RNase PH catalyzes a synthetic reaction, the addition of nucleotides to the 3' end of RNA.

Escherichia coli RNase PH is a phosphate-dependent exoribonuclease that has been implicated in the 3' processing of tRNA precursors. It degrades RNA chains in a phosphorolytic manner releasing nucleoside diphosphates as products. Here we show that RNase PH also catalyzes a synthetic reaction, the addition of nucleotides to the 3' termini of RNA molecules. The synthetic activity co-purifies with RNase PH throughout an extensive enrichment indicating that it is due to the same enzyme. The synthetic activity can incorporate all nucleoside diphosphates, but not triphosphates, and is strongly inhibited by Pi, but not PPi. Various RNA molecules stimulate nucleotide incorporation, and with tRNA the 3' end of the molecule serves a primer function. RNA chains as long as 40 residues can be synthesized in this system. As with polynucleotide phosphorylase, the synthetic activity of RNase PH apparently represents the reversal of the degradative reaction.