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Prokaryotic regulation

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REGULATION OF GENE EXPRESSION

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Prokaryotic regulation

  1. 1. REGULATION OF GENE EXPRESSION
  2. 2. DEFINITION  Gene expression refers to the multistep process that ultimately results in the production of a functional gene product, either ribonucleic acid (RNA) or protein Regulated genes are expressed only under certain conditions It is the basis for cellular differentiation, morphogenesis, and adaptability of any organism
  3. 3. LEVEL OF GENE REGULATION  Prokaryotes: Transcription  Eukaryotes : Transciption and post transcriptional and post-translational processes
  4. 4. Control of gene expression.
  5. 5. REGULATORY SEQUENCES AND MOLECULES Regulation of transcription is controlled by regulatory sequences of DNA  Embedded in the noncoding regions of the genome. The interaction between these DNA segments and regulatory molecules (transcription factors) can engage or repress the transcriptional machinery
  6. 6. TYPES OF TRANSCRIPTION FACTORS a) Cis acting elements:  Regulatory sequences flanking a gene  Influence gene expression only on the same chromosome b) A trans-acting factor  Proteins which transit through the cell from its site of synthesis to its DNA-binding site
  7. 7. Cis-acting elements and trans-acting molecules
  8. 8. REGULATION OF PROKARYOTIC GENE EXPRESSION  Primarily at the level of transcription  By the binding of trans-acting proteins to cis-acting regulatory elements on their single DNA molecule  Affects the initiation or premature termination of transcription
  9. 9. OPERON  The structural genes that code for proteins involved in a particular metabolic pathway are often found sequentially grouped together along with the cis- acting regulatory elements  The genes are, thus, coordinately turned on or off as a unit.  This entire package is called as operon  Eg, Lactose operon, tryptophan operon
  10. 10. Role of operators in prokaryotic transcription  OPERATOR: A segment of DNA that regulates the activity of the structural genes of the operon  REPRESSOR: As long as the repressor is bound to the operator, no proteins are made.  INDUCER: binds to the repressor, causing the repressor to change shape so that it no longer binds the operator.
  11. 11. COMPONENTS OF REGULATION RNA Poly. Promoter Operator Repressor Gene/ Operon
  12. 12. OPERATOR REPRESSOR BINDS OPERATOR R.P III CANT PROCEED HALTS TRANSCRIPTION INDUCER CHANGES REPRESSOR SHAPE REPR.CANT BIND OPERATOR TRANSCRIPTION PROCEEDS
  13. 13. LACTOSE OPERON It contains the genes that code for three proteins involved in the catabolism of the disaccharide lactose 1. lacZ gene: β-galactosidase 2. lacY gene : permease 3. lacA gene : transacetylase
  14. 14. THE REGULATORY COMPONENTS The regulatory portion has 3 sites: 1. Promoter : Binds RNA polymerase 2. Operator (O) site : Binds repressor encoded by lac-l gene 3. CAP site: Binds cAMP & CAP (Catabolite Activator Protein)
  15. 15. Regulatory Sites & Their Binding Factors Lac-l CAP Site Promoter Operator Gene RepressorRNA Poly. C AMP + CAP
  16. 16. PRE-REQUISITES FOR GENE EXPRESSION The lacZ, lacY, and lacA genes are expressed only when:  The O site is empty  The CAP site is bound Lac-l CAP site Promoter Operator Genes RNA Poly. C AMP +CAP
  17. 17. THREE SITUATIONS 1. ONLY GLUCOSE AVAILABLE 2. ONLY LACTOSE AVAILABLE 3. BOTH AVAILABLE
  18. 18. I.ONLY GLUCOSE Glucose  inactive adenyl cyclase no cAMP CAP remains unbound CAP site remains empty promoter cannot initiate transcription OPERON REPRESSED
  19. 19. I.ONLY GLUCOSE
  20. 20. II.ONLY LACTOSE 1. Absence of glucose, adenylyl cyclase is active cAMP made  bind CAP protein  The cAMP–CAP complex binds CAP site  RNA polymerase initiates transcription 2. Lactose allolactose (Inducer) induces repressor’s shape change cant bind opertaor operator empty RNA polymerase can proceed OPERON INDUCED
  21. 21. II.ONLY LACTOSE
  22. 22. III.BOTH LACTOSE AND GLUCOSE ALTHOUGH Repressor not bound to the operator region BUT Glucose  inactive adenyl cyclase no cAMP CAP remains unbound CAP site remains empty promoter cannot initiate transcription OPERON REPRESSED
  23. 23. III.BOTH GLUCOSE & LACTOSE
  24. 24. Regulation at the level of ribosomes I.Stringent response in Ecoli:  Regulation in response to amino acid starvation is known as the stringent response.  The binding of an uncharged transfer RNA (tRNA) to the A site of a ribosome  Triggers a series of events that leads to the production of a polyphosphorylated guanosine, ppGpp.  Catalyzed by stringent factor (RelA)  Elevated levels of ppGpp result in inhibition of rRNA synthesis
  25. 25. Events after empty tRNA binds A site
  26. 26. II.Regulatory ribosomal proteins  Operons for ribosomal proteins (r-proteins)  Can be inhibited by an excess of their own protein products  For each operon, one specific r-protein functions in the repression of translation  It binds Shine-Dalgarno (SD) sequence and inhibits binding of the small ribosomal subunit to the SD sequence.  Also binds to rRNA and with a higher affinity than for mRNA again inhibiting translation  If the concentration of rRNA falls, the r-protein then is available to bind its own mRNA and inhibit its translation.

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