controlling protein synthesis: operon model

housekeeping genes

genes that are continuously transcribed and translated because the proteins are constantly needed

operons

sets of related genes dealing with the same function and portion of DNA that controls those genes. prokaryotes regulate their gene expression using operons

lac operon

a cluster of genes regulating the metabolism of lactose, 2 genes code for it. expressed only when lactose is present and glucose is absent

lac repressor

acts as a lactose sensor

catabolite activator protein (CAP)

acts as a glucose sensor

no lactose present

Lac repressor binds to operator → RNA polymerase cannot bind, and transcription is blocked

lactose is present

allolactose binds to lac repressor, repressor cant bind to operator, RNA polymerase binds to promoter and transcription occurs, though it needs help from CAP to bind

allolactose

isomer of lactose, acts as an inducer

cyclic AMP

cAMP, universal cell signaling molecule. CAP cannot activate without cAMP

low glucose

cAMP is produced and attaches to CAP, which helps CAP bind to its site. CAP helps RNA polymerase bind to promoter to allow transcription

high glucose

no cAMP → no CAP binding → low transcription

tryptophan (trp)

an amino acid used to make proteins

tryptophan (trp) operon

E. Coli can take up trp from the surrounding environment if available, or can synthesize it. if available, operon is turned off. trp repressor is coded by a gene called trp R upstream of operon

tryptophan absent

trp repressor inactive - does not bind to operator. RNA polymerase binds to promoter and transcription occurs

tryptophan present

trp repressor is activated by trp binding to it, acts a prepressor, helping repressor bind to operator and stop transcription