THEME 3 MOD 2 Flashcards
(19 cards)
What proteins are only detectable when lactose is the only energy source for e coli bacteria cells
beta galactosidase enzyme and lactose permease proteins, expressed for transport and metabolism of lactose, only when glucose is fully depleted
What happens once glucose is depleted from the medium for e coli cells
e coli growth stops
if lactose is present, lactose becomes energy source
for this to happen, beta galactosidase and lactose permease protein genes need to be expressed
once lactose becomes the energy source for these cells, a steady increase in concentration of the two proteins involved in lactose metabolism is apparent
lactose permease protein
transport protein that sits if the cell membrane of bacteria cells to allow transport of lactose into the cell
beta galactosidase
enzyme in cytoplasm of bacteria cells that cleaves lactose into glucose and galactose monosaccharides
what is advantageous about the organization of genes in prokaryotes
related genes are grouped together into operons and are transcribed as one unit
what is an operon and who established the operon model
francois jacob and jaques monod discovered this gene expression control
operon is a promoter, operator, and clustered gene sequence that acts as a mechanism for control of gene expression from control of transcription
What is the operator of an operon
“on/off” switch of gene
LacO coding sequence
sequence of nucleotides at start of operon that can allow/inhibit transcription
if the operator is not bound to an inhibitor, rna polymerase can transcribe the gene cluster
operator is binding site for repressor protein (lacI coding sequence) which can inhibit transcription
operon and repressor protein are a negative transcriptional regulation process
what operon controls transcription of beta galactosidase and lactose permease proteins?
lac operon
role of promoter
where rna polymerase binds to transcribe gene cluster sequence
what is polycistronic mrna
mrna molecule that can code for many proteins and can produce seperate polypeptide products within translation as each gene has stop and start codons that signal the beginning and end of each coding sequence
identify LacO, LacY, LacZ, Lac I
LacO: operon sequence
LacY: codes for lactose permease protein
LacZ: codes for beta galactosidase
LacI: codes for repressor protein that binds to operator, controls expression of LacY and LacZ genes
what does negative regulation encapsulate?
- lactose operon is negatively regulated, where transcription for the genes needed to metabolize lactose is turned off
the lac operon is turned off most of the time there is glucose present for e coli bacteria as lacI repressor protein is expressed at low levels
repressor protein of lac operon in e coli
tetrameric protein (four identical protein subunits) binds tightly to operator regions. when all 4 subunits bind to lac operon dna, dna is twisted into a loop and rna polymerase cannot bind to the promoter region since it has undergone conformational change
glucose in the growth media for e coli facilitates expression of the repressor protein
allosteric inhibition of repressor protein
lactose, when present in the cell binds to the repressor protein on specific binding sites as an inducer molecule and conformationally changes the repressor protein so it can no longer bind to the lac operon dna
positive regulation of the lac operon
decrease in environmental glucose levels causes an increase in intracellular cAMP (due to increased enzyme activity of adenylyl cyclase)
high glucose levels inhibit the enzyme adenylyl cyclase which catalyzes production of cAMP from ATP
CRP (cAMP receptor protein) or CAP (catabolite activator protein) bound by cAMP will bind to a CRP-cAMP binding site of bacterial DNA
CRP-cAMP complex
cAMP binds to CRP as allosteric activator
conformational change in transcriptional activator protein
complex can bind to bacterial dna to further activate transcription
even in the precence of lactose (which unbinds the repressor protein), the CRP-cAMP complex wont bind to the DNA until there are low levels of glucose, meaning CRP-cAMP makes sure there are low levels of transcription
activator/binding site can be upstream, downstream or within promoter
once activator binds, rna polymerase can be recruited to the promoter
methods of positive regulation of lac operon
- low glucose signal high cAMP to bind to CRP-cAMP (positive regulator)
- lactose binds to repressor protein LacI so it cannot bind to operator
what summarizes the lac operon two part control mechanism
negative and positive transcriptional regulation
Where may lac operon mRNA levels vary
highest: lactose is actively used as energy source (lac operon genes are being transcribed to metabolize lactose)
very low: lac operon transcription initiating, glucose has just been used up
no lac operon mrna: lac operon transcription is being repressed as glucose is present in environment
