RNA polymerase III

In eukaryote cells, RNA polymerase III (also called Pol III) is a protein that transcribes DNA to synthesize 5S ribosomal RNA, tRNA and other small RNAs. The genes transcribed by RNA Pol III fall in the category of "housekeeping" genes whose expression is required in all cell types and most environmental conditions. Therefore, the regulation of Pol III transcription is primarily tied to the regulation of cell growth and the cell cycle, and thus requires fewer regulatory proteins than RNA polymerase II. Under stress conditions however, the protein Maf1 represses Pol III activity. Rapamycin is another Pol III inhibitor via its direct target TOR. The process of transcription (by any polymerase) involves three main stages: Initiation, requiring construction of the RNA polymerase complex on the gene's promoter Elongation, the synthesis of the RNA transcript Termination, the finishing of RNA transcription and disassembly of the RNA polymerase complex Initiation: the construction of the polymerase complex on the promoter. Pol III is unusual (compared to Pol II) by requiring no control sequences upstream of the gene, instead normally relying on internal control sequences - sequences within the transcribed section of the gene (although upstream sequences are occasionally seen, e.g. U6 snRNA gene has an upstream TATA box as seen in Pol II Promoters). There are three classes of Pol III initiation, corresponding to 5S rRNA, tRNA, and U6 snRNA initiation. In all cases, the process starts with transcription factors binding to control sequences, and ends with TFIIIB (Transcription Factor for polymerase III B) being recruited to the complex and assembling Pol III. TFIIIB consists of three subunits: TATA binding protein (TBP), a TFIIB-related factor (BRF1, or BRF2 for transcription of a subset of Pol III-transcribed genes in vertebrates), and a B-double-prime (BDP1) unit. The overall architecture bears similarities to that of Pol II.