Max-Planck-Institut für terrestrische Mikrobiologie and Laboratorium für Mikrobiologie des Fachbereichs Biologie der Philipps-Universität, Marburg, Germany.
The energy-conserving corrinoid-containing MtrA-H complex from Methanobacterium thermoautotrophicum is composed of eight different subunits of which MtrA harbors the corrinoid prosthetic group. EPR spectroscopic evidence has recently been presented for a histidine residue as a cobalt ligand of the cobamide [Harms, U. & Thauer, R. K. (1996a) Eur. J. Biochem. 241, 149-154]. This active site histidine was now identified by site-directed mutagenesis to be His84 in the MtrA sequence that contains three histidines. This result was substantiated by sequence comparison of MtrA from M. thermoautotrophicum, Methanococcus jannaschii, and Methanopyrus kandleri and of MtxA from Methanosarcina harkeri showing that only His84 is conserved. For comparison, the DNA sequences of the mtrEDCBAGH operon in M. kandleri and of the mtxXAH operon in M. barkeri were determined.