RNA Polymerase, also known as RNAP, RNApol, or DNA-dependent RNA polymerase, synthesize ribonucleotides into an RNA chain. Unlike DNA polymerases, they can initiate synthesis without a primer and also lack exonuclease activity. The bacterial RNA polymerase core enzyme, a 400-kDa five-subunit complex, synthesizes both coding and noncoding RNA. Eukaryotic RNA polymerases are more complex, contain more subunits, and are more specialized. Five different types have been classified, each synthesizing different types of RNA: rRNA (Pol I); mRNA, snRNA and miRNA (Pol II); tRNA and 5S rRNA (Pol III); plant siRNA (Pol IV and Pol V). Viral polymerases are diverse and many types do not closely resemble the bacterial or eukaryotic polymerases. However, a few have become distinguished for use in in vitro RNA synthesis, such as the SP6, T7, and T3 RNA polymerases.