<p>RNA polymerases catalyse the DNA dependent polymerisation of RNA from DNA, using the four ribonucleoside triphosphates as substrates. Prokaryotes contain a single RNA polymerase compared to three in eukaryotes (not including mitochondrial and chloroplast polymerases). Eukaryotic RNA polymerase I is essentially used to transcribe ribosomal RNA units, polymerase II is used for mRNA precursors, and III is used to transcribe 5S and tRNA genes. Each class of RNA polymerase is assembled from nine to fourteen different polypeptides. Members of the family include the largest subunit from eukaryotes; the gamma subunit from Cyanobacteria; the beta'subunit from bacteria; the A' subunit from archaea; and the B'' subunit from chloroplast RNA polymerases.</p>