RNA Polymerases and the Transcription Event
Introduction
Simply stated transcription is the synthesis of RNA from a DNA template. Also important is the concept that transcription, whether prokaryotic or eukaryotic, has three main events.
- Initiation - binding of RNA polymerase to double-stranded DNA; this step involves a transition to single-strandedness in the region of binding; RNA polymerase binds at a sequence of DNA called the promoter. Initiation is the most important step in gene expression!!!
- Elongation - the covalent addition of nucleotides to the 3' end of the growing polynucleotide chain; this involves the development of a short stretch of DNA that is transiently single-stranded
- Termination - the recognition of the transcription termination sequence and the release of RNA polymerase
Although transcription is performed by RNA Polymerase, the enzyme needs other proteins to produce the transcript. These factors are either associated directly with RNA Polymerase or add in building the actual transcription apparatus. The general term for these associated proteins is transcription factor.
Transcription factor - any protein other than RNA Polymerase that is required for transcription
Functions of Transcription Factors
- bind to RNA Polymerase
- bind another transcription factor
- bind to cis-acting DNA sequences
RNA Polymerase and the group of protein that directly interact with it are called the basal transcription apparatus. This is the apparatus that is directly responsible for transcription.
Basal transcription apparatus - RNA polymerase + general factors; both needed to initiate transcription
Other factors, those that interact directly or through a coactivator with the proteins of the basal transcription apparatus, are also important for transcription. These generally have a positive effect on transcription, but occasionally they can repress gene expression through transcripion. These factors are called upstream factors.
Upstream factors - ubiquitous factors that increase the efficiency of transcription initiation; set of factors unique to each promoter
Functions of Upstream Factors
- influence the initiation of transcription by contacting members of the basal apparatus
- promotes assembly of the apparatus
- may bind coactivators that interact with the basal apparatus
- typically bind to TFIID, TFIIB or TFIIA
- TFIID provides various TAFs that can be interacted with; if TAFs are unique to a specific promoter, then the interaction can control promoter specific transcription
- most interactions are positive in nature and induce transcription
- repressors may prevent the building of the basal apparatus
Finally, some factors are turned in a temporal or spatial manner, or directly in response to the environment. These factors provide the final link in controlling gene expression. These are termed inducible factors.
Inducible factors - act in the same manner as an upstream factor but their synthesis is regulated in a temporal or spatial manner
Given all of the discussion regarding the basal transcription apparatus, and upstream and inducible factors, we can know arrive at a definition of a promoter. The definition is reflects the interaction of the all of the important proteins and the DNA to which the bind.
Promoter - all the DNA sequences containing binding sites for RNA polymerase and the transcription factors necessary for normal transcription
Copyright © 1998. Phillip McClean