Chapter 12 Flashcards
Types of gene expression in bacteria
- Constitutive transcription
- Regulated Transcription
- Post-transcriptional regulation
Constitutive transcription
Essentially constant expression of genes that are needed continuously (housekeeping genes)
Regulated transcription
Expression that occurs under certain conditions (when there is an environmental change or a lack of a crucial enzyme; requires DNA-protein interactions)
Posttranscriptional regulation
After mRNA is synthesized, its abundance can be modified in various ways to influence the amount of protein that is translated
Negative control: Repressors and inducers and corepressors
- In the absence of an inducer the repressor blocks transcription
- the inducer causes an allosteric change in the repressor, causing it to release from DNA and allow transcription
- with corepressor present, the repressor blocks transcription
Allostery
When interaction between proteins change their conformation and function
Positive control: activators and effectors
- Without effector, no transcription even in presence of activator
- with effector, transcription is activated
Positive control: activator and inhibitor
- With inhibitor, no transcription - binds to activator preventing function
- Without inhibitor, activator can bind and transcription is activated
Lac Operon
- Wildtype function: a repressor binds when inducer is absent and prevent transcription
- Prevents synthesis of enzymes that are not needed
Beta-galactosidase
- Breaks the beta-galactosidase linkage of lactose to produce glucose and galactose
- Can also convert lactose to allolactose which acts as an induced compound
Permease
Imports lactose
Operon
A cluster of genes undergoing coordinated transcriptional regulation by a shard regulatory region
lacl
a regulatory gene that is adjacent to but not part of the lac operon
lac operon
Inducible polycistronic mRNA
- includes regulatory region, lacZ (beta-galactosidase gene), lacY (permease gene), lacA (transacetylase gene)
Lac operon promotor region
- CAP binding site
- lacP (promoter sequence): RNA polymerase binds to the promoter sequence
- lacO (operator): lacl repressor binds to the operator sequence and blocks transcription of the lac operon
Lactose unavailable, glucose available
- lacl transcribes mRNA for repressor protein
- repressor protein binds to operator (lacO) inhibiting transcription
Lactose and glucose available
- Allolactose acts an inducer to inhibit repressor protein
- lac operon is transcribed at a basal level, which leads to allolactose production for lactose
Cis-regulatory elements
Only influence transcription of genes to which they are physically connected on the same chromosome
Trans-Regulatory elements
Influence transcription via diffusible proteins and do not have to be on the same chromosome
Repressor mutation
prevents repressor from binding to operator; leads to constitutive expression, even in the absence of the inducer
Operator constitutive mutation
Prevents repressor from binding; leads to constitutive expression, even in the absence of the inducer
Super repressor mutation
Prevents inducer from suppressing repressor; transcription repressed even in presence of the inducer
Catabolite repression by glucose
In the presence of cAMP, CAP binds the promoter and increases RNA polymerase activity
- Catabolites produced by breakdown of glucose prevent cAMP production
- cAMP binds to CAP, complex then binds to chromosome enhancing RNA polymerase binding
- Low glucose, lots of cAMP = higher expression
- high glucose, low cAMP = lower expression
trp operon
polycistronic and includes 5 gens that work together to synthesize the amino acid tryptophan
- trpE, trpD, trpC, trpB, trpA
- the order of these genes corresponds to sequential steps of tryptophan synthesis
trp operon repressor and co-repressor
- Tryptophan acts as a co-repressor
- repressor is activated by the corepressor and binds to trpO to block gene expression
- level of expression influenced by the level of tryptophan (attenuated)
trp operon attenuation
In prokaryotes translation and transcription occur in same space
- during early transcription, the mRNA of the trp operon forms multiple secondary structures that influence if transcription continues or not
- if secondary structure forms between regions 3 and 4 trpL is transcribed by the rest is not
- if between 2 and 3 the rest of the operon is transcribed
What influences which secondary structure forms
- Slow translation of trpL due to a lack of tryptophan causes the hairpin which allows the continuation of transcription
- Rapid translation of trpL due to lots of tryptophan causes other structure which leads to termination
Alternative sigma factors for stress responses
- Normally (37 degrees) a subunit of the RNA polymerase holoenzyme call sigma 70 is translated and incorporated into the holoenzyme. Together they drive expression of a specific subset of genes
- At high temperatures (42) a difference sigma factor is translated and is incorporated into the RNA polymerase holoenzyme, and drives expression of another specific subset of genes
Riboswitches
- riboswitches in mRNA can cause inhibition of translation of transcripts involved with thiamin production
- When thiamin is low, translation of the operon continues: when thiamin is high, riboswitches cause ribosomal recognition sequences and the start codon to be inaccessible to 16S rRNA and other translation machinery
- Riboswitches can also affect transcription and RNA stability
