Microbiology Procedure

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Metabolic Regulation Regulation of Enzyme Synthesis Catabolite Repression Enzyme With Repressor Function Attenuation

Attenuation

	Attenuation - To ensure that no over production occurs, regulation of transcription at more than one control regions in the DNA have also been reported. For example, the synthesis of the five enzymes of the tryptophan operon in E.coli is regulated primarily at the promoter operator region. These enzymes are synthesized when cells are grown in a limiting concentration of tryptophan.
Excess tryptophan functions as a corepressor by activating specific repressor molecule which then binds to the trp operator and prevents transcription. In the presence of a low concentration of tryptophan, m-RNA synthesis begins at the promotor region but does not go to completion and the majority of m-RN A molecules stop growing before the transcription of the first gene Detailed biochemical analysis of this system has shown that a region of 161 bases (termed as leader region) separates the promoter operator region from the first gene of the operon (trp E, anthranilate synthetase gene) . In this region, most of the RNA polymerase molecules stop synthesis of m-RNA to yield fragments of about 140 nucleotides.
This site is caned the attenuator and consists of an A+T rich region immediately preceeded by a G+C rich region displaying a dyad symmetry. The G+C rich region forms a base paired structure called the terminator which is recognized as the termination signal by the transcribing RNA polymerase. This structure occurs when the cell has an excess of the regulatory amino acid. When there is a deficiency of the appropriately charged t-RNA,
It will allow the formation of an alternative RNA secondary structure called anti terminator which will not allow the formation of the terminator preventing transcription termination at the attenuator.Thus, there are two control regions the attenuator region and the operator, for the transcription of the trp operon The existance of these two control regions allows a fine tuning of the system which is responsive to the intracellular trytophan concentration.Under conditions of tryptophan starvation, attenuation is relaxed allowing trp m-RNA synthesis
	The regulation of enzymes of trp operon is an example where the end product of the biosynthetic pathway controls the amount of enzyme synthesized and this type of control is also called "end product repression". Similar mechanisms also exist in controlling the biosynthesis of other amino acids such as histidine, leucine, isoleucine, threonine and phenylalanine.

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Regualtion of Enzyme Activity Other Mechanisms Regulatory Mechanisms in Microorganisms Regulation of DNA and RNA Sysnthesis

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