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Genetic Regulation Mechanisms and Control of Gene Expression

Explore the transcriptional control, clustering of genes, and coordinate control within polycistronic mRNA inducible genes using the operon model. Learn about lactose and catabolic pathways, regulatory genes, and attenuation in gene expression.

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Genetic Regulation Mechanisms and Control of Gene Expression

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  1. Genetic Regulatory Mechanisms

  2. Control of Gene Expression • Transcriptional control • Clustering of genes with related function • Coordinate control of genes with related function • Polycistronic mRNA

  3. Inducible Genes - Operon Model • Definition: Genes whose expression is turned on by the presence of some substance • Lactose induces expression of the lac genes • An antibiotic induces the expression of a resistance gene • Catabolic pathways

  4. RegulatoryGene Operon DNA i p o z y a m-RNA Protein -Galactosidase Transacetylase Permease Lactose Operon • Structural genes • lac z, lac y, & lac a • Promoter • Polycistronic mRNA • Regulatory gene • Repressor • Operator • Operon • Inducer - lactose

  5. Absence of lactose z y a i p o Active No lac mRNA Presence of lactose z y a i p o Inactive -Galactosidase Permease Transacetylase Lactose Operon • Inducer -- lactose • Absence • Active repressor • No expression • Presence • Inactivation of repressor • Expression • Negative control

  6. - glucose Glucose added Units of -galactosidase + glucose Time (hr) + lactose Catabolite Repression (Glucose Effect) • Definition: Control of an operon by glucose • Catabolic operons

  7. Adenyl cyclase Absence of glucose c-AMP ATP CAP i z y a p o Active Inactive -Galactosidase Permease Transacetylase Maximum expression Mechanism of Catabolite Repression • c-AMP • CAP (CRP) protein • CAP-cAMP complex • Promoter activation • Positive control

  8. Presence of glucose CAP Adenyl cyclase X ATP Low level expression z y a i p o Inactive -Galactosidase Permease Transacetylase Mechanism of Catabolite Repression • Glucose:cAMP • CAP (CRP) protein • No CAP-cAMP complex • No Promoter activation

  9. Repressible Genes - Operon Model • Definition: Genes whose expression is turned off by the presence of some substance (co-repressor) • Tryptophan represses the trp genes • Biosynthetic pathways • Co-repressor is typically the end product of the pathway

  10. RegulatoryGene Operon A B L R P O E D C Inactive repressor (apo-repressor) 5 Proteins Tryptophan Operon • Structural genes • trp E, trpD, trpC trpB & trpA • Common promoter • Regulatory Gene • Apo-Repressor • Inactive • Operator • Leader • Operon • Co-repressor • Tryptophan

  11. Absence of Tryptophan A B L R P O E D C Inactive repressor (apo-repressor) 5 Proteins Presence of Tryptophan A B L R P O E D C No trp mRNA Inactive repressor (apo-repressor) Trp (co-repressor) Tryptophan Operon • Co-repressor -- tryptophan • Absence of tryptophan • Gene expression • Presence of tryptophan • Activates repressor • No gene expression • Negative control

  12. L DNA 3 4 P O 1 2 E RNA ATG TGA 3 4 1 2 2 trp codons Attenuation • Definition: Premature termination of transcription • Leader region • Leader transcript • Translation start • Translation stop • Tryptophan codons

  13. 4 1 2 3 UUUUUUU 3 2 UUUUUUU 4 1 Attenuation • Mutually exclusive mRNA secondary structure • region 1 : region 2 • region 2 : region 3 • region 3 : region 4 • Coupled transcription and translation

  14. High tryptophanyl-t-RNA Low tryptophanyl-t-RNA 3 3 4 4 1 1 2 2 ATG TGA ATG TGA UUUUUUU 2 1 4 2 trp codons 3 2 trp codons 3 2 UUUUUUU 4 1 No Attenuation Attenuation Attenuation

  15. Regulation of Enzyme Activity • Feed back inhibition • Epigenetic modification • Post translational modifications • Phosphorylation/dephosphorylation • Adenylation/deadenylation

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