Polynucleotide phosphorylase functions both as a 3 '-> 5 ' exonuclease anda poly(A) polymerase in Escherichia coli

in vitro, polynucleotide phosphorylase of Escherichia coil can both synthes ize RNA by using nucleotide diphosphates as precursors and exonucleolytical ly degrade RNA in the presence of inorganic phosphate. However, because of the high in vivo concentration of inorganic phosphate in exponentially grow ing cells, it has been assumed that the enzyme works exclusively as an exon uclease. Here we demonstrate that, contrary to this prediction, polynucleot ide phosphorylase not only synthesizes long, highly heteropolymeric tails i n vivo, but also accounts for all of the observed residual polyadenylylatio n in poly(A) polymerase I deficient strains. In addition, the enzyme is res ponsible for adding the C and U residues that are found in poly(A) tails in exponentially growing cultures of wild type E. coil.