Involved in mRNA degradation. Catalyzes the phosphorolysis of single-stranded polyribonucleotides processively in the 3'- to 5'-direction. Necessary for competence development in Bacillus subtilis. May be necessary for modification of the srfA transcript (stabilization or translation activation). Involved in processing precursor type I toxin-antitoxin RNAs antitoxin SR4 and SR5 RNAs to their mature forms (PubMed:22229825, PubMed:26940229). phosphate + RNA(n+1) = a ribonucleoside 5'-diphosphate + RNA(n) Homodimer (Probable). Component of a possible RNA degradosome complex composed of rny, rnjA, rnjB, pnp, pfkA and eno (although rnjA and rnjB's presence is unclear) (PubMed:19193632). RNA helicase CshA may also be a member of this complex (PubMed:20572937). Not essential, doubling time increased slightly. About 800-fold less compentent for plasmid transformation, no effect on sporulation efficiency. Grows poorly at 18 degrees Celsius. Increased sensitivity to several translation inhibiting antibiotics such as tetracycline, erythromycin and chloramphenicol, but increased resistance to streptomycin and nalidixic acid. Forms long filamentous cells, probably due to defective septum formation, cell walls are altered with looser, less dense peptidoglycan. Double pnp-rny mutants grow very slowly, while pnp-rnjA mutants could not be isolated (PubMed:23504012). Accumulation of precursor forms of type I toxin-antitoxin system antitoxin RNAs SR4 and SR5 (PubMed:22229825, PubMed:26940229). Belongs to the polyribonucleotide nucleotidyltransferase family.