the same extent by leucine deprivation. With leucine-deprived rel+ cells, the rate of [1-14C]acetate incorporation into phosphatidylethanolamine and cardiolipin was more drastically curtailed than into phosphatidylglycerol, while the incorporation pattern in reZcells (A leucine) was similar to that of nonstarved reZ+ cells. An elevated turnover rate of fatty acyl groups due to amino acid starvation cannot account for the decreased rate of lipid labeling by various precursors, since no detectable loss of 14C-activity from fatty acyls labeled during growth or leucine deprivation occurs during subsequent growth or leucine starvation in the presence of unlabeled precursor. Only minor amounts of labeled lipid are secreted by stringent or relaxed cells grown in the presence of [1-r4C]acetate; leucine deficiency has no significant effect on the rate of labeling of these extracellular lipids. These results lead to the conclusion that fatty acid synthesis per se is subject to stringent control being partially suppressed, i.e. 50 to 60%, by amino acid starvation in rel+ strains of E. coli. Amino acid starvation of stringent, but not relaxed, strains of E. coli is known to activate ribosomal synthesis of ppGpp and pppGpp. The resultant accumulation of (p)ppGpp appears to mediate stringent control of fatty acid synthesis by blocking the initial committed step of the pathway, i.e. the carboxylation of acetyl-CoA. Of the two catalytic components of the acetyl-CoA carboxylase system, namely biotin carboxylase and carboxyltransferase, only the latter component is inhibited by physiological concentrations (up