During growth under conditions of
phosphate limitation,
suspension-cultured cells of tomato (Lycopersicon esculentum Mill.) secrete
phosphodiesterase activity in a similar fashion to
phosphate starvation-inducible
ribonuclease (
RNase LE), a cyclizing
endoribonuclease that generates 2':3'-cyclic
nucleoside monophosphates (NMP) as its major monomeric products (T. Nürnberger, S. Abel, W. Jost, K. Glund [1990] Plant Physiol 92: 970-976). 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 has a molecular mass of 70 kD, a pH optimum of 6.2, and an isoelectric point of 8.1. The
phosphodiesterase preparation is free of any detectable
deoxyribonuclease,
ribonuclease, and
nucleotidase activity. Tomato extracellular
phosphodiesterase is insensitive to
EDTA and hydrolyzes 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 are 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 indicates that
cyclic nucleotide phosphodiesterase functions as an accessory ribonucleolytic activity that effectively hydrolyzes primary products of
RNase LE to substrates for
phosphate-
starvation-inducible
phosphomonoesterases. Biosynthetical labeling of
cyclic nucleotide phopshodiesterase upon
phosphate starvation suggests de novo synthesis and secretion of a set of nucleolytic
enzymes for scavenging
phosphate from extracellular
RNA substrates.