The mRNA encodes RNA dependent RNA polymerase. Reverse transcriptase is the virally encoded RNA-dependent DNA polymerase. The eukaryotic core RNA polymerase II was first purified using transcription assays. The main replicase-transcriptase protein is the RNA-dependent RNA polymerase (RdRp). Eukaryotic RNA interference requires a cellular RNA-dependent RNA polymerase (cRdRP). Mediator complexes interact with transcription factors and RNA polymerase II. These genes are transcribed by viral RNA polymerase and associated transcription factors. UV radiation can cause dimerization of RNA, which prevents transcription by RNA polymerase. Prokaryotic polymerases exist in two forms&colon; core polymerase and holoenzyme. Phosphodiester bonds are formed between ribonucleotides by the enzyme RNA polymerase. Those genes code for a protein subunits shared between RNA polymerase I and III. They are transcribed by either RNA polymerase II or RNA polymerase III. RPB3 is involved in RNA polymerase II assembly. The exoribonuclease provides a proofreading function to the complex which the RNA-dependent RNA polymerase lacks. DNA polymerase that alter substrate sensitivity.