Question

1. Fill in the blanks in this table with “yes” or “no” for each condition of lac operon regulation. The strain is wild type, with no partial diploidy. The first line is filled in for reference. Please explain your answers!

Condition	Lactose levels high?	lac repressor bound to operator?	cAMP levels high?	CAP bound to CAP binding site?	Transcription at highest level?
high glucose no lactose	no	Yes	no	no	no
no glucose high lactose
high glucose high lactose
no glucose no lactose

Answer 1

Bacteria can grow on various nutrition sources. Glucose is the major source for growth.

• In presence of a single nutrition source (only glucose), the standard sigmoid growth curve is obtained.
• If bacteria are grown on two sugar sources, one being glucose, and other sugar (like lactose), two-phase growth or diauxic growth (diauxie) is observed.
• The bacteria utilize the simple sugar (class A substrate), like glucose first, followed by the other complex sugar (class B substrate). This ability of the bacteria to preferentially utilize one carbon source over the other, is called carbon catabolite repression or “Glucose effect”.
• This repression created by class A substrate, is mediated by: Phosphoenolpyruvate (PEP) dependent carbohydrate Phospho-Transferase System (PTS).
• PTS lowers cAMP, in turn repressing CAP-dependent operons of bacteria
• CAP along with cAMP senses glucose level.
• Such regulation is observed in control of gene expression in bacteria, such as Lactose (LAC) operon.
• LAC operon of E. coli is an example of inducible gene expression system.
• Lactose acts as inducer molecule in lac operon.

• The structural genes of lac operon includes-

1. lac Z- encodes enzyme β- galactosidase, which breaks lactose
2. lac Y- encodes permease enzyme
3. lac A- encodes transacetylase enzyme

• Controlling genes are-
• Promoter- Region for binding of RNA polymerase to initiate transcription of the structural genes present downstream of the promoter.
• Operator- Region where the repressor binds
• Lac i- represents encoding gene. The repressor and is under a separate promoter and not under lac promoter control.

• Lactose acts as inducer, which on binding to repressor, prevents it to bind to the operon and induces expression.
• When there is low glucose concentration, high levels of cAMP are generated that binds to CAP, promoting transcription initiation of lac operon genes.
• CRP on binding to cAMP alters its conformation which allows tight binding with DNA at the promoters. This activates transcription by protein-protein interaction with RNA polymerase.

Condition	Lactose levels high?	lac repressor bound to operator?	cAMP levels high?	CAP bound to CAP binding site?	Transcription at highest level?
high glucose no lactose	no	Yes	no	no	no
no glucose high lactose	Yes	no	Yes	no	Yes
high glucose high lactose	Yes	no	no	Yes	no
no glucose no lactose	no	yes	no	Yes	no