In the case of PNPase, which is a phosphorolytic RNA-degrading protein found in bacteria and the chloroplasts and mitochondria of some eukaryotic organisms, two RNase PH domains, and both an S1 and KH RNA binding domain are part of a single protein, which forms a trimeric complex that adopts a structure almost identical to that of the exosome. Because of this high similarity in both protein domains and structure, these complexes are thought to be evolutionarily related and have a common ancestor. The RNase PH-like exosome proteins, PNPase and RNase PH all belong to the RNase PH family of RNases and are phosphorolytic exoribonucleases, meaning that they use inorganic phosphate to remove nucleotides from the 3' end of RNA molecules.