Bacterial cells regulate gene expression in response to environmental changes through transcriptional control mechanisms. The lac and arg operons are two examples that are regulated in Escherichia coli. The lac operon encodes enzymes for lactose metabolism and is induced in the presence of lactose. The arg operon encodes enzymes for arginine biosynthesis and is repressed in the presence of arginine. Operons allow functionally related genes to be coordinately regulated at the level of transcription through repressors and inducers binding to operator sites on the DNA.

1. Microbial Genetics The Operon: Regulation of bacterial gene expression

2. Controlling enzymes We know that … Bacterial cells carry out a huge number of chemical reactions catalyzed by enzymes And ….. Bacterial cells respond rapidly to changing environments e.g., presence of a new carbon source So …. control of Enzymes is VERY important and must be efficient

3. Enzyme activity can be regulated by feedback inhibition of synthesized enzyme BUT making enzyme requires energy a a bacterium cannot make ALL enzymes ALLTHE TIME * SO it is better to stop synthesizing enzymes that are not needed THUS, many genes such as those for enzymes involved in lactose metabolism or amino acid production are regulated and only expressed when required Enzyme Regulation * However, 60-80% of genes are not regulated and are termed Constitutive

4. Regulation of gene expression Constitutive Enzymes – No regulation Enzyme Induced Enzyme Repressed

5. Regulation of gene expression 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. Thus, the expression levels of certain proteins can be controlled at the level of transcription (mRNA synthesis).

6. Transcriptional control Two genetic control mechanisms which regulate transcription of mRNA and, therefore the synthesis of corresponding protein Induction -turning on gene expression Repression -turning off gene expression

7. Repression Inhibition of gene expression Often in response to an excess of endproduct, shuts down synthesis of the enzyme Mediated by regulatory proteins called repressors Block RNA polymerase from initiating transcription Repressible genes are transcribed until they are repressed

8. Induction Turns on transcription of a gene A substance which acts to induce transcription of a gene is an inducer , often the substrate Enzymes synthesized in the presence of inducers are called inducible enzymes Example: lactose utilization genes Inducible genes are not transcribed until they are induced

9. The operon Multiple genes are arranged in the same orientation and are closely linked on the DNA Genes in an operon are transcribed on a single RNA transcript, but are translated individually to form multiple proteins A mechanism for coordinate control of genes involved in a single process by regulating transcription of the operon mRNA promoter operator mRNA proteins

10. The operon (cont…) The term “operon” not only includes the structural genes in the operon but also the regulatory sequences controlling transcription Promoter -site of RNA polymerase binding Operator -site of binding of a regulatory protein promoter operator mRNA

11. Operons and regulation About 27% of E. coli genes are in operons Many of the genes arranged in operons are regulated For example, genes for the lactose utilization are controlled by the presence of lactose genes for pathogenesis are often controlled by temperature or physiologic factor

12. Two regulated operons Two E. coli operons reflect how operons in general are regulated The lac operon Encodes inducible catalytic enzymes involved in the lactose utilization and uptake The arg operon Encodes repressible anabolic enzymes involved in the production of the amino acid arginine

13. The lac operon Three structural genes lacZ : β -galactosidase: splits lactose to glucose and galactose lacY : a permease involved in transport of lactose lacA : a transacetylase, function unknown A regulatory gene lacI The LacI protein is a repressor of the lac operon A promoter where RNA polymerase binds to transcribe the operon An operator site where the LacI repressor binds to block transcription

14. Arrangement of the lac operon

15. In the absence of lactose In the absence of lactose, the repressor binds to operator site preventing binding of RNA polymerase and transcription

16. In the presence of lactose The lac operon is an inducible operon Lactose acts as the inducer Lactose induces enzyme expression by binding to the LacI repressor preventing its binding to the operator site

17. The arg operon A repressible operon: genes are transcribed until turned off or repressed Three structural genes argCBH encoding enzymes in the arginine biosynthetic pathway

18. In the absence of arginine The arg operon is transcribed and enzymes for the synthesis of arginine are produced

19. In the presence of arginine Arginine acts as a corepressor and represses its own synthesis Like feedback inhibition but acts on enzyme synthesis rather than activity

20. Summary of gene regulation The production of many bacterial proteins are controlled by regulating transcription endproduct Functionally related genes are arranged in operons to allow coordinate regulation Operons can be Induced in the presence of substrate Repressed in the presence of endproduct Transcripition mechanisms can be more complex with DNA sequences known as activators and attenuators.