Chapter 16 Flashcards
Define inducible enzyme
Enzyme whose expression is environment-sensitive.
Define constitutive enzyme
Enzyme whose expression is constantly expressed at the same level
How are inducible enzymes organized?
Into operons
Define operon
Clusters of functionally related genes that are all regulated/expressed together
What 2 main things do operons have?
Repressor genes and operator sequences
Define operator sequence
Regions near the promoter that help regulate the operon
What does the lac operon do?
Catabolize lactose for energy.
Induced ON when lactose is present
What genes are found in the lac operon and what do they produce?
lacZ -> B-Galactosidase
LacY -> permease
LacA -> transacetylase
What does B-Galactosidase do
Breaks down lactose into galactose and glucose
What does permease do?
Codes or a pore that allows lactose into the cell
What does transectylase do?
Largely unknown - may possibly remove toxic byproduct
Define polycistronic mRNA
Multiple genes coded in the same mRNA - found only in prokaryotes
lac mRNA has 3 start & stop codons, making 3 proteins from 1 mRNA.
What is the repressor gene for the lac operon?
LacI , produces a repressor transcription factor that binds to the operator to silence the operon.
What happens when lactose is present?
Lac operon becomes active. Lactose binds to the repressor, causing it to detach and allowing RNA polymerase to transcribe the gene. The gene products then catabolize lactose.
What happens is glucose is abundant?
Lac operon is kept off. This is due to adenyl cyclase being inactive, which keeps cAMP Levels low. With no cAMP-CAP dimer present, they do not bind to the promoter and causes less lac transcription.
What happens if glucose is absent?
Adenyl cyclase is active, causing cAMP levels to rise. cAMP binds to CAP, which this complex then binsd to te promoter causing more lac transcription
What is adenyl cyclase?
Enzyme that turns ATP to cAMP
What is cAMP?
Cyclic adenosine monophosphate. The “spent” version of ATP
What is CAP?
Catabolize-activating protein. Transcriptional activator coded by different genes.
What does the trp operon do?
Tryptophan synthesis. Turns off when tryptophan is abundant.
mRNA shape influences operon activation.
What happens to trp operon with high tryptophan?
Operon begins transcription. Ribosome binds to new mRNA while its still being produced. If there is enough tryptophan in the cell, the ribosome can translate the new mRNA quickly. Once it has translated past a certain point, the mRNA forms a terminator hairpin, stopping RNA polymerase from continuing synthesis.
Define terminator hairpin
Causes transcription to stop early
What happens to trp operon with low tryptophan
The ribosome is translating new mRNA, but stalls because there isn’t enough tryptophan to add to the polypeptide. This causes mRNA to form an anti-terminator hairpin, which allows RNA polymerase to continue synthesizing. The continued synthesis allows tryptophan biosynthesis genes to be expressed
Define anti-terminator hairpin
mRNA shape that allows transcription to occur.
Define riboswitch
When an RNA molecule’s shape can shift between alternative forms, altering its function (aka altering translation/degradation)
What is an example of a riboswitch?
trp operon mRNA
What are the 3 steps to riboswitch?
- A molecule/ligand binds to RNA (usually a nutrient or metabolite)
- Causes a shape change of RNA
- Activation/inactivation of RNA occurs.
