Abstract
Polynucleotide phosphorylase (polyribonucleotide:orthophosphate nucleotidyltransferase, EC 2.7.7.8) activity has been found in many prokaryotes and studied in detail since 1955. Such enzymes have been detected also in plants. We now describe the purification of polynucleotide phosphorylase from cucumber cotyledons and leaves. This enzyme is a complex of three subunits, possibly not identical, of about M(r) 50,000. Its enzymatic properties are similar to those of the tobacco enzyme. Unlike the prokaryotic enzymes, the plant enzyme shows activity in the absence of primer but is to various extents stimulated by various ribopolynucleotides or RNAs. RNA-dependent RNA polymerase, not previously shown to exist in non-virus-infected cucumber, has been found to be present at a low level and was separated from the much greater amount of polynucleotide phosphorylase, although some of the physical properties of the two enzymes are rather similar.