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- In the absence of precise information about the mechanisms that regulate gene expression in eukaryotes, many models were proposed. One of the more popular early models known as Britten Davidson model or gene battery model was that given by R.J. Britten and E.H.
Davidson in 1969. This model even though widely accepted, is only a theoretical model and lacks sound practical proof. The model predicts the presence of four types of sequences.
Producer gen: It is comparable to a structural gene in prokaryotes. It produces pre mRNA, which after processing becomes mRNA. Its expression is under the control of many receptor sites
.Receptor site (gene):It is comparable to the operator in bacterial operon. At least one such receptor site is assumed to be present adjacent to each producer gene. A specific receptor site is activated when a specific activator RNA or an activator protein, a product of integrator gene, complexes with it.
Integrator gene:Integrator gene is comparable to regulator gene and is responsible for the synthesis of an activator RNA molecule that may not give rise to proteins before it activates the receptor site. At least one integrator gene is present adjacent to each sensor site.
Sensor site:A sensor site regulates activity of an integrator gene which can be transcribed only when the sensor site is activated. The sensor sites are also regulatory sequences that are recognized by external stimuli, e.g. hormones, temperature
According to the Britten Davidson model, specific sensor genes represent sequence-specific binding sites (similar to CAP-cAMP binding site in the E. coil) that respond to a specific signal. When sensor genes receive the appropriate signals, they activate the transcription of the adjacent integrator genes. The integrator gene products will then interact in a sequence specific manner with receptor genes.
Britten and Davidson proposed that the integrator gene products are activator RNAs that interact directly with the receptor genes to trigger the transcription of the continuous producer genes.
It is also proposed that receptor sites and integrator genes may be repeated a number of times so as to control the activity of a large number of genes in the same cell. Repetition of receptor ensures that the same activator recognizes all of them and in this way several enzymes of one metabolic pathway are simultaneously synthesized.
Transcription of the same gene may be needed in different developmental stages. This is achieved by the multiplicity of receptor sites and integrator genes. Each producer gene may have several receptor sites, each responding to one activator. Thus, though a single activator can recognize several genes, different activators may activate the same gene at different times.
A set of structural genes controlled by one sensor site is termed as a battery. Sometimes when major changes are needed, it is necessary to activate several sets of genes. If one sensor site is associated with several integrators, it may cause transcription of all integrators simultaneously thus causing transcription of several producer genes through receptor sites.
The repetition of integrator genes and receptor sites is consistent with the reports that state that sufficient repeated DNA occurs in the eukaryotic cells. The most attractive features of the Britten and Davidson model is that it provides a plausible reason for the observed pattern of interspersion of moderately repetitive DNA sequences and single copy DNA sequences.
Direct evidence indicates that most structural genes are indeed single copy DNA sequences. The adjacent moderately repetitive DNA sequences would contain the various kinds of regulator genes (sensor, integrator and receptor genes).
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