Ionic control of enzymic degradation of double-stranded RNA

The pattern of the degradation of various double-stranded polyribonucleotides by several ribonucleases (bovine RNAase A and its cross-linked dimer, bovine seminal RNAase, and pike-whale pancreatic RNAase) has been studied as a function of ionic strength and pH. It appears that (1) there is no direct correlation between the secondary structure of double-stranded RNA and its resistance against enzymatic breakdown, i.e., the stability of the secondary structure of double-helical RNA is not the main variable in the process. (2) The acstivity responses of the enzymes examined to changes of ionic strength and pH suggest that enzymic degradation of double-stranded RNA is mainly controlled by ion concentration, and that the process may fall within the phenomena interpreted by the theory of the ionic control of biochemical reactions advanced by Douzou and Maurel (Douzou, P. and Maurel, P. (1977) Proc. Natl. Acad. Sci. U.S.A. 74, 1013--1015). (3) The activity curves of the enzyme studied show, at a given pH, a shift toward higher ionic strengths as a function of the basicity of the enzyme protein. This finding explains the already observed correlation between number and/or density of positive charges of a ribonuclease molecule and its ability to attack double-stranded RNA in 0.15 M sodium chloride/0.015 M sodium citrate (SSC). (4) A careful analysis of the influence of ionic strength and pH on the reaction appears to be necessary in order to characterize a ribonuclease which shows activity towards double-stranded RNAs, and to allow a meaningful comparison between different enzymes capable of attacking these substrates.