Polynucleotide phosphorylase

Polynucleotide Phosphorylase (PNPase) is a bifunctional enzyme with a phosphorolytic 3' to 5' exoribonuclease activity and a 3'-terminal oligonucleotide polymerase activity. That is, it dismantles the RNA chain starting at the 3' end and working toward the 5' end. It also synthesizes long, highly heteropolymeric tails in vivo. It accounts for all of the observed residual polyadenylation in strains of Escherichia coli missing the normal polyadenylation enzyme. Discovered by Ochoa in 1951, the RNA-polymerization activity of PNPase was initially believed to be responsible for DNA-dependent synthesis of messenger RNA, a notion that got disproved by the late 1950s (http://www.jbc.org/content/281/15/e12.full.pdf).