Exoribonuclease phosphorolytic (IPR001247) PH (phosphorylation) domain is responsible for 3'-5' exoribonuclease activity. The PH domain is present in bacterial/organellar RNase and PNP enzyme (polynucleotide phosphorylase). The active PH domain, using inorganic phosphate as a nucleophile, releases ribonucleoside 5'-diphosphate (rNDP) from the 3' end of the RNA substrate and adds it to the phosphodiester bond between the two terminal nucleotides.