Regulation of transcription in prokaryotes and eukaryotes

REGULATION OF TRANSCRIPTION IN PROKARYOTES & EUKARYOTES

Course ContentsI. Regulation of lactose metabolism & tryptophan

synthesis in Prokaryotes. II. Gene silencing in Eukaryote.

Gurumayum Suraj Sharma

REGULATION OF GENE EXPRESSION

PROKARYOTES

REGULATED GENES: Genes whose activity is controlled in response to needs of cell or organism.

All organisms have a large number of genes whose products are essential to normal functioning of growing & dividing cell, no matter what the conditions are.

These genes are always active in growing cells & are known as Constitutive Genes Or Housekeeping Genes

Eg. genes that code for enzymes needed for protein synthesis & glucose metabolism. All genes are regulated on some level. If normal cell function is impaired, expression of all genes, including constitutive genes, is reduced by regulatory mechanisms.

Gurumayum Suraj Sharma

Genes which encode proteins that work together in the cell typically are organized into OPERONS

The genes are adjacent to each other and are transcribed together onto a Polycistronic mRNA, so called because it contains the information from more than one gene.

Cistron - A Gene

Regulation of synthesis of such mRNA depends on interactions between regulatory proteins & regulatory sequences that are next to the gene array.

Gurumayum Suraj Sharma

Certain enzymes only when specific chemical substrates are present.

These were referred to as Adaptive Enzymes Also called Facultative In contrast, enzymes that are produced continuously, regardless of the chemical makeup of the environment [Constitutive enzymes]

Term adaptive has been replaced with the more accurate term INDUCIBLE

Reflects the role of substrate, which serves as the inducer in enzyme production.

Gurumayum Suraj Sharma

A contrasting system, whereby presence of a specific molecule inhibits gene expression.

o Such molecules are usually end products of anabolic biosynthetic pathways. o Eg. Tryptophan can be synthesized by bacterial cells. o If a sufficient supply of Trp present in culture medium

o No reason to expend energy in synthesizing enzymes necessary for Trp production. o A mechanism thus evolved whereby Trp plays a role in repressing transcription of mRNA needed for producing Trpsynthesizing enzymes. In contrast to the inducible system controlling lactose metabolism, the system governing tryptophan expression is said to be REPRESSIBLE.

Gurumayum Suraj Sharma

Regulation of both inducible or repressible, may be under either Negative or Positive Control

Under negative control, genetic expression occurs unless it is shut off by some form of a regulator molecule.

In contrast, under positive control, transcription occurs only if a regulator molecule directly stimulates RNA production.

Either type of control or a combination of the two can govern inducible or repressible systems.

Gurumayum Suraj Sharma

POSITIVE & NEGATIVE CONTROL OF GENE EXPRESSION

Regulation of gene expression [Induction, or turning genes on, & Repression, or turning genes off] can be accomplished by both Positive & Negative Control Mechanisms.

Both involve participation of Regulator Genes [genes encoding products that regulate the expression of other genes].

o In Positive Control Mechanisms, the product of the regulator gene is required to turn on expression of one or more Structural Genes. o In negative control mechanisms, the product of the regulator gene is necessary to shut off the expression of Structural Genes.

Structural Genes

Genes specifying the amino acid sequences of enzymes or structural proteins

Gurumayum Suraj Sharma

RNA Pol- Binds to promoter & synthesizes RNA transcript containing coding region of the gene.

The product of the regulator gene acts by binding to a site called the Regulator Protein-binding Site [RPBS] adjacent to promoter of structural genes.

When product of regulator gene is bound at RPBS, transcription of structural genes is turned on in a positive control system or turned off in a negative control system.

The regulator gene products are called asACTIVATORS [because they activate gene expression] in

positive control systems REPRESSORS [because they repress gene expression] in

negative control systems.

Gurumayum Suraj Sharma

Binding of a regulator protein to RPBS depends on presence or absence of another molecule in the cell- The Effector Molecules.

Effectors- Usually small molecules [amino acids, sugars & similar metabolites] Effector molecules involved in induction of gene expression are called INDUCERS

Those involved in repression of gene expression are called COREPRESSORS Effectors bind to regulator gene products [Activators & Repressors] & cause changes in 3D structures of these proteins.

Conformational changes in protein structure resulting from binding of small molecule- Called Allosteric Transitions

Result in alterations in their activity. In case of activators & repressors, allosteric transitions caused by binding of effector molecules usually alter their ability to bind to regulator protein-binding sites adjacent to the promoters of the structural genes they control.

Gurumayum Suraj Sharma

THE OPERON MODEL

Jacob & Monod [1961] Co-ordinated regulation of transcription of genes involved in specific metabolic pathways. The operon is a unit of gene expression and regulation. Typically includes:

The structural genes [any gene other than a regulator] for enzymes involved in a specific biosynthetic pathway whose expression is co-ordinately controlled.

Control elements such as an operator sequence, which is a DNA sequence that regulates transcription of the structural genes.

Regulator genes whose products recognize the control elements.

Eg. A repressor which binds to & regulates an operator sequence.

Gurumayum Suraj Sharma

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