The chain termini of polynucleotides formed by limited enzymic fragmentation of wheat embryo ribosomal RNA. Part 1. Studies of snake venom phosphodiesterase

Abstract

Snake venom phos-phodiesterase induces about 15 exonucleolytic cleavages for each endonucleolytic cleavage during the 1st hour of hydrolysis of wheat embryo ribosomal RNA, under the conditions of hydrolysis used in this present investigation. The polynucleotide chains in the ribosomal RNA preparation have an average degree of polymerization in the neighborhood of 1300 nucleotide residues, and there is a mean of between 5 and 10 endonucleolytic breaks per chain during this 1st hour of phosphodiesterase-induced hydrolysis. The cleavages occur widely throughout most of the polynucleotide chains in the ribosomal RNA preparation, as judged by the sharp decrease in mean sedimentation rate which accompanies a limited degree (about 10[degree]/o) of exonucleolysis of the RNA. Studies of phosphodiesterase-induced endonucleolysis of wheat embryo soluble RNA are reported, but because of the much lower initial degree of polymerization (about 80 nucleotide residues per polynucleotide chain), the results of endonucleolysis are less pronounced in terms of the proportional increment in chain termini and the proportional decrease of mean sedimentation rate. The endonucleolysis of RNA is discussed in terms of the minor nucleotide components in both ribosomal and soluble RNA, and particular reference is made to pseudouridylate which has been found in relatively high proportion among the chain termini after limited hydrolysis with venom phosphodiesterase. Purified venom phosphodiesterase preparations, devoid of RNase or 5[image]-nucleotidase contamination, were found to convert nucleoside 2[image](3[image]), 5[image]-diphosphates to 5[image]-nucleotides under conditions which had virtually no effect on nucleoside 3[image]-phosphates or nucleoside 5[image]-phosphates. The possibility that this reaction may be catalyzed by the venom phosphodiesterase itself is discussed.