pääRNApolymeraasi

pääRNApolymeraasi, also known as RNA polymerase I (Pol I), is one of the three core RNA polymerases found in eukaryotic cells. Its primary function is the transcription of ribosomal RNA (rRNA) genes, with the exception of 5S rRNA, which is transcribed by RNA polymerase III. Pol I is responsible for synthesizing the vast majority of cellular RNA, as ribosomes are essential for protein synthesis and are highly abundant.

The enzyme complex is a large, multi-subunit protein that operates within the nucleolus. This specialized compartment within the nucleus is the site of ribosome biogenesis. Pol I requires a set of accessory transcription factors to initiate transcription, including SL1/TBP and UBF. These factors bind to specific DNA sequences within the promoter region of rRNA genes, recruiting Pol I and facilitating its assembly into a transcription-competent complex.

The transcription process by Pol I involves the synthesis of a large precursor rRNA molecule (45S pre-rRNA in humans and mice). This pre-rRNA undergoes extensive processing, including cleavage and modification, to yield the mature 18S, 5.8S, and 28S rRNAs. These mature rRNAs then assemble with ribosomal proteins to form the small and large ribosomal subunits. Due to the critical role of Pol I in ribosome production, its activity is tightly regulated and closely linked to cellular growth and metabolic state. Dysregulation of Pol I transcription has been implicated in various diseases, including cancer.