DNase A, a poly(dA) and poly(dT)-specific deoxyribonuclease from Achatina fulica. Further investigation on the specificity.

Abstract

The degradation of heat-denatured and native calf thymus DNAs, poly(dA-dT), poly(dA-dC) . poly(dG-dT), and poly(dG-dC) by DNase A has been investigated with the main aim of providing background information for studying the specificity of the enzyme. The specificity of the DNase A was studied by determining the base compositions of 5'- and 3'-terminal nucleotides of oligonucleotides released by the enzyme. The 5'- and 3'-terminal nucleotide compositions were found to vary in the average chain length (Pn) range of 45 to 7 for degradations of heat-denatured calf thymus DNA at pH 4.0, 4.5, 5.0, and 5.5. When the heat-denatured DNA was digested at the optimal pH (pH 5.0), at the Pn = 45 dAdo was predominant (39%) and dThd was minor (9%) in 5'-terminals, whereas dThd was predominant (43%) and dAdo was minor (9%) in 3'-terminals. At Pn = 7, dAdo was even more predominant (49%) and there was very little dThd (7%) in 5'-terminals. No preference was seen in 3'-terminals. This finding indicates that change in the specificity takes place during digestion. The compositions of 5'- and 3'-terminal nucleotides of oligonucleotides released from poly(dA-dT) by exhaustive digestion of the enzyme showed pronounced preferences for dAdo in 5'-terminals (81%) and dThd in 3'-terminals (78%). The hexamer or above deoxyadenylate and the nonamer or above of thymidylate were good substrates of the enzyme. It can be concluded that DNase A is a novel cluster-specific DNase which recognizes and cleaves sequences of oligodeoxyadenylate and/or sequences of oligothymidylate.