IDENTIFICATION OF THE ACTIVE-SITE HISTIDINE IN THE CORRINOID PROTEIN MTRA OF THE ENERGY-CONSERVING METHYLTRANSFERASE COMPLEX FROM METHANOBACTERIUM-THERMOAUTOTROPHICUM

The energy-conserving corrinoid-containing MtrA-H complex from A Metha nobacterium thermoautotrophicum is composed of eight different subunit s of which MtrA harbors the corrinoid prosthetic group. EPR spectrosco pic evidence has recently been presented for a histidine residue as a cobalt ligand of the cobamide [Harms, U. & Thauer, R. K. (1996a) fur. J. Biochem. 241, 149-154]. This active site histidine was now identifi ed by site-directed mutagenesis to be His84 in the MtrA sequence that contains three histidines. This result was substantiated by sequence c omparison of MtrA from M. thermoautotrophicum, Methanococcus jannaschi i, and Methanopyrus kandleri and of MtxA from Methanosarcina barkeri s howing 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.