An RNase activity probably involved in the maturation of 16‐S pre‐ribosomal RNA in Escherichia coli has been partially purified from crude cell extracts. When 27‐S ribosome precursor particles are incubated with this enzyme preparation in vitro, their 17‐S RNA is converted to a product with the same electrophoretic mobility as mature 16‐S rRNA. Fingerprint analysis of this product shows that it contains the 3′‐OH but not the 5′‐P terminus of mature 16‐S rRNA. Generation of the normal 5′‐P terminus seems to require a factor present in cell extracts since incubation of the 27‐S precursor particle in an extract obtained after centrifugation at 30000 g causes conversion of the 17‐S RNA to a 16‐S species containing both termini of mature 16‐S rRNA. Preliminary experiments suggest that correct maturation of the 5′ end of the 17‐S precursor RNA requires a system in which protein synthesis can take place.

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Proteolytic degradation of ribosomal proteins occurs during the preparation of subunits of the cytoplasmic ribosomes of the protozoa Tetrahymenu thernioplrila and the isolated subunits are inactive. Addition of 5 mM iodoacetamide to cell suspensions before extraction inhibits proteolytic activity and permits isolation of active subunits. The protein complements of these subunits have been characterized in two different two-dimensional elcctrophoretic systems, and their molecular weights have been determined.Limited degradation of ribosomal proteins has been observed during isolation of ribosomes and ribosomal subunits from a variety of eukaryotic cells [l -61. This phenomenon was not observed in our earlier studies [7] with ribosomes of an amicronucleate Tetraliymeiza strain of unknown phenoset (Tetrahyniena 'pyr{forn~is' CGL) but has subscquently been shown to occur during preparation of ribosomes from several other amicronucleate strains [XI (and B. Petridou and F. Hayes, unpublished results). It also occurs, as shown here, during preparation of ribosomes from the micronucleate strain Tetruhymenu thermophila and leads to isolation of inactive ribosomal subunits unless protease inhibitors are present throughout the preparation process. Of a number of inhibitors tested, iodoacetamide was found to be the most effective. 40s and 60s ribosomal subunits prepared from T . thermophila in its presence are active in vitro and retain their activity during storage at low tcmperature. Alkylation of a small number of proteins in these subunits revealed by experiments with radioactive iodoacetamide appears not to impair their capacity to reassociate or to catalyse polyphenylalanine synthesis in vitro. The protein complements of active and inactive 40s and 60s ribosomal subunits of T. thernmphilu prepared in the presence and absence respectively of iodoacetamide have been characterized by polyacrylamide gcl electrophoresis in two different systems. The molecular weights of the proteins of active subunits have been estimated by SDS gcl electrophoresis.

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