Genetics c


Published on

Published in: Technology
1 Like
  • Be the first to comment

No Downloads
Total views
On SlideShare
From Embeds
Number of Embeds
Embeds 0
No embeds

No notes for slide

Genetics c

  1. 1. Microbial Genetics The Operon: Regulation of bacterial gene expression
  2. 2. Controlling enzymes <ul><ul><li>We know that </li></ul></ul><ul><ul><li>… Bacterial cells carry out a huge number of chemical reactions catalyzed by enzymes </li></ul></ul><ul><ul><li>And ….. Bacterial cells respond rapidly to changing environments e.g., presence of a new carbon source </li></ul></ul><ul><ul><li>So …. </li></ul></ul><ul><ul><li>control of Enzymes is VERY important and must be efficient </li></ul></ul>
  3. 3. <ul><li>Enzyme activity can be regulated by feedback </li></ul><ul><li>inhibition of synthesized enzyme </li></ul><ul><ul><li>BUT making enzyme requires energy a a bacterium cannot make ALL enzymes ALLTHE TIME * </li></ul></ul><ul><ul><li>SO it is better to stop synthesizing enzymes that are not needed </li></ul></ul><ul><li>THUS, many genes such as those for enzymes involved in lactose metabolism or amino acid production are regulated and only expressed when required </li></ul>Enzyme Regulation <ul><li>* However, 60-80% of genes are not regulated and are termed Constitutive </li></ul>
  4. 4. Regulation of gene expression Constitutive Enzymes – No regulation Enzyme Induced Enzyme Repressed
  5. 5. Regulation of gene expression <ul><li>The process of transcription plays an important role in the ability of bacteria to respond to changing environments: mRNA compared to DNA is inherently unstable. </li></ul><ul><li>Thus, the expression levels of certain proteins can be controlled at the level of transcription (mRNA synthesis). </li></ul>
  6. 6. Transcriptional control <ul><li>Two genetic control mechanisms which regulate transcription of mRNA and, therefore the synthesis of corresponding protein </li></ul><ul><li>Induction -turning on gene expression </li></ul><ul><li>Repression -turning off gene expression </li></ul>
  7. 7. Repression <ul><li>Inhibition of gene expression </li></ul><ul><li>Often in response to an excess of endproduct, shuts down synthesis of the enzyme </li></ul><ul><li>Mediated by regulatory proteins called repressors </li></ul><ul><ul><li>Block RNA polymerase from initiating transcription </li></ul></ul><ul><li>Repressible genes are transcribed until they are repressed </li></ul>
  8. 8. Induction <ul><li>Turns on transcription of a gene </li></ul><ul><li>A substance which acts to induce transcription of a gene is an inducer , often the substrate </li></ul><ul><li>Enzymes synthesized in the presence of inducers are called inducible enzymes </li></ul><ul><ul><li>Example: lactose utilization genes </li></ul></ul><ul><li>Inducible genes are not transcribed until they are induced </li></ul>
  9. 9. The operon <ul><li>Multiple genes are arranged in the same orientation and are closely linked on the DNA </li></ul><ul><li>Genes in an operon are transcribed on a single RNA transcript, but are translated individually to form multiple proteins </li></ul><ul><ul><li>A mechanism for coordinate control of genes involved in a single process by regulating transcription of the operon mRNA </li></ul></ul>promoter operator mRNA proteins
  10. 10. The operon (cont…) <ul><li>The term “operon” not only includes the structural genes in the operon but also the regulatory sequences controlling transcription </li></ul><ul><ul><li>Promoter -site of RNA polymerase binding </li></ul></ul><ul><ul><li>Operator -site of binding of a regulatory protein </li></ul></ul>promoter operator mRNA
  11. 11. Operons and regulation <ul><li>About 27% of E. coli genes are in operons </li></ul><ul><li>Many of the genes arranged in operons are regulated </li></ul><ul><ul><li>For example, </li></ul></ul><ul><ul><ul><li>genes for the lactose utilization are controlled by the presence of lactose </li></ul></ul></ul><ul><ul><ul><li>genes for pathogenesis are often controlled by temperature or physiologic factor </li></ul></ul></ul>
  12. 12. Two regulated operons <ul><li>Two E. coli operons reflect how operons in general are regulated </li></ul><ul><li>The lac operon </li></ul><ul><ul><li>Encodes inducible catalytic enzymes involved in the lactose utilization and uptake </li></ul></ul><ul><li>The arg operon </li></ul><ul><ul><li>Encodes repressible anabolic enzymes involved in the production of the amino acid arginine </li></ul></ul>
  13. 13. The lac operon <ul><li>Three structural genes </li></ul><ul><ul><li>lacZ : β -galactosidase: splits lactose to glucose and galactose </li></ul></ul><ul><ul><li>lacY : a permease involved in transport of lactose </li></ul></ul><ul><ul><li>lacA : a transacetylase, function unknown </li></ul></ul><ul><li>A regulatory gene lacI </li></ul><ul><ul><li>The LacI protein is a repressor of the lac operon </li></ul></ul><ul><li>A promoter where RNA polymerase binds to transcribe the operon </li></ul><ul><li>An operator site where the LacI repressor binds to block transcription </li></ul>
  14. 14. Arrangement of the lac operon
  15. 15. In the absence of lactose <ul><li>In the absence of lactose, the repressor binds to operator site preventing binding of RNA polymerase and transcription </li></ul>
  16. 16. In the presence of lactose <ul><li>The lac operon is an inducible operon </li></ul><ul><li>Lactose acts as the inducer </li></ul><ul><li>Lactose induces enzyme expression by binding to the LacI repressor preventing its binding to the operator site </li></ul>
  17. 17. The arg operon <ul><li>A repressible operon: genes are transcribed until turned off or repressed </li></ul><ul><li>Three structural genes argCBH encoding enzymes in the arginine biosynthetic pathway </li></ul>
  18. 18. In the absence of arginine <ul><li>The arg operon is transcribed and enzymes for the synthesis of arginine are produced </li></ul>
  19. 19. In the presence of arginine <ul><li>Arginine acts as a corepressor and represses its own synthesis </li></ul><ul><li>Like feedback inhibition but acts on enzyme synthesis rather than activity </li></ul>
  20. 20. Summary of gene regulation <ul><li>The production of many bacterial proteins are controlled by regulating transcription endproduct </li></ul><ul><ul><li>Functionally related genes are arranged in operons to allow coordinate regulation </li></ul></ul><ul><ul><li>Operons can be </li></ul></ul><ul><ul><ul><li>Induced in the presence of substrate </li></ul></ul></ul><ul><ul><ul><li>Repressed in the presence of endproduct </li></ul></ul></ul><ul><li>Transcripition mechanisms can be more complex with DNA sequences known as activators and attenuators. </li></ul>