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To determine the role of ppGpp in both negative and positive regulation of transcription initiation during exponential growth in Escherichia coli, we examined transcription in vivoand in vitro from the growth-rate-dependent rRNA promoter rrnB P1 and from the inversely growth-rate-dependent amino acid biosynthesis/transport promoters PargI, PhisG, PlysC, PpheA,PthrABC, and PlivJ. rrnB P1 promoter activity was slightly higher at all growth-rates in strains unable to synthesize ppGpp (ΔrelAΔspoT) than in wild-type strains. Consistent with this observation and with the large decrease in rRNA transcription during the stringent response (when ppGpp levels are much higher), ppGpp inhibited transcription from rrnB P1 in vitro. In contrast, amino acid promoter activity was considerably lower in ΔrelAΔspoT strains than in wild-type strains, but ppGpp had no effect on amino acid promoter activity in vitro. Detailed kinetic …