Polyamines and glutamate decarboxylase-based acid resistance in Escherichia coli

The expression of gadA and gadB, which encode two glutamate decarboxylases ( GADs) of Escherichia coli, is induced by an acidic environment and participate in acid resistance. In this study, we constructed a polyamine-deficient mutant and investigated the role of polyamines in acid resistance. The expression of gadA and gadB was shown to be dependent on polyamines. For that reason, the polyamine- deficient mutant was completely devoid of GAD activity and was very susceptible to low pH if large amounts of polyamines were not provided. We also showed that the polyamine- deficient mutant contained higher cAMP levels than the isogenic polyamine- proficient wild type, and cAMP negatively regulated the expression of gadA and gadB. Therefore, introduction of the cya ( encoding adenylate cyclase) mutation allele into the polyamine- deficient mutant resulted in the increment of GAD activity and thus restored the reduced acid resistance of the mutant. The positive regulators, H- NS ( histone- like protein, encoded by the hns gene) and RpoS ( alternative RNA polymerase sigma subunit, encoded by rpoS gene), also significantly governed the expression of gadA and gadB, respectively. However, polyamines did not regulate either the intracellular H- NS level or rpoS expression under these culture conditions. These results strongly suggest that there are at least two different regulatory systems in acid resistance, one is positive regulation via a H- NS/ RpoS system and the other is negative regulation via a polyamine/ cAMP system.