RNA polymerase (RNAP or RNApol), also known as DNA-dependent RNA polymerase, is an enzyme that produces RNA. In cells, RNAP is necessary for constructing RNA chains using DNA genes as templates, a process called transcription. RNA polymerase enzymes are essential to life and are found in all organisms and many viruses. In chemical terms, RNAP is a nucleotidyl transferase that polymerizes ribonucleotides at the 3' end of an RNA transcript.

RNAP was discovered independently by Samuel B. Weiss, Audrey Stevens, and Jerard Hurwitz in 1960. By this time, one half of the 1959 Nobel Prize in Medicine had been awarded to Severo Ochoa for the discovery of what was believed to be RNAP, but instead turned out to be polynucleotide phosphorylase.

The 2006 Nobel Prize in Chemistry was awarded to Roger D. Kornberg for creating detailed molecular images of RNA polymerase during various stages of the transcription process.

Control of the process of gene transcription affects patterns of gene expression and, thereby, allows a cell to adapt to a changing environment, perform specialized roles within an organism, and maintain basic metabolic processes necessary for survival. Therefore, it is hardly surprising that the activity of RNAP is both long and complex and highly regulated. In Escherichia coli bacteria, more than 100 transcription factors have been identified, which modify the activity of RNAP.

Robert Tjian (Traditional Chinese: 錢澤南; born 1949 in Hong Kong) is a U.S. biochemist best known for his work on eukaryotic transcription. He is currently Professor of Biochemistry and Molecular Biology at the University of California, Berkeley and an investigator of the Howard Hughes Medical Institute (HHMI). On April 1, 2009, Tjian became the President of HHMI.

Tjian was born in Hong Kong in 1949. Tjian's father Tjian Tze-Ning (錢子寧), a native of Shaoxing, Zhejiang, was a famous tycoon in Shanghai and pioneer of China's modern paper industry. In April 1949, Tjian's family and business all moved to Hong Kong; in 1950 moved to São Paulo, Brazil; and later settled in New Jersey, the United States.

Tjian received a Bachelor of Arts degree from the University of California, Berkeley in 1971 and a Ph.D. degree from Harvard University in 1976. He was a postdoctoral fellow at Cold Spring Harbor Laboratory with James D. Watson for three years before returning to the University of California in 1979 when he was appointed Assistant Professor of Biochemistry. In 2008, Tjian joined the Board of Directors of the Lasker Foundation.