Assembly of RNA Polymerase-I Initiation Complex
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Assembly of RNA Polymerase-I Initiation Complex
The nuclei of all eukaryotic cells contain three different RNA Polymerases, designated I, II and III. Like the DNA Polymerase that catalyzes DNA replication, RNA Polymerases catalyze the formation of the phosphodiester bonds that link the nucleotides together to form a linear chain. The RNA Polymerase moves stepwise along the DNA, unwinding the DNA helix just ahead of the active site for polymerization to expose a new region of the template strand for complementary base-pairing. In this way, the growing RNA chain is extended by one nucleotide at a time in the 5’-to-3’ direction. The substrates are nucleoside triphosphates (ATP, CTP, UTP, and GTP); as for DNA replication, a hydrolysis of high-energy bonds provides the energy needed to drive the reaction forward. Each eukaryotic RNA Polymerase catalyzes transcription of genes encoding different classes of RNA. RNA Polymerase-I is dedicated to the synthesis of only one type of RNA molecule, called pre-rRNA (ribosomal RNA). The primary pre-rRNA transcript is processed into the 18S, 5.8S, and 28S rRNAs found in vertebrate ribosome or their functional equivalents in other eukaryotes (Ref.1).

RNA Polymerase-I transcribes the gene for rRNA from a single type of promoter. The transcript includes the sequences of both large and small rRNAs, which are later released by cleavages and processing. There are many copies of the transcription unit, alternating with nontranscribed spacers, and organized in a cluster. The promoter in human cells consists of a bipartite sequence in the region preceding the start point. The ‘core promoter element’ which is essential for transcription surrounds the start point, extending from -45 to +20, and is sufficient for transcription to initiate. However, its efficiency is very much increased by ~50-bp long UCE (Upstream Control Element), which extends from -180 to -107. Both regions have an unusual composition for a promoter, being rich in G*C base pairs; and they are -85% identical. RNA Polymerase-I requires two ancillary factors. UBF (Upstream Binding Factor) is a single polypeptide that binds to a G’C-rich element in the core promoter and UCE and the two bound molecules are thought to make protein-protein interactions causing the intervening DNA to loop out. Factor SL1 (Selectivity Factor-1) does not by itself have specificity for the promoter, but once UBF has bound, SL1 can bind cooperatively to extend the region of DNA that is covered (Ref.2 & 3). Once both factors are bound, RNA Polymerase-I can bind to the core promoter to initiate transcription. SL1 is a multimeric protein composed of TBP (TATA-Binding Protein) and three TAFI (TBP-Associated Factors) with molecular weights of 110, 63, and 48 kDa (Ref.4 & 5). The behavior of SL1 resembles a bacterial Sigma Factor. As an isolated protein complex, it does not bind specifically to the promoter, but in conjunction with other components, specific promoter regions are bound. It may have primary responsibility for ensuring that the RNA Polymerase is properly localized at the start point. After SL1 binding, RNA Polymerase-I binds completing assembly of the Initiation Complex. In contrast to initiation by RNA Polymerase-II, transcription initiation by RNA Polymerase-I does not require hydrolysis of the Beta-Gamma phosphate of ATP.