Biochemical properties and hormonal regulation of barley nuclease

Abstract

The amino acid composition and NH2-terminal amino acid sequence of barley nuclease (EC 3.1.30.2) were determined. The amino acid composition is similar to that of mung bean nuclease, and therefore the biochemical properties of barley nuclease were characterized and compared with those of mung bean and other plant nucleases. The 3''-nucleotidase activity of barley nuclease is greater for purine than for pyrimidine ribonucleotides. The enzyme has little activity towards ribonucleoside 2'' and 5''-monophosphates, and deoxyribonucleotides 3'' and 5''-monophosphates, and is also inactive towards the 3''-phosphoester linkage of nucleoside cyclic 2'',3'' and 3'',5''-monophosphates. The enzyme hydrolyzes dinucleoside monophosphates, showing strong preference for purine nucleosides as the 5'' residues. Barley nuclease shows significant base preference for homoribonucleic acids, catalyzing the hydrolysis of polycytidylic acid > polyuridylic acid > polyadenylic acid .mchgt. polyguanylic acid. The enzyme also has preference for single-stranded nucleic acids. Hydrolysis of nucleic acids is primarily endonucleolytic, whereas the products of digestion possess 5''-phosphomonoester groups. Nuclease activity is inhibited by ethylenediaminetetraacetic acid and zinc is required for reactivation. Secretion of nuclease from barley aleurone layers is dependent on the hormone gibberellic acid [Brown, P. H. and Ho, T.h. D. (1986) Plant Physiol. 82. 801-806]. Consistent with these results, gibberellic acid induces up to an eightfold increase in the de novo synthesis of nuclease in aleurone layers. The secreted enzyme is a glycoprotein having an apparent molecular mass of 35 kDa. It consists of a single polypeptide having an asparagine-linked, high-mannose oligosaccharide. The protein portion of the molecule has a molecular mass of 33 kDa.