A commonly observed coupling of sensory domains to GGDEF-class diguanylate cyclases and EAL-class phosphodiesterases has long suggested that c-di-GMP synthesizing and degrading enzymes sense environmental signals. Nevertheless, relatively few signal ligands have been identified for these sensors, and even fewer instances of in vitro switching by ligand have been demonstrated. Here we describe an Escherichia coli two-gene operon, dosCP, for control of c-di-GMP by oxygen. In this operon, the gene encoding the oxygen-sensing c-di-GMP phosphodiesterase Ec Dos (here renamed Ec DosP) follows and is translationally coupled to a gene encoding a diguanylate cyclase, here designated DosC. We present the first characterizations of DosC and a detailed study of the ligand-dose response of DosP. Our results show that DosC is a globin-coupled sensor with an apolar but accessible heme pocket that binds oxygen with a K d of 20 μM. The response of DosP activation to increasing oxygen concentration is a complex function of its ligand saturation such that over 80% of the activation occurs in solutions that exceed 30% of air saturation (oxygen >75 μM). Finally, we find that DosP and DosC associate into a functional complex. We conclude that the dosCP operon encodes two oxygen sensors that cooperate in the controlled production and removal of c-di-GMP.
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