RNA polymerase (RNAP), illustration. RNA (ribonucleic acid) polymerases are enzymes that catalyse all DNA (deoxyribonucleic acid) transcription. Eukaryotic RNA polymerases can be distinguished into RNAP I, II, and III, which convert the information in DNA into RNA molecules, that furthermore fulfil a variety of functions. These include messenger RNA (mRNA), transfer RNA (tRNA), ribosomal RNA (rRNA) for protein synthesis, ribozymes for catalysis, as well as microRNAs for the regulation of gene expression.