POLR1B
Description
The POLR1B (RNA polymerase I subunit B) is a protein-coding gene located on chromosome 2.
DNA-directed RNA polymerase I subunit RPA2 is an enzyme that in humans is encoded by the POLR1B gene.
The POLR1B gene encodes a protein that is a catalytic core component of RNA polymerase I (Pol I). Pol I is a DNA-dependent RNA polymerase that synthesizes ribosomal RNA precursors using the four ribonucleoside triphosphates. It transcribes 47S pre-rRNAs from multicopy rRNA gene clusters, leading to the production of 5.8S, 18S, and 28S ribosomal RNAs. The Pol I transcription cycle involves three stages: initiation, elongation, and termination. During initiation, Pol I pre-initiation complex (PIC) is recruited by the selectivity factor 1 (SL1/TIF-IB) complex bound to the core promoter preceding an rDNA repeat unit. PIC assembly bends the promoter, facilitating the formation of the transcription bubble and promoter escape. Once the polymerase has escaped the promoter, it enters the elongation phase, during which RNA is actively polymerized based on its complementarity to the template DNA strand. Highly processive, Pol I assembles in structures called 'Miller trees,' where many elongating Pol I complexes queue and transcribe the same rDNA coding regions. At terminator sequences downstream of the rDNA gene, PTRF interacts with Pol I, halting Pol I transcription and leading to the release of the RNA transcript and polymerase from the DNA. POLR1B/RPA2 forms the Pol I active center together with the largest subunit POLR1A/RPA1. It appends one nucleotide at a time to the 3' end of the nascent RNA. POLR1A/RPA1 contributes a Mg(2+)-coordinating DxDGD motif, while POLR1B/RPA2 participates in coordinating a second Mg(2+) ion and provides lysine residues believed to facilitate Watson-Crick base pairing between the incoming nucleotide and the template base. Typically, Mg(2+) ions direct a 5' nucleoside triphosphate to form a phosphodiester bond with the 3' hydroxyl of the preceding nucleotide of the nascent RNA, eliminating pyrophosphate. POLR1B/RPA2 exhibits proofreading activity: it pauses and backtracks to allow the cleavage of a missincorporated nucleotide via POLR1H/RPA12. The high processivity of Pol I is associated with decreased transcription fidelity. POLR1B/RPA2 is a component of the RNA polymerase I (Pol I) complex, which consists of 13 subunits. These include a ten-subunit catalytic core composed of POLR1A/RPA1, POLR1B/RPA2, POLR1C/RPAC1, POLR1D/RPAC2, POLR1H/RPA12, POLR2E/RPABC1, POLR2F/RPABC2, POLR2H/RPABC3, POLR2K/RPABC4, and POLR2L/RPABC5; a mobile stalk subunit POLR1F/RPA43 protruding from the core; and additional subunits homologous to general transcription factors POLR1E/RPA49 and POLR1G/RPA34. POLR1B/RPA2 is part of the Pol I pre-initiation complex (PIC), where the Pol I core assembles with RRN3 and promoter-bound UTBF and SL1/TIF-IB complex.
POLR1B is also known as A135, RPA135, RPA2, Rpo1-2, TCS4.
