The capacity of the various polymerases to synthesize different RNAs was shown with the toxin a -amanitin. At low concentrations of a -amanitin synthesis of mRNAs are affected but not rRNAs nor tRNAs. At high concentrations, both mRNAs and tRNAs are affected. These observations have allowed the identification of which polymerase synthesizes which class of RNAs. RNA pol III synthesizes the tRNAs, the 5 S rRNA and some snRNAs.
Transcription exhibits several features that are distinct from replication.
Transcription initiates, both in prokaryotes and eukaryotes, from many more sites than replication. There are many more molecules of RNA polymerase per cell than DNA polymerase. This is allowable since the aberrant RNA molecules can simply be turned over and new correct molecules made. These sequence elements are termed promoters . Promoter sequences promote the ability of RNA polymerases to recognize the nucleotide at which initiation begins.