FixL's are chimeric heme protein kinases from symbiotic nitrogen-fixing Rhizobia. We have overexpressed three FixL variants in Escherichia coli. Bradyrhizobium japonicum FixL, a soluble dimeric protein, is the first full-length FixL to be purified. The other two proteins are soluble truncations of Rhizobium meliloti FixL, which is a membrane protein. One contains both heme and kinase domains and is dimeric; the other has only the heme domain and is monomeric. We find that all the FixL's bind oxygen and carbon monoxide non-cooperatively, with very low affinities due entirely to slow association rates. FixL P50's for oxygen are 17-76 mmHg. FixL's may sense nitric oxide and carbon monoxide in addition to oxygen, especially at the low oxygen pressures encountered in vivo. Autoxidation rates are about 50 times faster than that of sperm whale myoglobin. The carbon monoxide affinity of FixL's is about 300 times lower than that of myoglobin, resulting in the unusually low values of 7.5-17 for the partition constant, M = P50(O2)/P50(CO), between carbon monoxide and oxygen. Met-FixL's have their Soret absorption maximum at 395 nm instead of the typical 408 nm and a steep hydroxymet transition at pH > or = 9.3; these properties indicate a pentacoordinated high-spin ferric heme and suggest a sterically hindered hydrophobic heme pocket lacking a distal (E7) histidine. FixL is the first member of a new class of heme proteins, the heme-based sensors, distinct from the oxygen carriers and electron transporters. We expect that some of the novel properties of FixL will be characteristic of the class.