DLW02 - The Lac Operon Flashcards
Define “regulator gene”
A regulator gene is a gene that codes for a product that controls the expression of other genes.
Define “operon”
An operon is a unit of bacterial gene expression and regulation, including structural genes and control elements in DNA recognised by regulatory gene products.
Describe negative control
In negative control, a repressor protein binds to an operator to prevent a gene from being expressed.
Describe positive control
In positive control, an activator (transcription factor) binds to the promoter to enable RNA polymerase to initiate transcription
How is induction achieved?
Induction can be achieved by activating an activator or inactivating a repressor.
How is repression achieved?
Repression can be achieved by activating a repressor or inactivating an activator
What is meant by the term “negative inducible”?
Negative - a repressor protein is involved.
Inducible - to activate the gene, the repressor protein must be inactivated.
The lacI gene shares the same promoter as the lac structural genes. TRUE or FALSE?
FALSE. The lacI gene has its own promoter and terminator.
State the product of the gene lacZ and its function
beta-galactosidase: metabolises complex beta-galactoside sugars such as lactose
State the product of the gene lacY and its function
permease: transports beta galactoside into cells
State the product of gene lacA and its function
transacetylase: transfers an acetyl group from acetyl-CoA to beta-galactoside
State the inducer of the lac operon
allolactose, the isomer of lactose
State the substrate of the lac operon
beta galactoside
beta-galactosidase (the protein) degrades at a similar rate as the mRNA encoding it. TRUE or FALSE?
FALSE. The lac mRNA is extremely unstable, and degrades before beta-galactosidase.
Define “gratuitous inducer”
A gratuitous inducer is an inducer that resembles authentic inducers of transcription but are not substrates for the induced enzymes. Hence, they cannot be metabolised and persist in the cell.
The lac repressor is a multimeric enzyme. How many subunits does it contain?
Four.
Briefly describe the formation of a tetrameric lac repressor.
The lac repressor is synthesised as a monomer. Two monomers form a dimer by making contacts between core 1 and 2.
Two dimers then form a tetramer by interactions between the tetramerisation (oligomerisation) helices.
Describe the structure of a monomer of the lac repressor
The monomer contains:
i) a helix-turn-helix DNA binding domain,
ii) two core subdomains
iii) an oligomerisation helix
iv) a hinge
v) an inducer binding site.
What is a helix-turn-helix?
A helix-turn-helix motif is one of the most common DNA binding motifs. All HTH DNA binding proteins bind DNA as dimers, in which the two copies are separated by exactly 3.4 nm (distance of the major groove)
Suggest 3 ways that may cause constitutive expression of the lac operon and explain why.
- Mutation to lac operator region which disallows lacR to bind and thus no repression of lac operator.
- Mutation to DNA-binding domain of lacR prevents it from binding to lac operator
- Mutation to CRP that allows it to become constitutively active without cAMP
What are cis-acting mutations?
Cis-acting mutations are mutations that affect only genes related to it on a contiguous stretch of DNA.
What are dominant mutations?
Dominance can be defined as the inability to restore the wild-type phenotype (of the mutated gene) if a wild-type copy is present.
What are trans-acting mutations?
Trans-acting mutations are mutations of the regulatory gene (i.e. trans-acting factors)
Give three examples of trans-acting mutations of the lac operon and describe them briefly
- lacI^(-): a recessive mutation in the lac repressor, causing it to be inactive.
- lacI^(-d): a dominant mutation in the lac repressor, preventing it from binding to the operator.
- lacI^(s): a mutation in the lac repressor, preventing it from binding to the inducer.
