The occurrence of methylated proteins in the ribosomes of Saccharomyces cerevisiue was investigated by tracing the transfer of radioactive methyl groups from S-adenosyl methionine, taken up by growing cells, into the protein moiety of ribosomes. It was estimated that the large subunit contained about 10 protein-bound methyl groups distributed mainly among proteins YL23, YL32 and YL1. The small subunit contained at most 2 -4 methyl groups in proteins. Methyl groups could be transferred in vifro to proteins YL23 and YL32 in extracts from cultures of an Sadenosyl methionine auxotroph deprived of the methyl-group donor. In the most heavily methylated proteins the methylated amino acids formed in vitro were the same as those found in vivo (monomethyllysine and dimethyllysine in YL32; dimethyl and trimethyllysine in YL23). It is concluded that the enzymatic reaction in vitro faithfully saturates with methyl groups the target amino acids which are normally fully methylated in zjiz>o.

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