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GENE EXPRESSION IN PROKARYOTES

BIOCHEMISTRY

mprasadnaidu
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GENE EXPRESSION IN PROKARYOTES

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  1. Gene expression in prokaryotes M.Prasad NaiduMSc Medical Biochemistry,Ph.D.Research Scholar

  2. Introduction • Genetic expression is the process by which inheritable information from a gene, such as the DNA sequence, is made into a functional gene product, such as protein or RNA • Non-protein coding genes (e.g. rRNA genes, tRNA genes) are transcribed, but not translated into protein

  3. Prokaryotes • Group of organisms lacking a cell nucleus or any other membrane-bound organelles. • Differ from the eukaryotes, which have a cell nucleus • Mode of division is binary fission • Prokaryotes exhibit efficient genetic mechanisms to respond to environmental conditions

  4. Rod-Shaped Bacterium, Escherichia colidividing by binary fission Rod-Shaped Bacterium, hemorrhagic E. coli, strain

  5. Types of regulation in Gene Expression

  6. Responses in Gene Expression

  7. Types of genes in Gene Expression • Inducible gene • Regulated by inducer/activator • Constitutive gene • Not subjected to regulation

  8. Regulation of Prokaryotic Gene Expression • Control at the level of transcription • Induction - the production of a specific enzyme/s in response to the presence of a substrate • Repression - the cessation of production of a specific enzyme/s in response to an increased level of a substrate

  9. All of the genes which encode the enzymes necessary for the pathway are found next to each other on the E. coli chromosome • A single mRNA carries information for multiple proteins • This type of mRNA is called a polycistronic mRNA and is totally unique to prokaryotes

  10. Operon Models • An operon model is a self-regulating series of genes found on DNA that work in concert • It includes a special segment of genes that are regulators of the protein synthesis, but do not code for protein, called the promoter and operator regions • Lactose (Lac), Tryptophan (Trp), L-Arabinose (Ara)

  11. Lac Operon Model • Inducible system • Three genes part of an operon that code for three separate enzymes • Needed for the breakdown of lactose, a simple sugar

  12. Components of Lac Operon

  13. Lac I Promoter gene Operator gene Lac Z Lac Y Lac A RNA polymerase Translation & Transcription Desired product R Repressor tetramer Inactive repressor

  14. Lac I Promoter gene Operator gene Lac Z Lac Y Lac A RNA polymerase No Gene Expression Translation & Transcription R

  15. Lac I Promoter gene Operator gene Lac Z Lac Y Lac A RNA polymerase Varying conditions No Gene Expression mRNA R R R R Lactose absent Repressor molecules Repressor tetramer

  16. Lac I Promoter gene Operator gene Lac Z Lac Y Lac A RNA polymerase Lactose/ Isopropyl Thiogalactoside (IPTG) present mRNA -galactosidase mRNA Inducer R R R R R Permease Thiogalactoside transacetylase Inactive repressor

  17. If there occurs no glucose metabolism

  18. If there occurs glucose metabolism CAP cAMP cAMP  Lac I Promoter gene Operator gene Lac Z Lac Y Lac A RNA polymerase Glucose pool gets depleted due to metabolism CAP-cAMP complex formed I -galactosidase mRNA R Permease IR Thiogalactoside transacetylase

  19. Trp Operon Model P/O Trp L Trp E Trp D Trp C Trp B Trp A

  20. Anthranilate synthetase Anthranilate synthetase Glycerol-phosphate synthetase Tryptophan synthetase (B protein) Tryptophan synthetase (A protein)

  21. Attenuation of Trp Operon Model

  22. AraOperon Model • L-arabinose operon model • Controlled by a dual positive and negative system • 3 structural genes: araB, araA, and araD • Encode the metabolic enzymes for breaking down arabinose for further metabolism via the HMP Shunt pathway

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