Phosphodiesterase A1, a Regulator of Cellulose Synthesis in Acetobacter xylinum, Is a Heme-Based Sensor

The phosphodiesterase A1 protein of Acetobacter xylinum, AxPDEA1, 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 O2 to the heme. Apo-AxPDEA1 has less than 2% of the phosphodiesterase activity of holo-AxPDEA1, and reconstitution with hemin restores full activity. O2 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 O2-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 O2 affinity of AxPDEA1 (K d ∼ 10 μM) is comparable to that of EcDos, but the rate constants for O2 association (k on = 6.6 μM-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 O2 sensors and provide the first example of O2 regulation of a second messenger.