Involved in mRNA degradation. Hydrolyzes single-stranded polyribonucleotides processively in the 3' to 5' direction (PubMed:11948193). RNases 2 and R (rnr) contribute to rRNA degradation during starvation, while RNase R and PNPase (pnp) are the major contributors to quality control of rRNA during steady state growth (PubMed:21135037). This RNase is required to decrease expression of RNase PH (rnp) at 42 degrees Celsius during starvation, which in turn represses rRNA degradation (PubMed:28625967). Exonucleolytic cleavage in the 3'- to 5'-direction to yield nucleoside 5'-phosphates. Stimulated by the presence of a monovalent cation (PubMed:11948193). Acetylation leads to decreased substrate binding and decreased catalytic activity (PubMed:26847092). Optimum pH is 7.5-9.5. Optimum temperature is 50 degrees Celsius. Acetylated at Lys-501 by PatZ. Deacetylated by CobB. In the presence of wild-type RNase PH (rph) 80-90% reduction in viability in stationary phase (96 hours of growth) at 31 and 42 degrees Celsius or when cells are rapidly starved at 42 degrees Celsius; if rph is also absent there is no visible phenotype under these 3 growth conditions (PubMed:28625967). Its absence leads to extensive degradation of rRNA (PubMed:28625967). In K12 strains that are derived from W1485 (including MG1655 and W3110) the rph gene has a frameshift that leads to loss of its ribonuclease PH activity. In strain K12 / MG1655(Seq)* the wild-type Rph protein has been restored (PubMed:28625967). Belongs to the RNR ribonuclease family. RNase II subfamily.