Phosphodiesterase A1, a regulator of cellulose synthesis in Acetobacter xylinum, is a heme-based sensor

The phosphodiesterase Al protein of Acetobacter xylinum, xPDEA1, is a key regulator of bacterial cellulose synthesis. This phosphodiesterase linearizes cyclic bis(3'-5')diguanylic acid, an allosteric activator of the bacterial cellulose synthase, to the ineffectual pGpG. Here we show that AxPDEA1 contains heme and is regulated by reversible binding of O-2 to the heme. Apo-AxPDEA1 has less than 2% of the phosphodiesterase activity of holo-AxPDEA1, and reconstitution with hemin restores full activity. O-2 regulation is due to deoxyheme being a better activator than oxyheme. AxPDEA1 is homologous to the Escherichia coli direct oxygen sensor protein, EcDos, over its entire length and is homologous to the FixL histidine kinases over only a heme-binding PAS domain. The properties of the heme-binding domain of AxPDEA1 are significantly different from those of other O-2-responsive heme-based sensors. The rate of AxPDEA1 autoxidation (half-life > 12 h) is the slowest observed so far for this type of heme protein fold. The O-2 affinity of AxPDEA1 (K-d similar to 10 muM) is comparable to that of EcDos, but the rate constants for O-2 association (k(on) = 6.6 muM(-1) s(-1)) and dissociation (k(off) = 77 s(-1)) are 2000 times higher. Our results illustrate the versality of signal transduction mechanisms for the heme-PAS class of O-2 sensors and provide the first example of O-2 regulation of a second messenger.