RNA polymerase II. Molecular model showing the structure of the enzyme RNA polymerase II (blue and white) in complex with Transcription Initiation Factor IIB ( TFIIB, pink). This enzyme synthesises a complementary mRNA (messenger ribonucleic acid) strand from a strand of DNA (deoxyribonucleic acid) during a DNA transcription. The RNA polymerase molecule is uses the original DNA strand as a template for the mRNA. mRNA is the intermediary between DNA and its protein product. TFIIB is one of several general transcription factors that make up the RNA polymerase II preinitiation complex