Goals
- Explain why it is important for cells to express different genes at different times, in prokaryotes and eukaryotes.
- Describe the general function of a transcriptional activator and transcriptional repressor
- Describe what an operon is and its basic regulation
- Explain how and under what conditions the lux operon is regulated
- Predict the effect of mutations in the lux operon or its regulatory components on gene expression
Gene Expression
According to Biology Online Gene Expression is defined as:
The conversion of the information from the gene into mRNA via transcription and then to protein via translation resulting in the phenotypic manifestation of the gene.
Biology Online
They also define Gene Expression Regulation as:
The modulation of any of the stages of gene expression, hence, it encompasses the various systems that control and determine which genes are switched on and off, and when, how long, and to what extent the genes are expressed.
Biology Online
Both Prokaryotes and Eukaryotes regulate gene expression, but why?
It could be, in the case of adapting to changes in the environment.
An example of this is with the amino acid Tryptophan. When prokaryotes have an abundance of Tryptophan. The prokaryotes do not produce any tryptophan. However, when the prokaryotes have low/ no levels of tryptophan then the tyrptophan biosynthesis gene is expressed.
And in eukaryotes as tissue specific and developmental requirements.
Mechanisms of Genetic Regulation
There are multiple steps that are taken which can be impacted positively or negatively to express gene regulation.
We’ll first touch on the process when DNA is transcribed into an RNA transcript. This is the transcriptional control/regulator
Transcriptional Control
Regulation of initiating transcription is common. This impacts the recruitment of RNA polymerase.
This is done by repressor gene bind to where an RNA polymerase normally binds to called the operator (promoter) region of the gene. This effectively prevents the initiation of transcription, and overall the transcribing of the gene.
However, when an activator is placed onto an activator binding site the rate of transcription is therefore increased.
The reason for these activators and repressors is to alter levels of one RNA, but how does that work?
Transcriptional Regulation (Prokaryotes)
Networks of genes are regulated by operons.
Operons are defined as clustered genes transcribed in one transcript, genes are transcriptionally co-regulated.
One regulator many affect many genes, and is independent on the affect it has on which gene it turns on and off.
An example of this is the bacterium Vibrio fischeri which has a mutualistic relationship with the Bobtail squid, allowing its organs to bioluminesce with the enzyme luciferase.
Luciferase is expressed as part on an operon.
Another example is the lux operon. The LuxR is activated when it is bound by an autoinducer. This autoinducer is a small molecule which is secreted by the cells, and is used to sense bacterial concentration.
When there is high cell density, the lux operon is expressed, but is not expressed in low cell density. The dependance of the little autoinducer is why. If the activator is not present, then it is not transcribed.