Transcription and RNA Processing

ID #2350

I understand that RNA polymerases I, II, and III transcribe rRNA, mRNA, and tRNA, respectively in eukaryotes. I just want to clarify this because the book seems to make RNA polymerase II the main RNA synthesizing enzyme. Is this just because of the added complexities of requiring transcription factors?

Yes, RNAP I transcribes rRNA genes, RNAP II transcribes mRNA genes, and RNAP III transcribes tRNA genes and some rRNA genes (which you will learn more about for the next exam). I would not necessarily say that RNAP II is the main RNA synthesizing enzyme, especially since rRNA is actually the predominant type of RNA in cells. RNAP I is probably the busiest of the RNA polymerases. The textbook says that because RNAP II is responsible for transcribing protein-coding genes, it has been the focus of the most research (so likely more is known about transcription of mRNA). I do want to stress that RNAP I, II, and III all use transcription factors. Each polymerase has its own set of transcription factors, so for example transcription factors for RNAP II will not be recognized by RNAP I and III. 

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