Preferential Resistance of Phosphodiester Bonds between Deoxycytidine and 5′-Adjacent Bases to Chlamydomonas Nuclease C

Chlamydomonas Ca2+-dependent nuclease (nuclease C) has been shown to be polymorphic (Ogawa and Kuroiwa 1985a). Aged preparations of crude extract obtained in the absence of protease inhibitor contained little nuclease C1&2. Instead, most of the activities appeared in association with smaller molecules similar in size to nuclease C3 and C4 that are members of nuclease C isozymes, suggesting that the polymorphism may be caused by the action of an endogenous protease. Both the original form and the active fragments showed base-specific endo-exonucleolytic activity, and liberated an extremely low level of 3′-dCMP as compared with the other three 3′-dNMPs. We compared the extent of hydrolysis of phosphodiester bonds between two bases of all possible combinations. The result showed preferential resistance of phosphodiester bonds between dC and 5′ -adjacent bases to nuclease C. This explains the paucity of 3′-dCMP in the digests of DNA. The base-specific action of nuclease C1&2 was preserved during the modification by endogenous protease.