RNA enzymes are postulated to have catalysed all chemical reactions in the earliest living cells. This idea is now investigated in a search for alkyl transferases from a pool of random sequence RNAs. Selection for self-biotinylation yields a transfer RNA-like ribozyme that efficiently catalyses carbon-nitrogen bond formation. Ribozymes can thus promote reactions other than those involving the RNA sugar-phosphate backbone, suggesting that RNA may be capable of a broad range of catalytic activities.