Comparison of Delta relA strains of Escherichia coli and Salmonella enterica serovar typhimurium suggests a role for ppGpp in attenuation regulation of branched-chain amino acid biosynthesis

The growth recovery of Escherichia coli K-12, and Salmonella enterica serov ar Typhimurium Delta relA mutants were compared after nutritional downshift s requiring derepression of the branched-chain amino acid pathways.. Becaus e wild-type E. coli K-12 and S. enterica serovar Typhimurium LT2 strains ar e defective in the expression of the genes encoding the branch point acetoh ydroxy acid synthetase II (ilvGM) and III (ilvIH) isozymes, respectively, D elta relA derivatives corrected for these mutations were also examined. Res ults indicate that reduced expression of the known global regulatory factor s involved in branched-chain amino acid biosynthesis cannot completely expl ain the observed growth recovery defects of the Delta relA strains. In the E. coli K-12 MG1655 Delta relA background, correction of the preexisting rp h-1 allele which causes pyrimidine limitations resulted in complete loss of growth recovery. S. enterica serovar Typhimurium LT2 Delta relA strains, w ere fully complemented by elevated basal ppGpp levels in an S. enterica ser ovar Typhimurium LT2 Delta relA spoT1 mutant or in a strain harboring an RN A polymerase mutation conferring a reduced RNA chain elongation rate. The r esults are best explained by a dependence on the basal levels of ppGpp, whi ch are determined by relA-dependent changes in tRNA synthesis resulting fro m amino acid starvations. Expression of the branched-chain amino acid opero ns is suggested to require changes in the RNA chain elongation rate of the RNA polymerase, which can be achieved either by elevation of the basal ppGp p levels or, in the case of the E. coli K-12 MG1655 strain, through pyrimid ine limitations which partially compensate for reduced ppGpp levels. Roles for ppGpp in branched-chain amino acid biosynthesis are discussed in terms of effects on the synthesis, of known global regulatory proteins and curren t models for the control, of global RNA synthesis by ppGpp.