RNA polymerase III
Encyclopedia
RNA polymerase III transcribes
DNA
to synthesize ribosomal 5S rRNA, 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
, thus requiring fewer regulatory proteins than RNA polymerase II
.
In the process of transcription
(by any polymerase) there are three main stages:
Pol III is unusual (compared to Pol II) 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, eg. U6 snRNA gene has an upstream TATA box
as seen in Pol II Promoters).
TFIIIB remains bound to DNA following initiation of transcription by Pol III (unlike bacterial σ factors and most of the basal transcription factors for Pol II transcription). This leads to a high rate of transcriptional reinitiation of Pol III-transcribed genes.
is not required, as it is in prokaryotes.
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
DNA
DNA
Deoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
to synthesize ribosomal 5S rRNA, 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
Cell growth
The term cell growth is used in the contexts of cell development and cell division . When used in the context of cell division, it refers to growth of cell populations, where one cell grows and divides to produce two "daughter cells"...
and the cell cycle
Cell cycle
The cell cycle, or cell-division cycle, is the series of events that takes place in a cell leading to its division and duplication . In cells without a nucleus , the cell cycle occurs via a process termed binary fission...
, thus requiring fewer regulatory proteins than RNA polymerase II
RNA polymerase II
RNA polymerase II is an enzyme found in eukaryotic cells. It catalyzes the transcription of DNA to synthesize precursors of mRNA and most snRNA and microRNA. A 550 kDa complex of 12 subunits, RNAP II is the most studied type of RNA polymerase...
.
In the process of transcription
Transcription (genetics)
Transcription is the process of creating a complementary RNA copy of a sequence of DNA. Both RNA and DNA are nucleic acids, which use base pairs of nucleotides as a complementary language that can be converted back and forth from DNA to RNA by the action of the correct enzymes...
(by any polymerase) there are three main stages:
- Initiation; requiring construction of the RNA polymerase complex on the gene's promoter.
- Elongation; the writing of the RNA transcript.
- Termination; the finishing of RNA writing and disassembly of the RNA polymerase complex.
Initiation
Initiation: the construction of the polymerase complex on the promoter.Pol III is unusual (compared to Pol II) 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, eg. U6 snRNA gene has an upstream TATA box
TATA box
The TATA box is a DNA sequence found in the promoter region of genes in archaea and eukaryotes; approximately 24% of human genes contain a TATA box within the core promoter....
as seen in Pol II Promoters).
Class I
Typical stages in 5S rRNA (also termed class I) gene initiation:- TFIIIA (Transcription Factor for polymerase III A) binds to the intragenic (lying within the transcribed DNADNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
sequence) 5S rRNA control sequence, the C Block (also termed box C). - TFIIIA Serves as a platform that replaces the A and B Blocks for positioning TFIIIC in an orientation with respect to the start site of transcription that is equivalent to what is observed for tRNA genes.
- Once TFIIIC is bound to the TFIIIA-DNA complex the assembly of TFIIIB proceeds as described for tRNA transcription.
Class II
Typical stages in a tRNA (also termed class II) gene initiation:- TFIIIC (Transcription Factor for polymerase III C) binds to two intragenic (lying within the transcribed DNADNADeoxyribonucleic acid is a nucleic acid that contains the genetic instructions used in the development and functioning of all known living organisms . The DNA segments that carry this genetic information are called genes, but other DNA sequences have structural purposes, or are involved in...
sequence) control sequences, the A and B Blocks (also termed box A and box B).. - TFIIIC acts as an assembly factor that positions TFIIIB to bind to DNA at a site centered approximately 26 base pairs upstream of the start site of transcription. TFIIIB (Transcription Factor for polymerase III B), consists of three subunits: TBPTATA Binding ProteinThe TATA-binding protein is a general transcription factor that binds specifically to a DNA sequence called the TATA box. This DNA sequence is found about 35 base pairs upstream of the transcription start site in some eukaryotic gene promoters...
(TATA Binding Protein), the Pol II transcription factor TFIIB-related protein, Brf1 (or Brf2 for transcription of a subset of Pol III-transcribed genes in vertebrates) and Bdp1.. - TFIIIB is the transcription factor that assembles Pol III at the start site of transcription. Once TFIIIB is bound to DNA, TFIIIC is no longer required. TFIIIB also plays an essential role in promoter opening.
TFIIIB remains bound to DNA following initiation of transcription by Pol III (unlike bacterial σ factors and most of the basal transcription factors for Pol II transcription). This leads to a high rate of transcriptional reinitiation of Pol III-transcribed genes.
Class III
Typical stages in a U6 snRNA (also termed class III) gene initiation (documented in vertebrates only):- SNAPc (SNRNA Activating Protein complex) (also termed PBP and PTF) binds to the PSE (Proximal Sequence Element) centered approximately 55 base pairs upstream of the start site of transcription. This assembly is greatly stimulated by the Pol II transcription factors Oct1 and STAF that bind to an enhancer-like DSE (Distal Sequence Element) at least 200 base pairs upstream of the start site of transcription. These factors and promoter elements are shared between Pol II and Pol III transcription of snRNA genes.
- SNAPc acts to assemble TFIIIB at a TATA box centered 26 base pairs upstream of the start site of transcription. It is the presence of a TATA box that specifies that the snRNA gene is transcribed by Pol III rather than Pol II.
- The TFIIIB for U6 snRNA transcription contains a smaller Brf1 paralogue, Brf2.
- TFIIIB is the transcription factor that assembles Pol III at the start site of transcription. Sequence conservation predicts that TFIIIB containing Brf2 also plays a role in promoter opening.
Termination
Polymerase III terminates transcription at small polyTs stretch (5-6). In Eukaryotes, a hairpin loopStem-loop
Stem-loop intramolecular base pairing is a pattern that can occur in single-stranded DNA or, more commonly, in RNA. The structure is also known as a hairpin or hairpin loop. It occurs when two regions of the same strand, usually complementary in nucleotide sequence when read in opposite directions,...
is not required, as it is in prokaryotes.
Transcribed RNAs
The types of RNAs transcribed from RNA polymerase III includes:- Transfer RNATransfer RNATransfer RNA is an adaptor molecule composed of RNA, typically 73 to 93 nucleotides in length, that is used in biology to bridge the three-letter genetic code in messenger RNA with the twenty-letter code of amino acids in proteins. The role of tRNA as an adaptor is best understood by...
s - 5S ribosomal RNA5S ribosomal RNA5S ribosomal RNA is a component of the large ribosomal subunit in both prokaryotes and eukaryotes .Eukaryotic 5S rRNA is synthesised by RNA polymerase III, whereas most other eukaroytic rRNAs are cleaved from a 45S precursor transcribed by RNA polymerase I...
- U6 spliceosomal RNAU6 spliceosomal RNAU6 snRNA is the non-coding small nuclear RNA component of U6 snRNP , an RNA-protein complex that combines with other snRNPs, unmodified pre-mRNA, and various other proteins to assemble a spliceosome, a large RNA-protein molecular complex upon which splicing of pre-mRNA occurs...
- RNase PRNase PRibonuclease P is a type of ribonuclease which cleaves RNA. RNase P is unique from other RNases in that it is a ribozyme – a ribonucleic acid that acts as a catalyst in the same way that a protein based enzyme would. Its function is to cleave off an extra, or precursor, sequence of RNA on tRNA...
and RNase MRPRNase MRPRNase MRP is an enzymatically active ribonucleoprotein with two distinct roles in eukaryotes. In mitochondria it plays a direct role in the initiation of mitochondrial DNA replication. In the nucleus it is involved in precursor rRNA processing, where it cleaves the internal transcribed spacer 1...
RNA - 7SL RNA7SL RNAThe signal recognition particle RNA, also known as 7SL, 6S, , or 4.5S RNA, is the RNA component of the signal recognition particle ribonucleoprotein complex. SRP is a universally conserved ribonucleoprotein that directs the traffic of proteins within the cell and allows them to be secreted...
(the RNA component of the signal recognition particleSignal recognition particleThe signal recognition particle is an abundant, cytosolic, universally conserved ribonucleoprotein that recognizes and targets specific proteins to the endoplasmic reticulum in eukaryotes and the plasma membrane in prokaryotes....
) - Vault RNAVault RNAVault RNAs are an RNA family found as part of the vault ribonucleoprotein complex first discovered in 1986. The complex consists of a major vault protein , two minor vault proteins , and several small untranslated RNA molecules. Each vault particle contains 8-16 vRNA molecules. The vault complex...
s - Y RNAY RNAY RNAs are small non-coding RNA components of the Ro ribonucleoprotein particle . The Ro RNP was first identified by Lerner et al.. as a target of autoimmune antibodies in patients with systemic lupus erythematosus.-Function:...
- SINESinéMaurice Sinet , known as Siné, is a French cartoonist.As a young man he studied drawing and graphic arts, while earning a living as a cabaret singer. After his military service he started publishing his drawings and also worked as a photo-retoucher for porn magazines. His first published drawing...
s (short interspersed repetitive elements) - 7SK RNA7SK RNA-External links:...
- Several microRNAs
- Several small nucleolar RNAs
- Several gene regulatory antisense RNAs