Single-strand-preferring RNases degrade double-stranded RNAs by destabilizing its secondary structure

To establish the mechanism of dsRNA degradation by mammalian single-stranded-preferring ribonucleases, and, in particular, the influence of their positively charged non-catalytic amino acid residues, we have studied the kinetic parameters of the depolimerization of single- and double-stranded polyribonucleotides such as poly(U), poly(U).poly(A), poly(C) and poly(C).poly(I) by the action of human seminal RNase, bovine seminal RNase and ox pancreas RNase A. While the activities of these RNases on poly(I).poly(C) were definitely lower than those on poly(C), the activities of human seminal and bovine seminal RNases on poly(U).poly(A) and poly(U) were of the same order of magnitude under physiological salt conditions. The ratio of the RNase A degrading activities towards poly(U) and poly(U).poly(A) at I = 0.16 M is ten times higher than the corresponding ratios determined with bovine seminal and human seminal ribonucleases. The high activities of these two RNases towards poly(U).poly(A) are discussed on the basis of their efficient estabilishing action on this double-helical nucleic acid due to their high affinity for poly(A). The destabilizing action of human seminal RNase and bovine seminal RNase on the poly (U).poly(A) duplex is higher than that measurable with bovine RNase A because of the higher number of positive charges present on those enzyme molecules. This may therefore explain why human seminal and bovine seminal ribonucleases are more efficient than RNase A in the depolymerization of poly(U).poly(A) at physiological ionic strength.