Association of ribonuclease I with ribosomes and their subunits.

RNase I of Salmonella typhimurium has been purified to a homogeneous state. It has a molecular weight of 2 x 104. Association of RNase I with the 30 S subunits of ribosomes has been investigated by using an inhibition assay. 70 S ribosomes and 30 S subunits, but not the 50 S subunits, inhibit the hydrolysis of polyribonucleotides (only poly adenylic acid was used in the present investigation) by RNase I. This inhibition is Mg++-dependent. Each 30 S subunit appears to bind approximately 3 molecules of enzyme. The three-dimensional configuration of the ribosomes is essential for the inhibition since removal of even a small amount of protein by salt extraction leads to considerable loss of inhibitory activity; the detached proteins have no inhibitory activity. Although ribosomal RNA has some inhibitory activity, it is much less than that of the intact ribosomes. Reconstitution of the ribosomes from the salttreated particles and detached proteins leads to complete recovery of the inhibitory activity. This inhibition assay can be very conveniently used in measuring the purity of subunit preparations from 70 S ribosomes as well as in the study of the reconstitution of the ribosomes.