Abstract
Most naturally competent bacteria tightly regulate the window of the competent state to maximize their ecological fitness under specific conditions. Development of competence by Haemophilus influenzae strain Rd KW20 is stimulated by cAMP and inhibited by purine nucleotides, respectively. In contrast, cAMP inhibits cell growth, but nucleotides are important for KW20 growth. However, the mechanisms underlying the abovementioned reciprocal effects are unclear. Here, we first identified a periplasmic acid phosphatase AphA(Ec) of Escherichia coli as a new cAMP-binding protein. We show cAMP competitively inhibits the phosphatase activities of AphA(Ec) and its homolog protein AphA(Hi) in the KW20 strain. Furthermore, we found cAMP inhibits two other periplasmic nonspecific phosphatases, NadN(Hi) (which provides the essential growth factor V, NAD) and Hel(Hi) (eP4, which converts NADP to NAD) in KW20. We demonstrate cAMP inhibits cell growth rate, especially via NadN(Hi). On the other hand, the inhibitory effect of purine nucleotide AMP on competence was abolished in the triple deletion mutant Δhel(Hi)ΔnadN(Hi)ΔaphA(Hi), but not in the single, double deletion or complemented strains. Adenosine, however, still inhibited the competence of the triple deletion mutant, demonstrating the crucial role of the three phosphatases in converting nucleotides to nucleosides and thus inhibiting KW20 competence. Finally, cAMP restored the competence inhibited by GMP in a dose-dependent manner, but not competence inhibited by guanosine. Altogether, we uncovered these three periplasmic phosphatases as the key players underlying the antagonistic effects of cAMP and purine nucleotides on both cell growth and competence development of H. influenzae.