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Ribonucleic acid (RNA) polymerase I is an enzyme that is found in most eukaryotic cells and that transcribes ribosomal RNA (rRNA). RNA polymerase I is found in all organisms except for bacteria and viruses, which do not have nuclei. This enzyme is a macromolecule composed of several chains of amino acids, and it includes several components, including a region responsible for melting DNA strands and another permitting the entry of RNA onto the separated strands. The transcription process involves matching nucleotides on a single strand of DNA to corresponding RNA nucleotides. This enzyme transcribes only from DNA gene templates that code for rRNA, and it transcribes in a three-step process involving initiation, elongation of the RNA chain and termination of the process.
When transcription occurs, a molecule known as selectivity factor 1 can bind to a region of DNA, known as a promoter, right before the RNA gene. This action creates a complex to which RNA polymerase I easily binds, which is a way of encouraging rRNA transcription. Hydrophobic areas of the enzyme then attach to hydrophobic nucleotides on the DNA, which separates the two strands of DNA from one another and opens up the region of the gene so it can be transcribed. The active site of RNA polymerase I then allows molecules called ribonucleoside-triposphates to diffuse into the open region and bind to complementary nucleotides on the DNA until the enzyme reaches its termination site, a bend in the DNA bound to the thyroid transcription factor-1 (TTF-1) protein. This forms an RNA chain that is temporarily bound to the DNA, which is peeled off after completion, allowing ribosomes to translate the RNA into proteins.
RNA polymerase I is the only enzyme that can transcribe rRNA, and rRNA accounts for about half of RNA in a cell, so it is an important enzyme involved in the cellular growth process. The rate of cell growth is limited by how quickly proteins can be synthesized, which itself depends on how quickly ribosomes are synthesized and how quickly RNA polymerase I transcribes rRNA. The enzyme therefore can regulate its transcription rate through an unknown mechanism as well as by responding to the cell, decreasing the number of available rRNA genes to be transcribed. The several hundred available rRNA genes are turned off as cells become more differentiated in an organism, because they do not need to grow as rapidly and therefore do not need as high a rate of rRNA transcription as undifferentiated cells.