Ribosomal RNA transcription in vitro is species specific

Eukaryotic cells possess three distinct nuclear DNA-dependent RNA polymerases which are responsible for transcription of different sets of genes (for review see refs 1, 2). Recently, cell-free transcription systems have been developed which faithfully initiate transcription of isolated genes by the corresponding RNA polymerase in the presence of crude cellular extracts. These cellular extracts supply additional components required for specific transcription 3–6 . Successful in vitro systems for transcription of RNA polymerase II or III genes were developed using either heterologous or homologous components 7–11 . In contrast, an analogous cell-free system for the RNA polymerase I transcription unit from mouse has been shown to be active only with homologous extracts from mouse cells 6 . Data presented here show that in vitro transcription of ribosomal DNA isolated from mouse, human and a protozoan requires completely homologous components. None of the three active cell-free systems is capable of correct initiation on the nonhomologous templates. Further, supplementation of mouse extracts with purified protozoan RNA polymerase I failed to result in specific transcription of the protozoan rDNA, suggesting that the species specificity of pre-ribosomal RNA synthesis resides, in part, in the transcription factors.