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BIOL 221 Microbiology The lac operon KEY 2008 Name: ____________________________ 1. The “box” below represents the lac operon. Indicate with boxes the relative positions of all the regulatory regions and the three genes. Label the regulatory regions and use genes 1 – 3 for the coding genes. CAP binding site Promoter Operator Gene1 Gene2 Gene3 CAP 2. What role does –galactosidase play in lactose metabolism? It breaks down lactose (a disaccharide) into galactose and glucose (monosaccharides). 3. What is the inducer of the lac operon? __Lactose or allolactose__ (note: cAMP is an activator) 4. What role does the repressor (product of the lacI gene) play in control of transcription of the lac operon? It is inhibits transcription when physically bound to the regulatory region. 5. What effect does the inducer have on the lacI gene product? It has an allosteric effect on the repressor, changing its shape so it can no longer bind to DNA (the operator site). 6. What is the region of the lac operon where the repressor protein binds to the DNA called? _operator_ 7. Where is this in relation to the promoter region of the lac operon? ___downstream___ 8. Why is transcription blocked when the lacI inhibitor is bound to DNA? It physically prevents the RNA polymerase from moving downstream and transcribing the coding (structural) genes of the lac operon. 9. Why is this type of gene regulation called “negative”? Transcription is “turned off” by the presence of a repressor. 10. What is the activator protein called that positively regulates the lac operon? Spell it out. Catabolite activator protein or cAMP receptor protein 11. Fill in the following table. Growth Medium presence of glucose, no lactose presence of glucose and lactose presence of lactose, no glucose 12. lacI bound to DNA? (Y or N) Y CAP bound to DNA? (Y or N) N lac operon turned on? (Y or N) N N N N Y N leaking expression Y effective –galactosidase enzyme in the constitutive mutants different from normal E. coli cells? The enzyme is expressed all the time, regardless of the environment. 13. What genetic experiment did Jacob & Monod perform to demonstrate that the constitutive mutants were lacI-, i.e. defective in the lacI gene? (Think about the transfer of DNA). Conjugation of a bacterium containing an F’ plasmid that has a normal lacI gene with the constitutive mutant (female) restored normal regulation of the lac operon in the mutant.