It is believed that histones under certain situations bring about super coiling of DNA preventing the movement of RNA- polymerase and thus blocking transcription. This is known by the fact RNA precursors are incorporated only in those regions where chromatin is diffusing (no super coiling).
Three models have been proposed by various scientists to account for gene regulation in eukaryotes. These are – 1. Frenster’s model, 2. Non histone derepressor model and Britten and Davidson’s model.
1. Frenster’s model:
This is also called gene specific derepressor RNA model. As has been pointed out earlier, histones bring about non specific repression of transcription.
Derepression (initiation of transcription) is brought about when nuclear polyions separate histones from DNA at certain specific parts (of DNA) and form complexes with histones.
2. Non histones derepressors:
The model proposed by Paul et al (1971), is similar to Frenster’s model except the derepression is brought about by NHC acid proteins instead gene specific RNA.
The fact that NHC proteins are of immense variety in different organisms, and also at different stages of growth even in the same individual goes to show that they can undertake selective derepression at specific gene loci.
Derepression is brought about when NHC proteins bind themselves at specific loci to histones forming complexes. This complex separates itself from the DNA and allows it to transcribe, only at those specific loci.
3. Britten Davidson Model:
Also known as operon operator model, this is somewhat different from the other two models. As per this model, four types of genes are involved in the regulation process. These are sensor genes, producer genes, integrator genes and receptor genes.
i. Sensor genes:
Comparable to the promoter genes these are sensitive to the metabolic
Status of the cell. The sensor gene is stimulated by various metabolic Substances.
ii. Producer genes:
These regulate the output of metabolism and control the formation of enzymes, cell organelle, membranes etc.
iii. Integrator gene:
As the name suggests, the gene integrates the activities of other genes. The integrator gene sends specific molecular signals to other genes as to whether the sensor gene is activated or not. Each sensor gene may be associated with a single or a group of integrator genes.
In the latter case, a sensor gene may send a variety of molecular signals to other genes through its various integrator genes.
iv. Receptor gene:
This acts as an intermediate between the integrators and the producers. When the molecular signal reaches the receptor gene, it in turn activates the netics Producer gene.
A producer may have one or a group of receptor genes associated with it in the same way as sensors have several integrators.
This helps, not only in the variety of origin of molecular signals, (from sensors) but also variety in reception of these signals (by receptors). Perhaps multi integrator gene complex and multi receptor gene complex may have a specific multichannel relationship.