Important Mechanisms Which Can Regulate the Transcription of a Gene by RNA polymerase are listed below:

1. Specificity factors alter the specificity of RNA polymerase for a given promoter or set of promoters, making it more or less likely to bind to them (i.e. sigma factors used in prokaryotic transcription).

2. Repressors bind to non-coding sequences on the DNA strand that are close to or overlapping the promoter region, impeding RNA polymerase’s progress along the strand, thus impeding the expression of the gene.

RNA

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3. General transcription factors these transcription factors po­sition RNA polymerase at the start of a protein-coding se­quence and then release the polymerase to transcribe the mRNA.

4. Activators enhance the interaction between RNA polymerase and a particular promoter, encouraging the expression of the gene.

5. Activators do this by increasing the attraction of RNA polymerase for the promoter, through interactions with sub- units of the RNA polymerase or indirectly by changing the structure of the DNA.

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6. Enhancers are sites on the DNA helix that are bound to by activators in order to loop the DNA bringing a specific pro­moter to the initiation complex.

There are only two types of gene regulation: positive regula­tion and negative regulation. When the expression of genetic in­formation is quantitatively increased by the presence of a specific regulatory element, regulation is said to be positive and when the expression of genetic information is diminished by the presence of a specific regulatory element, regulation is said to be negative.

The element or molecule mediating negative regulation is said to be a negative regulator or repressor; that mediating positive regu­lation is a positive regulator or activator. However, a double nega­tive has the effect of acting as a positive.

Thus, effectors that inhibit the function of a negative regulator will appear to bring about a positive regulation. Many regulated systems that appear to be induced are in fact depressed at the molecular level.