Illustration of the RNA polymerase (RNAP, white and grey) from Escherichia coli complexed with a promoter DNA sequence (yellow). Bacterial RNA (ribonucleic acid) polymerases are enzymes that catalyse the transcription of DNA (deoxyribonucleic acid) to messenger RNA (mRNA) for protein synthesis. The sigma region of the RNAP recognises and binds to promotor sequences, regions just before the start of a gene, to initiate transcription.