Propriétés des ribosomes et du RNA synthétisés par Escherichia coli cultivé en présence d'éthionine

Escherichia coli EA1 (met − , bio − , RC str ) grown in the absence of methionine but in the presence of ethionine synthesises ribosomes containing ethionine in their proteins, and whose RNA is sub‐methylated. When the sedimentation behaviour of such ribosomes is examined in the presence of low concentrations (100 μM) of Mg ++ they are found to consist mainly of 50 S and 30 S particles which resemble normal ribosomes, but to contain some particles which sediment between 30 S and 50 S. Furthermore, the ratio of 30 S to 50 S particles in the ethionine ribosomes is higher than that observed for normal ribosomes. These modifications, in the synthesis of ribosomes seem to result more from the slowing of bacterial growth in the presence of ethionine than from a specific inhibition of the formation of 50 S particles. 50 S and 30 S subunits formed in the presence of ethionine are defective in the capacity to associate at high Mg ++ concentration (10 mM) to form 70 S particles. Reconstruction experiments suggest that this defect is inherent in the structure of ribosomes synthesised in the presence of ethionine. When bacteria, cultured in the presence of ethionine, are sub‐cultured in the presence of methionine, or in the presence of methionine and chloroamphenicol their defective ethionine ribosomes regain normal association properties without loss of incorporated ethionine. These experiments led us to study the level of methylability of the RNA of these ribosomes. Using an in vitro system, with an S 100 as a source of methylases, we have shown that the RNA, isolated from ethionine ribosomes, accepts methyl groups from S ‐adenosyl‐[ Me ‐ 14 C]‐methionine while normal ribosomal RNA or RNA isolated from bacteria grown successively in the presence of ethionine and methionine does not. Moreover, we have shown that ethionine ribosomes can be methylated in vitro to a level corresponding to 5–6 methyl groups per 70 S particle. When in vitro methylated ethionine ribosomes are analysed in sucrose gradients the methyl groups are found to be associated both with particles which sediment approximately at 30 S and 50 S and with particles which sediment between 30 S and 50 S. These results suggest that the defect in the association properties of ethionine ribosomes results from the submethylation of their RNA moieties. Preparations of ethionine ribosomes have been found to contain methylases since methylation of these particles occurs when external methylases (S 100) are omitted.