RNA Polymerase

 RNA Polymerases are enzymes involved in the production of RNA. After the annealing of a primer, RNA polymerases build a complementary strand of RNA from free nucleotides using a strand of DNA as a template. The active form is a complex consisting of multiple subunits known as the RNA polymerase holoenzyme (Molecular mass ~400 kilodaltons).

In Eukaryotes some RNA polymerases include:

• RNA Polymerase I: Involved in the transcription of ribosomal RNA.
• RNA Polymerase II: Transcribes all genes that code for proteins.
• RNA Polymerase III: Involved in the transcription of transfer RNA (tRNA)
  

References 

1. Hartl. D, Jones. E (2009), Genetics - Analysis Of Genes And Genomes, 7th Edition, Sudbury: Jones and Bartlett Publishers p346