Specifically methylates position 8 of adenine 2503 in 23S rRNA. Can also methylate position 2 of A2503 after the primary methylation at position 8 is complete, to form 2,8-dimethyladenosine; however, C8 is its preferred target. Confers resistance to several classes of antibiotics such as phenicols, lincosamides, oxazolidinones, pleuromutilins, and streptogramin A. The antibiotic resistance conferred by Cfr is provided by methylation at the 8 position and is independent of methylation at the 2 position of A2503. adenosine(2503) in 23S rRNA + 2 reduced [2Fe-2S]-[ferredoxin] + 2 S-adenosyl-L-methionine = 5'-deoxyadenosine + 8-methyladenosine(2503) in 23S rRNA + L-methionine + 2 oxidized [2Fe-2S]-[ferredoxin] + S-adenosyl-L-homocysteine Binds 1 [4Fe-4S] cluster. The cluster is coordinated with 3 cysteines and an exchangeable S-adenosyl-L-methionine. Reaction proceeds by a ping-pong mechanism involving intermediate methylation of a conserved cysteine residue. Belongs to the radical SAM superfamily. RlmN family. Cfr subfamily.