Specificity of cleavage by ribonuclease III

The specificity of RNase III for various synthetic homopolymeric doublestranded RNA substrates have been examined. Although RNase III appears to cleave all homopolymeric RNA duplex structures, with Poly (U)·Poly (A) as the substrate, the enzyme cleaves the Poly (U) strand much faster than it cleaves the Poly (A) strand. Under conditions where the Poly (U) strand is quantitatively cleaved into acid-soluble fragments ranging in size between 5-8 nucleotides in length, the poly (A) strand is cleaved into large fragments 40-60 nucleotides in length. These results indicate that RNase III recognizes duplex RNA structures for binding, and makes single-stranded scissions and suggests that the enzyme has a preference for cleaving adjacent to UMP residues over AMP residues in polynucleotide chains.