Inhibition of synthetic and natural messenger translation. I. Purification and properties of a protein isolated from Escherichia coli MRE 600 ribosomes.

Escherichia coli MRE 600 ribosomes was found to inhibit translation of R17 RNA or poly(U) and initiation complex formation with R17 RNA. The effect of this protein on poly(U) translation was used as the basis of a simple and specific assay for its purification. The factor was purified to homogeneity by chromatography on DEAE-cellulose, hydroxyapatite, and Sephadex G-150, and its molecular weight was determined to be 69,000. Factor activity was rapidly lost above SO”, and was not inactivated by treatment with p-hydroxymercuribenzoate. An affinity of the purified protein for RNA was shown by the formation of an RNA-protein complex which could be retained on Millipore filters. On the basis of molecular weight, mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis, and inhibitory activity in R17 RNA and poly(U) translation, the factor was identified as subunit I of Q/3 replicase.