Modulation of RNA polymerase specificity by ppGpp.

ppGpp alters the initiation specificity of RNA polymerase holoenzyme in vitro in a direction which mimics the stringent response in vivo. The transition temperature for opening rRNA promoters is increased by the nucleotide, that for opening phi80 promoters is unaffected. This implies that RNA polymerase can discriminate between different types of promoter. ppGpp may act by effecting a structural change in the enzyme.