Radical SAM methyltransferase that is responsible for the C(5)-methylation of 'Arg-285' of the methyl-coenzyme M reductase (MCR) subunit alpha McrA. This post-translational methylation, despite being not essential in vivo, plays a role for the stability and structural integrity of MCR. Binds at least 1 [4Fe-4S] cluster. This cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Protein modification. Cells lacking this gene produce an active MCR lacking the C(5)-methylation of the respective arginine residue. The mutant strain shows impaired growth under stress conditions, such as the presence of 0.2 mM hydrogen peroxide (H2O2) or elevated temperature (42 degrees Celsius). Belongs to the radical SAM superfamily. MCR alpha subunit C-methyltransferase family.