Prokaryotic Gene Regulation

genetic and epigenetic regulation

regulation of gene expression

genome analogy

it is like a cookbook with 20,000 recipes, and different people use different recipes, cells are like this and will not use all of the recipes

regulation of gene expression in prokaryotes

mostly transcriptional regulation because as soon as mRNA is available, before transcription is finished, ribosome starts to synthesize protein in 5’ to 3’ direction (fast)

transcriptional regulation

can be positive and negative

repressor protein

• binds to repressor binding site

• negative regulator

• in absence of this regulator transcription does occur

negative regulation

off switch

polymerase binding site

• promoter

• tells RNA polymerase where the start of the gene is

positive regulation

“on” switch

activator protein

• binds to activator binding site which encourages RNA polymerase to initiate transcription

• in absence of this regulator, transcription does not occur

trp operon

example of negative regulation in prokaryotes

tryptophan

small amino acid that bacteria and humans need

operon

portion of chromosome made of DNA; genes, promoter, and regulatory sites

operator

repressor binding site

trp operon polycistronic mRNA

• 5 genes that code for enzymes that make tryptophan

• they make mRNA that contains the codes for the genes

• each has a Shine-Delgarno and AUG at the beginning of each cell that a ribosome plops down at

trp operator process

• tryptophan binds to repressor which then binds to the operator and blocks transcription

• if there is no trp then the repressor will not bind and transcription will occur which will make the enzymes that make tryptophan

what type of binding occurs between tryptophan and repressor

allosteric binding because the tryptophan changes the repressors shape so it can bind to the operator

trp operon cycle

if there is no tryptophan, transcription occurs and makes tryptophan, that tryptophan binds to the repressor which will stop transcription so there is no more tryptophan- then the cycle continues

lac operon

positive regulation in prokaryotes

lac

lactose which is sugar that gives energy

CAP protein

• activator and positive regulator

• only binds if there is no glucose

• when it binds to the promoter it turns transcription “on”

• part of lac operon

lac operon genes

genes coded by operon that code for enzymes that break down lactose

lac operon process

• CAP binds and starts transcription to produce lactose

• lactose binds to repressor and prevents repressor from binding to operator so transcription occurs, when the repressor binds and no transcription occurs

promoter

• DNA

• where transcription factors bind that helps RNA polymerase know where to bind and which direction to initiate transcription

activator

• protein

• binds to activator binding sequence to increase transcription

repressor

• protein

• binds to operator and shuts down transcription

operator

• DNA

• where repressor binds

operon

• DNA

• promoter, operator, and the genes

what do small molecules often regulate

binding of repressors and activators when they bind allosterically

examples of small molecules

amino acids, sugar molecules, etc

• NOT a protein or something like that

what type of regulation does a repressor protein have

negative regulation because it is repressing transcription, and in the absence of its binding, transcription will occur

what type of regulation does an activator protein have

positive regulation because when binding it increases/turns on transcription

if a mutation in the operator prevents the repressor from binding, what will happen in the absence of lactose?

the operon will be transcribed

if a mutation in the repressor prevents lactose from binding, what will happen in the presence of lactose from binding, what will happen in the presence of lactose

the operon will not be transcribed

what happens when the operon is transcribed

production of polycistronic mRNA

a scientist discovers a mutant E. coli strain in which the trp operon is transcribed even when levels of tryptophan are high, where is the mutation

the operator

if there is high levels of tryptophan, will transcription occur

no because tryptophan binds to the repressor, which binds to the operator which blocks transcription; if there are already high levels we do not need to make more

if there are low levels of tryptophan, will transcription occur

yes because there is no tryptophan to bind to the repressor, which means the repressor cannot bind to the operator, then the operator cannot bind so transcription will occur; if there are low levels of tryptophan, more needs to be made

if there is high levels of glucose and lactose will transcription occur

no because the CAP protein will not bind because there is high glucose and the repressor will not bind because there is high lactose; if the CAP protein does not bind then no transcription will occur

if there is high levels of glucose but low levels of lactose will transcription occur

no because the CAP protein will not bind because there are high levels of glucose and the repressor will bind so no transcription will occur

if there are low levels of glucose but high levels of lactose, will transcription occur

yes because the CAP protein will bind since there are low levels of glucose and the repressor will not bind because there are high levels of lactose so transcription will occur

if there are low levels of glucose and low levels of lactose will transcription occur

no because while the CAP protein can bind, the repressor does too due to the low levels of lactose which blocks transcription

a mutant E. coli strain that when grown in conditions of low glucose and low lactose, the lac operon is strongly transcribed, where might the mutation be

repressor because typically blocks transcription when bound so since transcription is strong, it is not working

a mutant E. coli strain is grown in high glucose and high lactose, and the lac operon is strongly transcribed, where is the mutation?

the CAP protein may be mutated because if there is high glucose it should not be binding, but it still is and is leading to transcription