Radical S-adenosylmethionine (SAM) enzyme responsible for the first step of the biosynthesis of the enzyme cofactor mycofactocin (MFT). Catalyzes two reactions at the C-terminus of the mycofactocin precursor (the MftA peptide). The first one is the oxidative decarboxylation of the C-terminal L-tyrosine of MftA, forming an unsaturated tyramine moiety (PubMed:27158836). The second reaction is the cross-linking of the tyramine with the penultimate L-valine residue, forming a five-membered lactam ring (By similarity). Its activity requires the presence of the MftB chaperone (PubMed:27158836). Is required for the in vivo ethanol assimilation in M.smegmatis (PubMed:31113891). [mycofactocin precursor peptide]-C-terminal glycyl-L-valyl-L-tyrosine + S-adenosyl-L-methionine = 5'-deoxyadenosine + [mycofactocin precursor peptide]-C-terminal glycyl-N-{[2-(4-hydroxyphenyl)ethenyl]-3-methylbutanamide} + CO2 + L-methionine [mycofactocin precursor peptide]-C-terminal glycyl-N-{[2-(4-hydroxyphenyl)ethenyl]-3-methylbutanamide} + AH2 + S-adenosyl-L-methionine = 5'-deoxyadenosine + [mycofactocin precursor peptide]-C-terminal glycyl-N-{5-[(4-hydroxyphenyl)methyl]-4,4-dimethyl-2-oxopyrrolidin-3-yl}acetamide + A + H(+) + L-methionine Binds 3 [4Fe-4S] clusters. One cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine (Probable). All three [Fe-S] clusters are required for MftC modification of MftA (By similarity). Cells lacking this gene lose the ability to utilize ethanol as the sole growth substrate (PubMed:31113891). Glycosylated mycofactocins are not detected in this mutant (PubMed:33014324). Belongs to the radical SAM superfamily. MftC family.