The lac operon

1. The lac operon

2. In bacteria, the genes involved in the same process are often clustered together. For example, the genes that allow E. coli to break down milk sugar (lactose) to produce energy. Lactose catabolism

3. lacY encodes lactose permease -transports lactose into the cell lacZencodes -galactosidase – enzyme that catalyses the reaction: lactose  glucose + galactose lacA encodes lactose transacetylase – biological function unclear. Lactose catabolism

4. These genes are controlled. E. coli is a successful competitor in the gut because it doesn’t waste time and energy making mRNA and proteins that are not needed. The lac genes are only transcribed if lactose is present in the growth medium. These genes are expressed co-ordinately. Either they are all switched on or they are all switched off. Lactose catabolism

5. The coordinate regulation arises from the clustering of the genes (strictly called CISTRONS) into a structure called an OPERON. There is also a regulatory gene, the lacIgene, that is not part of the operon. This produces a repressor protein that controls the operon. Lactose catabolism

6. The lac operon P lacI P lacO lacZ lacY lacA DNA RNA polymerase Transcription mRNA (polycistronic message) Active repressor binds to operator translation LacI repressor protein lactose permease lactose trans-acetylase -galactosidase enzyme Inactive repressor-effector complex lactose

7. The lac operon P lacI P lacO lacZ lacY lacA DNA RNA polymerase Transcription The three coding sequences lie side by side but there is only one promoter mRNA (polycistronic message) Active repressor binds to operator translation LacI repressor protein lactose permease lactose trans-acetylase -galactosidase enzyme Inactive repressor-effector complex lactose

8. The lac operon This means that there is only one mRNA that encodes three proteins. Each coding region has its own start and stop codon P lacI P lacO lacZ lacY lacA DNA RNA polymerase Transcription mRNA (polycistronic message) Active repressor binds to operator translation LacI repressor protein lactose permease lactose trans-acetylase -galactosidase enzyme Inactive repressor-effector complex lactose

9. The lac operon The separate lacI gene is not controlled. It has Its own promoter and encodes a repressor protein. It is not part of the operon P lacI P lacO lacZ lacY lacA DNA RNA polymerase Transcription mRNA (polycistronic message) Active repressor binds to operator translation LacI repressor protein lactose permease lactose trans-acetylase -galactosidase enzyme Inactive repressor-effector complex lactose

10. In the absence of lactose, the repressor protein binds to a special site in the operon called the OPERATOR and prevents RNA polymerase from moving along the DNA The lac operon P lacI P lacO lacZ lacY lacA DNA RNA polymerase Transcription mRNA (polycistronic message) Active repressor binds to operator translation LacI repressor protein lactose permease lactose trans-acetylase -galactosidase enzyme Inactive repressor-effector complex lactose

11. The lac operon P lacI P lacO lacZ lacY lacA DNA In the presence of lactose (effector), the repressor protein binds to the lactose and changes shape. It now falls off the operator and RNA polymerase can transcribe the operon RNA polymerase Transcription mRNA (polycistronic message) Active repressor binds to operator translation LacI repressor protein lactose permease lactose trans-acetylase -galactosidase enzyme Inactive repressor-effector complex lactose

12. The lac operon P lacI P lacO lacZ lacY lacA DNA RNA polymerase Lactose absent: operon switched off mRNA Active repressor binds to operator LacI repressor

13. The lac operon P lacI P lacO lacZ lacY lacA DNA RNA polymerase Transcription mRNA (polycistronic message) translation LacI repressor protein lactose permease lactose trans-acetylase -galactosidase enzyme Inactive repressor-effector complex lactose

14. The lac operon Jacob and Monod Jacob François Jacob and Jacques Monod worked out how the lac operon functioned and they formulated the operon hypothesis. Monod

15. The lac operon lac mutants The properties of various mutants allowed Jacob and Monod to work out how operons work.

16. DNA DNA The lac operon P O lacZ lacY lacA DNA mRNA Active -galactosidase enzyme protein Lactose catabolism X mRNA X X lacZ mutations are recessive protein

17. Mutants where the lacI gene has mutated, will grow on lactose. However they make β-galactosidase all of the time. These mutants that have lost the ability to control gene expression are called constitutive mutants. They are also recessive. The lac operon Constitutive mutants

18. The lac operon P lacI P lacO lacZ lacY lacA DNA RNA polymerase Active repressor binds to both operators LacI repressor P lacI P lacO lacZ lacY lacA X No active repressor to bind to operator X lacI mutations are recessive

19. Jacob & Monod realised that if their operon hypothesis was right, there should be another type of constitutive mutant – one where the operator has mutated so that the repressor cannot recognise it. Such mutants should be dominant and it should be possible to isolate them in a diploid. The lac operon A testable prediction

20. The lac operon P lacI P lacO lacZ lacY lacA DNA RNA polymerase Active repressor binds only to wild type operator LacI repressor P lacI P lacO lacZ lacY lacA DNA X mRNA lacOc mutations are dominant

21. Jacob & Monod mutated a diploid wild type to see whether they could get constitutive mutants. They did get them, and showed that they mapped to the operator region. This supported their hypothesis. The lac operon

22. The lac operon The lac repressor is an example of a negative regulatory protein, whose action prevents expression of the genes under its control and whose function is controlled by an effector molecule (in this case, lactose).

23. The lac operon Catabolic repression • The lac operon is also under the control of a positive regulatory protein. • E. coli’s preferred carbon source is glucose. • Glucose inhibits transcription of the lac operon, even in the presence of lactose. • Inhibition occurs in lacI and lacO mutants, as well as wild type, indicating the effect of glucose is NOT via the repressor-operator interaction.

24. The lac operon • The effect of glucose is mediated by a nucleotide, cyclic AMP (cAMP). • The intracellular concentration of cAMP is high in the absence of glucose and low in its presence. • cAMP binds to a catabolic activator protein (CAP), upstream of the lac promoter driving the lac operon. • When bound to cAMP, CAP enhances lac transcription.

25. The lac operon P lacO lacZ lacY lacA CAP DNA RNA polymerase Transcription mRNA (polycistronic message) translation protein Glucose cAMP lactose permease lactose trans-acetylase -galactosidase enzyme

26. The lac operon Summary • Regulation of expression of the lac operon is under two sets of controls, both of which are governed by environmental factors. • The repressor-operator interaction provides an “all or none” level of control (lactose  on). • [CAP-cAMP]-CAP-binding site interaction provides a modulatory control. (glucose levels control rate of mRNA initiation)

27. Complementation • Diploid, Haploid • Dominant, Recessive • Homozygous, Heterozygous • Cistron • Cross-feeding

28. Colinearity of the gene and protein • Protein structure • Haemoglobin • Genetic code  amino acid sequence

29. The Genetic code • Codon • Dictionary of the genetic code • How the code was deciphered • How the code works

30. tRNA and Translation • RNA translation (5'  3') • Ribosomes • Structure of tRNA • Anticodon • Mechanism of translation • Wobble hypothesis

31. Inosine (I) is a rare base found in tRNA, often in the anticodon, capable of binding to adenine, uracil or cytosine.

32. RNA Translation •  RNA growth (5'  3') • RNA polymerase, structure and properties • Promoter, consensus • Mechanism of translation • Termination

33. Suggested reading Regulation of gene transcription (2000) In: An Introduction to Genetic Analysis. pp 336-344. Griffiths, A. J. F,. Miller, J. H., Suzuki, D. T., Lewontin, R. C. and Gelbart, W. M. (Eds). Freeman and Company, New York.