Activation of the glnA, glnK, and nac promoters as Escherichia coli undergoes the transition from nitrogen excess growth to nitrogen starvation

The nitrogen-regulated genes and operons of the Ntr regulon of Escherichia coli are activated by the enhancer-binding transcriptional activator NRIsimilar toP (NtrCsimilar toP). Here, we examined the activation of the glnA, glnK, and nac promoters as cells undergo the transition from growth on ammonia to nitrogen starvation and examined the amplification of NRI during this transition. The results indicate that the concentration of NRI is increased as cells become starved for ammonia, concurrent with the activation of Ntr genes that have less-efficient enhancers than does glnA. A diauxic growth pattern was obtained when E, coli was grown on a low concentration of ammonia in combination with arginine as a nitrogen source, consistent with the hypothesis that Ntr genes other than glnA become activated only upon amplification of the NRI concentration.