<p> All mature tRNAs contain a terminal CCA sequence essential for function [<cite idref="PUB00010739"/>]. CCA-adding enzyme ensures that each tRNA contains a terminal CCA, either by de novo addition or by repair of a damaged end [<cite idref="PUB00010740"/>]. CCA-adding enzyme of this group is a class I nucleotidyltransferase unrelated, except at the active site, to class II CCA-adding enzymes (<db_xref db="INTERPRO" dbkey="IPR012006"/>) [<cite idref="PUB00010741"/>]). Interestingly, despite the fact that class I CCA-adding enzyme combines tRNA cytidylyltransferase (<db_xref db="EC" dbkey="2.7.7.25"/>) and tRNA adenylyltransferase (<db_xref db="EC" dbkey="2.7.7.21"/>) activities, it possesses only one active site per monomer [<cite idref="PUB00010742"/>]. However, stoichiometric analysis shows formation of a tetramer upon binding of only two tRNA molecules, raising the possibility that two active sites, each responsible for a different nucleotide, form adjacent to each substrate molecule.</p> tRNA nucleotidyltransferase, archaea