Structure of the RNase PH hexamer
RNase PH is an 3'-5' exoribonuclease and nucleotidyltransferase, present in archaebacteria and eubacteria, that is involved in tRNA processing. Contrary to hydrolytic enzymes, it is a phosphorolytic enzyme, meaning that it uses inorganic phosphate as a cofactor to cleave nucleotide-nucleotide bonds, releasing diphosphate nucleotides. The active structure of the proteins is actually a homohexameric complex, consisting of three RNase PH dimers.  RNase PH has homologues in many other organisms, which are referred to as RNase PH-like proteins. When a part of another larger protein has a domain that is very similar to RNase PH, this is called an RNase PH domain (RPD).
Two highly related exoribonuclease complexes:
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