Evolution of Eukaryotic RNA Polymerases

Abstract

Abstract Ribonucleic acid (RNA) polymerases are responsible for the transcription of deoxyribonucleic acid (DNA) into RNA. The number of protein subunits making up these enzymes has increased during evolution from 5 in Eubacteria to 12 in the common ancestor of Archaea and eukaryotes. Eukaryotes have three RNA polymerases , each transcribing a particular subset of genes. RNA polymerase I transcribes ribosomal RNA genes, RNA polymerase II transcribes messenger RNA genes and RNA polymerase III transcribes 5S and transfer RNA genes. Although RNA polymerase II is still made up of 12 subunits, the number of subunits has increased to 14 in RNA polymerase I and to 17 in RNA polymerase III . These new subunits originated from the permanent recruitment of pre‐existing general transcription factors. Interestingly, the evolution of the three eukaryotic RNA polymerases has been affected not only by adaptive forces but also by the nonadaptive forces due to the concerted evolution of the genes they transcribe. Key Concepts: The larger numbers of protein subunits found in RNA polymerases I and III, relative to RNA polymerase II, are due to the permanent recruitment of general transcription factors. The three universal eukaryotic RNA polymerases have specific functional differences near their respective active sites. During evolution, the three eukaryotic RNA polymerase have been selected to better perform their specific tasks. The rate of evolution of RNA polymerases is proportional to the amount of homogenisation (concerted evolution) experienced by the genes they transcribe. Nonadaptive processes therefore also affect the rate of evolution of RNA polymerase genes.