Involved in mRNA degradation. Catalyzes the phosphorolysis of single-stranded polyribonucleotides processively in the 3'- to 5'-direction (By similarity). Involved in decay of speF mRNA (PubMed:30742606). phosphate + RNA(n+1) = a ribonucleoside 5'-diphosphate + RNA(n) Component of the RNA degradosome, which is a multiprotein complex involved in RNA processing and mRNA degradation. Increased stability of speF mRNA. Belongs to the polyribonucleotide nucleotidyltransferase family.