Comparison of 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 serovar Typhimurium relA mutants were compared after nutritional downshifts requiring derepression of the branched-chain amino acid pathways. Because wild-type E. coli K-12 and S. enterica serovar Typhimurium LT2 strains are defective in the expression of the genes encoding the branch point acetohydroxy acid synthetase II (ilvGM) and III (ilvIH) isozymes, respectively, relA derivatives corrected for these mutations were also examined. Results indicate that reduced expression of the known global regulatory factors involved in branched-chain amino acid biosynthesis cannot completely explain the observed growth recovery defects of the relA strains. In the E. coli K-12 MG1655 relA background, correction of the preexisting rph-1 allele which causes pyrimidine limitations resulted in complete loss of growth recovery. S. enterica serovar Typhimurium LT2 relA strains were fully complemented by elevated basal ppGpp levels in an S. enterica serovar Typhimurium LT2 relA spoT1 mutant or in a strain harboring an RNA polymerase mutation conferring a reduced RNA chain elongation rate. The results are best explained by a dependence on the basal levels of ppGpp, which are determined by relAdependent changes in tRNA synthesis resulting from amino acid starvations. Expression of the branched-chain amino acid operons 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 ppGpp levels or, in the case of the E. coli K-12 MG1655 strain, through pyrimidine 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 current models for the control of global RNA synthesis by ppGpp.