regulation of prokaryotic gene expression Flashcards
what are housekeeping genes
genes that are always switched on
(genes that produce proteins involved in making amino acids, transcription, translation etc)
most regulation of gene expression in bacteria is:
transcriptional control - no mRNA synthesis
negative control of transcription
binding of a repressor protein to a regulatory DNA sequence preventing transcription of a gene
positive control of transcription
binding of an activator protein to a regulatory DNA sequence, inititiates transcription
what is an operon
cluster of bacterial genes undergoing transcriptional regulation
-genes in an operon participate in the same metabolic/biosynthetic pathway
how is lac operon an inducible operon system
turned on only when inducer compound is available = lactose
lac operon - what is the structural gene region -
has 3 protein coding genes - lacZ, lacY, lacA
lac operon - what is the regulatory region -
-CAP binding site
-promoter that binds RNA polymerase
-operator that binds lac repressor protein
structural gene region - what do the 3 genes encode
- lac Z - encodes B galactosidase, cleaves bond between lactose to form galactose and glucose
- lac Y - encodes lactose permease - uptake of lactose from the environment
- lac A - encodes transacetylase
3 genes transcribed as mRNA, then translated to 3 polypeptides
lacl gene produces the ……….. protein next to the lac operon
lac repressor protein that then binds to the operator of lac operon and allolactose binds to it (inducer)
breakdown of lactose produces small amount of ….. - inducer compound. slightly modified version of lactose
allolactose
when there’s no lactose present - lac operon expressed at very low levels
what happens
lac repressor protein binds to operator (lacO) sequence inhibiting transcription
when there’s lactose present - lac operon expressed - transcription
what happens
B-galactosidase produces allolactose which binds to lac repressor forming inducer-repressor complex
-repressor protein is now inactive because it is bound to allolactose so can’t bind to the operator
-RNA polymerase binds to promotor and carries out transcription
when there’s lactose present (no glucose) - cAMP is high or low
high cAMP binds to CAP
CAP-CAMP then binds to the lac promoter region stimulating a high level of transcription
when there’s no lactose present (glucose) - cAMP is high or low
low cAMP - prevents transcription
mutation in lacl gene -
mutant lac repressor protein can’t bind to operator region
-lac operon consitutively expressed - B-galactosidase always present
mutation in operator region (lacO) -
lac repressor protein can’t bind to mutated operator sequence
-lac operon constitutively expressed - B galactosidase always present
mutation in laclS (super lac repressor)
mutant repressor protein cannot bind to allactose
-so binds to operator constitutively
-SO transcription never happens even when there’s lactose
arabinose released in intestine after plant is eaten but not absorbed by intestine - it is a
carbon source for gut E.coli if glucose is absent
E.coli arabinose operon - contains
genes required for metabolism of arabinose
ara operon has a …. regulatory protein (araC) that carries out both positive and negative transcriptional regulation
single
arabinose operon - what happens when there’s no arabinose
araC protein monomers bind to araI and araO2, monomers link to one another forming DNA loop
-DNA loop stops RNA polymerase and CAMP-CAP binding to promoter, so ara operon is not expressed
arabinose operon - what happens when there is arabinose
arabinose breaks the connection between araC proteins at araI and araO2
-opens up DNA loop
-2nd araC-arabinose complex binds to araI and interacts with RNA pol
-CAP-CAMP binds CAP binding site - RNA pol binds to promotor and initiates transcription
correct order of steps that can be used to identify a gene whose mutation causes a genetic disorder
map the mutation to the arm of a chromosome, fine map the position of the mutation using other known genetic markers on that arm, identify candidate genes, use DNA sequencing to determine mutations
