Dissociation of ribosomal ribonucleic acid from silkmoth pupae by heat and dimethylsulfoxide: Evidence for specific cleavage points

Phenol treatment of wing epidermal tissue from Hyalophora cecropia silkmoth pupae yields RNA with ribosomal components having slightly lower sedimentation rates than those from rat liver. These are designated 26 s and 17s. Heating for three minutes at 50 or 60 °C abolishes the 26 s component, the material of which then appears quantitatively in the 17 s position in sucrose gradient analyses. Isolated 26 s RNA breaks down in the same manner, whereas native 17 s RNA is unaltered by brief heat treatment. Gradual degradation attributable to nuclease action, exhibited by both 26 s and 17 s components, is kinetically distinct from the sharp thermal dissociation. The thermal dissociation is inhibited by NaCl (0.4 m) or magnesium ions. Similar conversion of 26 s RNA to a 17 s product is brought about by 80% dimethylsulfoxide at 0 °C. It is attributed to separation of base-paired secondary structure at points of pre-existing cleavage in the polynucleotide chain. The homogeneity of the dissociated product indicates specificity in the points of cleavage, which, it is suggested, may be due to nuclease action in the intact ribosome. RNA heavier than 26 s, demonstrated in the same preparations by pulse labeling and presumed to represent ribosomal precursor, is stable during brief heating.