This model describes the pcnB family of poly(A) polymerases (also known as plasmid copy number protein). These enzymes sequentially add adenosine nucleotides to the 3' end of RNAs, targeting them for degradation by the cell. This was originally described for anti-sense RNAs, but was later demonstrated for mRNAs as well. Members of this family are as yet limited to the gamma- and beta-proteobacteria, with putative members in the Chlamydiacae and spirochetes. This family has homology to tRNA nucleotidyltransferase (cca).