Involved in fosfomycin biosynthesis (PubMed:17220970, PubMed:24370735, PubMed:28678474). Catalyzes the C-methylation of cytidylyl-2-hydroxyethylphosphonate (HEP-CMP) to form cytidylyl-2-hydroxypropylphosphonate (HPP-CMP). The C-methylation is not stereoselective and the ratio of (S)- to (R)-HPP-CMP is almost equal in vitro (PubMed:28678474). AH2 + cytidine 5'-{[hydroxy(2-hydroxyethyl)phosphonoyl]phosphate} + 2 S-adenosyl-L-methionine = 5'-deoxyadenosine + A + cytidine 5'-{[hydroxy(2-hydroxypropyl)phosphonoyl]phosphate} + 2 H(+) + L-methionine + S-adenosyl-L-homocysteine Binds 1 [4Fe-4S] cluster. The cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. kcat is 0.89 h(-1). Antibiotic biosynthesis; fosfomycin biosynthesis. Disruption mutant does not produce fosfomycin. Was originally thought to catalyze the C-methylation of 2-hydroxyethylphosphonate (2-HEP) to form 2-hydroxypropylphosphonate (2-HPP). Belongs to the radical SAM superfamily.