prokaryotic transcription regulation Flashcards
why is gene expression regulated?
prokaryotes
conservation of energy
housekeeping genes - constituitively expressed
induced or repressed genes
how is gene expression regulated?
prokaryotes
transcription regulation
mRNA hydrolysis > prevents translation
protein degradation
inhibition of protein function
components of transcription regulation
prokaryotes
the promoter
the sigma factor
RNA polymerase
transcription factors - activators and repressors
operons
operator
inducers and co-repressors
the promoter
prokaryotes
a sequence of DNA adjacent to the gene where transcription factors bind and RNA polymerase is recruited to initiate transcription
strong promoters conform to the consensus sequence and initiate transcription frequently
contains specific motifs, the -10 and -35 regions (Pribnow box)
the sigma factor
prokaryotes
subunits of RNA polymerase that direct the enzyme to specific promoters
specific -10 and -35 sequences bound by specific sigma factors allow coordinated gene expression
transcription factors
prokaryotes
repressor proteins > negative regulation, blocks transcription
activator proteins > positive regulation, stimulates transcription
regulation of a metabolic pathway
prokaryotes
allosteric regulation of enzyme-catalysed reactions > allows rapid fine-tuning
regulation of gene expression > slower but conserves resources
an operon
prokaryotes
a cluster of genes with a single promoter are transcribed together into a single mRNA to allow genes to be co-regulated
consists of:
* a promoter
* two or more structural genes
* an operator > a short stretch of DNA between the promoter and the structural genes
the lac operon
prokaryotes
an inducible system
the lac operon
lactose absent
prokaryotes
genes are ‘off’
the repressor protein encoded by gene i prevents transcription by binding to the operator
RNA polymerase cannot bind to the promoter so transcription is blocked
no mRNA is produced, so no enzyme for lactose breakdown is available
the lac operon
lactose present
prokaryotes
genes are ‘on’
lactose induces transcription by binding to the repressor, which then cannot bind to the operator, RNA polymerase binds to the promoter
RNA polymerase can then transcribe the genes for the enzymes β-galactosidase, permease, and transacetylase
inducible operon
control of operons
regulated by a repressor protein and a metabolic substrate
e.g. the lac operon
controls catabolic pathways
*only on when induced *
repressible operon
control of operons
regulated by a repressor protein following binding of a co-repressor
e.g. trp operon
control anabolic pathways
*always on until repressed *
low glucose, lactose present
regulation of the lac operon by lactose and glucose
low glucose levels cause CRP to bind to cAMP and the CRP-cAMP complex then binds to the promoter, allowing more efficient binding of RNA polymerase and the sigma factor to the promoter which increases transcription and the lac operon is activated.
high glucose, lactose present
regulation of the lac operon by lactose and glucose
high glucose levels mean cAMP levels are low so CRP doesnt bind to the promoter resulting in inefficient binding of RNA polymerase to the promoter so transcription levels and the lac operon is reduced
the promoter is weak so requires CRP to stimulate transcription
