Radical SAM methyltransferase that is responsible for the C(5)-methylation of 'Arg-274' 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. Belongs to the radical SAM superfamily. MCR alpha subunit C-methyltransferase family.