The characterization of LeNUC1, a nuclease associated with leaf senescenceof tomato

Induction of nuclease and RNase activities, together with decreases in nucl eic acid content are considered to be characteristics of senescence in high er plants. However, little is known about the specific identities or functi ons of the enzymes involved or the mechanisms controlling their activation. Here me report the identification of a 41-kDa-tomato nuclease, LeNUC1, whi ch is specifically induced during tomato leaf senescence but not in ripenin g fruits. LeNUC1 is a glycoprotein, which can degrade both RNA and DNA and has optimal activity at pH 7.5-8, EDTA inhibits the activity of LeNUC1, whi le the addition of Co2+ or Mn2+ can restore its activity in the presence of the chelating agent. Interestingly, the activity of LeNUC1 is also induced in young leaves upon treatment with ethylene, which is known to be a senes cence-promoting hormone in tomato. Constitutive activity of a 39-kDa nuclea se, LeNUC2, similar in its biochemical requirements to LeNUC1, was also det ected, LeNUC2 is not induced by ethylene and does not seem to be glycosylat ed. Based on their characteristics, LeNUC1 and LeNUC2 can be classified as Nuclease I enzymes. LeNUC1 may be involved in nucleic acid metabolism durin g tomato leaf senescence.