Fig. 5.

Trinucleotide/dinucleotide ratio for the cleavage of the oligocytidylic acid (Cp)₃C>p substrate by RNase A, K7Q/R10Q-RNase A, and K66Q-RNase A. With this substrate, trinucleotide formation indicates exonucleolytic cleavage by the enzyme, whereas dinucleotide formation indicates endonucleolytic cleavage. The extrapolation of the ratios to zero time was determined with the program GraFit v. 5 (Leatherbarrow 2001) and indicates the preference of the enzymes on the intact substrate. The ratio decreases with time because the trinucleotide produced at the initial stages of the reaction is used later as a substrate by the enzyme producing more dinucleotide. The dinucleotide concentration has been divided by two because each endonucleolytically cleaved bond in the tetranucleotide yields two dinucleotide molecules, whereas each exonucleolytically broken bond only produces one trinucleotide molecule and one mononucleotide.