Specifically methylates position 8 of adenine 2503 in 23S rRNA. Confers resistance to some classes of antibiotics. 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.