Specifically methylates position 8 of adenine 2503 in 23S rRNA. Confers resistance to some classes of antibiotics, such as chloramphenicol, florfenicol, clindamycin and linezolid. 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. In details, the methyl group from one SAM molecule is initially transferred to Cys-338 in a typical SN2 displacement. Then a 5'-dA radical formed from a second molecule of SAM abstracts a hydrogen from the methyl group of mCys-338, and the resulting Cys-appended methyl radical attacks the substrate adenine ring. Methyl transfer to C8 thus results in a covalent adduct between the substrate and Cys-338, which is resolved by formation of a disulfide bond between Cys-338 and a second conserved Cys residue (Cys-105) (Probable). Belongs to the radical SAM superfamily. RlmN family. Cfr subfamily.