Induction of an extracellular cyclic nucleotide phosphodiesterase as an accessory ribonucleolytic activity during phosphate starvation of cultured tomato cells

Induction of an extracellular cyclic nucleotide phosphodiesterase as an accessory ribonucleolytic activity during phosphate starvation of cultured tomato cells

2000

A:PS

During growth under conditions of phosphate limitation, suspension cells of tomato secreted phosphodiesterase activity in a similar fashion to phosphate starvation-inducible ribonuclease (RNase LE), an endoribonuclease that generated 2':3'-cyclic nucleoside monophosphates (NMP) as its major monomeric products. Tomato extracellular phosphodiesterase was purified to homogeneity from the spent culture medium of phosphate-starved cells and was characterized as a cyclic nucleotide phosphodiesterase. The purified enzyme had a MW of 70 kDa, a pH optimum of 6.2, and an isoelectric point of 8.1. The phosphodiesterase preparation was free of any detectable deoxyribonuclease, ribonuclease and nucleotidase activity. Tomato extracellular phosphodiesterase was insensitive to EDTA and hydrolysed with no apparent base specificity 2':3'-cyclic NMP to 3'-NMP and the 3':5'-cyclic isomers to a mixture of 3'-NMP and 5'-NMP. Specific activities of the enzyme were 2-fold higher for 2':3'-cyclic NMP than for 3':5'-cyclic isomers. Analysis of monomeric products of sequential RNA hydrolysis with purified RNase LE, purified extracellular phosphodiesterase, and cleared - Pi culture medium as a source of 3'-nucleotidase activity indicated that cyclic nucleotide phosphodiesterase functioned as an accessory ribonucleolytic activity that effectively hydrolysed primary products of RNase LE to substrates for phosphate-starvation-inducible phosphomonoesterases. Labelling of cyclic nucleotide phosphodiesterase upon phosphate starvation suggested de novo synthesis and secretion of a set of nucleolytic enzymes for scavenging phosphate from extracellular RNA substrates.

122(2):543-552, PLANT PHYSIOLOGY

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