RNA polymerase. Molecular model of RNA polymerase (blue) transcribing a strand of mRNA (messenger ribonucleic acid, red) from a DNA (deoxyribonucleic acid) template (green and orange). This is T7 RNA polymerase from the T7 bacteriophage, a virus that infects Escherichia coli bacteria. mRNA is the intermediary between DNA and its protein product. This enzyme produces mRNA from viral DNA inserted into the bacteria's genome.