A direct sensor of O2, the Dos protein, has been found in Escherichia coli. Previously, the only biological sensors known to respond to O2 by direct and reversible binding were the FixL proteins of Rhizobia. A heme- binding region in Dos is 60% homologous to the O2-sensing PAS domain of the FixL protein, but the remainder of Dos does not resemble FixL. Specifically, the C-terminal domain of Dos, presumed to be a regulatory partner that couples to its heme-binding domain, is not a histidine kinase but more closely resembles a phosphodiesterase. The absorption spectra of Dos indicate that both axial positions of the heme iron are coordinated to side chains of the protein. Nevertheless, O2 and CO bind to Dos with K(d) values of 13 and 10μM, respectively, indicating a strong discrimination against CO binding. Association rate constants for binding of O2 (3 mM-1 s-1), CO (1 mM-1 s-1) and even NO (2 mM-1 s-1) are extraordinarily low and very similar. Displacement of an endogenous ligand, probably Met 95, from the heme iron in Dos triggers a conformational change that alters the activity of the enzymatic domain. This sensing mechanism differs from that of FixL but resembles that of the CO sensor CooA of Rhodospirillum rubrum. Overall the results provide evidence for a heme-binding subgroup of PAS-domain proteins whose working range, signaling mechanisms, and regulatory partners can vary considerably.
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