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

Two isogenic derivatives of Escherichia coli K12: strains EA 1 (Met^-, Biot^-, RC^str and EA 2 (Mei^-, Biot^-, RC^rel) when grown in the presence of ethionine in place of methionine, synthesize ribosomes whose RNA is submethylated and whose proteins contain ethionine. Net ribosome synthesis in the presence of ethionine is greater in the RC^rel strain than in the RC^str strain. The precedinig paper describes results obtained with the latter strain. Here we present further studies carried out with the isogenic RC^rel strain EA 2 As in the case of the RC^str strain (previous paper) ribosomes synthesized by the RC^rel strain grown in the presence of ethionine display defective association properties. A large fraction of these "ethionine" ribosomes is found as free subunits in crude extracts containing 10 mM Mg⁺ prepared from strain EA 2 grown in an ethionine-containing medium. These free "ethionine" subunits have sedimentation coefficients of 28 S and 45 S approximately while the "ethionine" subunits found associated as 70 S particles in crude bacterial extracts sediment at 30 S and 50 S respectively. Both classes of "ethionine" subunits (free, and associated) can be methylated in vitro in the presence of S-adenosyl methionine with or without the addition of an external source of methylases (100000 x g supernatant of a homologous crude bacterial extract) The latter observation shows that the "ethionine" subunits contain bound methylases. These enzymes do not seem to be normal ribosomal proteins nor to be bound specifically to "ethionine" ribosomes since (a) they are removed by washing the "ethionine" ribosomes with 1 M NH₄Cl, and (b) they are found bound to normal methionine ribosomes from which they are also removed by washing with 1 M NH₄Cl. [ABSTRACT FROM AUTHOR]