Unit 6 - Prokaryotic Gene Control: Operons

prokaryotic metabolism

prokaryotes must use substances and synthesize macromolecules fast enough to meet their needs, if enzymes aren’t needed genes for their production must be “off”, conservation of resources

operons

control synthesis of same protein, cluster of proteins, genes grouped together operating together in prokaryotic cells

what is required for metabolism

coordinated control of genes

operons: inducible

can be inducible, normally “off” but able to turn “on” when needed

operons: repressible

can be repressible, normally “on” but able to turn “off” when not needed

promoter

region of DNA that signals for RNA polymerase to bind, 1 for all structural genes

operator

region of chromosome to which the repressor binds when the operon is turned “off”, on and off switch, allow or don’t allow, RNA polymerase binds to promoter and starts transcription, if blocked won’t get to genes to start transcription

repressor

a protein that suppresses the transcription of a gene, binds to operator, always being synthesized

regulatory gene

codes for production of the repressor protein

structural gene

codes for enzyme product

inducer

molecule that binds to and changes repressor’s shape

trp operon: repressible

genes are normally “on” and produce tryptophan for the cell, if tryptophan is present genes are turned “off” and tryptophan in the environment is utilized

trp operon: normal “on” position

cell requires tryptophan for metabolism, typically has to make if for itself, to do this: repressor is inactive and not bound to the operator, promoter is available to RNA polymerase, structural gene is transcribed, mRNA is translated into enzymes, enzymes produce tryptophan

trp: “on” position

no tryptophan present in environment, cell makes it own tryptophan producing enzymes

if trp is abundant: turning “off”

if the cell has adequate supply of tryptophan it turns the operon “off” and uses what is available, save resources don’t make it if it’s already there, to do this: tryptophan binds to the repressor activating it, activated repressor binds to operator, this blocks RNA polymerase from transcribing mRNA, no genes are transcribed means no proteins produced

trp: “off” position

tryptophan is present in the environment, no need to produce tryptophan producing enzymes

lac operons: inducible

genes are normally turned “off” in absence of lactose, if lactose is present genes are turned “on” and enzymes to digest it are produced

lac: normal “off” position

if no lactose is present in the cell no lactose digesting enzymes are produced, save resources don’t make it if it’s not needed, to do this: active repressor binds to operator, this blocks RNA polymerase from transcribing mRNA, no genes are transcribed means no proteins produced

lac: “off” position

no lactose in environment, no need to produce lactose digesting enzymes, can’t break it down because it is not there

if lac is present: turning “on”

lactose is not normally present, if it is bacteria will digest it and use it, to do this: allolactose inducer binds to repressor inactivating it, inactivated repressor cannot bind to operator, RNA polymerase can bind to promoter, genes coding for lactose digesting enzymes transcribed, mRNA is translated into enzymes that digest lactose, when lactose supply is depleted inducer separates from repressor allowing it to bind to operator again and block transcription

lac: “on” position

lactose is present in environment, enzymes to digest it are produced