Control of gene expression in prokaryotes Flashcards
Define the initiation rate
number of mRNA molecule whose synthesis is initiated per unit of time
What two factors impact the initiation rate
- consensus sequences
2. Proteins that change the affinity of the RNA polymerase for the promoter
What are the two consensus sequences in the promoter region
- TATAA
2. TTGACA
What are the two types of transcription control
Positive and negative control
Define positive control
Genes under positive control will only be expressed when an activator protein is present
Define Negative control
Genes under negative control are expressed unless a repressor protein is present to block their transcription
Define an inducer (in context of repressors)
A ligand that binds to a repressor protein, causing it to detach
Define a co-repressor
A ligand that is needed to help bind repressor to DNA
Define an inducer (in context of activators)
It binds the activator, allowing it to bind DNA and machinery
Define catabolic inhibition
The process by which genes coding for certain catabolic machinery is turned off when not needed.
What are the two main components of the lac operon?
- cis-acting regulator elements
2. protein-coding structural genes
What are the four products of the lac operon?
LacI
Lac Z
Lac Y
LacA
Define the lac Z gene
codes for β-galactosidase, the enzyme that breaks the glycoside bond in
lactose
Define the lacY gene
codes for lactose permease, the protein that transports lactose in the cell
Define the lacA gene
Codes for sulfogalactoside transacetylase; this enzyme plays a role detoxification of
thiogalactosides that are also transported into the cell by the lacY gene product.
What type of control is the lactose repressor an example of?
Negative control:
- controlled by both a repressor and an activator
Define the repressor of the lactose operon
- coded in the lacI gene
- binds to the operator sequence, which overlaps with the start sequence, inhibiting it
- Forms a tetramer
Define allolactose
- Intermediate of lactose metabolism
2. binds to repressor protein, causing it to dissociate from the DNA
What kind of control is demonstrated in the activator of the lactose operon?
Positive control
What is the activator for the lactose operon?
CAP (catabolic activator protein)
-acts as a dimer, increases affinity of RNA pol. for promoter
cAMP must be present for CAP to bind DNA. What influences the relative presence of cAMP?
intracellular concentrations of glucose. As glucose conc. goes up, cAMP conc. goes down.
Status of operon when glucose and lactose present
Operon is off, because glucose inhibits cAMP, therby preventing the binding of CAP to DNA
Status of operon when only glucose present
repressor is bound, due to absence of allolactose. Therefore, operon is off
Status of operon when both lactose and glucose are absent
operon is off, lac repressor is still bound. However, CAP is bound as well, since absence of glucose allows presence of cAMP
Status of operon when only lactose present
CAP binds, allolactose removes repressor, operator is ON
