BIOL 2010 - Control of transcription in prokaryotes

types of expression

• constitutive - always on

• regulated genes - can be turned on/off in response to changes in environment

operons

• genes encoding for proteins in the same pathway are located adjacent to one anther and controlled as a single unit that is transcribed into a polycistronic RNA

• polycistronic RNA is RNA with no introns and is specific to prokaryotes

LAC OPERON

• lac z - beta galactosidase breaks down lactose

• lac y - permease encodes the transporter that allows lactose in

• lac a -transacetylase is involved in acetylating pyranosides so that they cant enter cells and become toxic

• consensus sequence gives directionality to RNA pol

• down/up mutations decrease or increase the affinity of the promoter to the RNA pol to increase or decrease the expression of the gene

• lacI repressor is constitutively expressed but when lactose is present it binds to the lactose repressor

• the lac repressor is a tetramer made of 2 dimers

• the lac operon has 3 operator sites and in order to repress transcription O1 must be bound with either O2 or O3

• in order for the repressor to be bound to 2 regions the DNA must loop in different ways —> this looping of DNA physically prevents RNA polymerase from binding to DNA

• catabolite activating protein acts as a transcriptional activator

NOTE: IPTG mimics allolactose

bacterial growth curve

1. lag phase: cells are getting used to the environment producing diff proteins so theres no oncrease

2. log phase: use sigma 70 to exponentially increase

3. stationary phase: plateau in rate of growth —> the death and division rate is equal

4. death/decline phase: the bacteria run out of resource

regulon

• a group of genes that are regulated as a unit

• its different from an operon because a regulon could be multiple operons and some individual genes

transcription factors

• negative regulation: transcriptional repressors that bind at an operator site within the promoter region

• positive regulator: transcriptional activators which interacts with the RNA polymerase binding close by and increases affinity

CAP site

• some promoters have low affinity

• cap increases affinity of RNA for promoter site

• CAP functions when glucose is unavailable

• when glucose is low there’s an increase in cAMP which causes the activation of CAP —> lac operon turns on so lactose can be used instead of glucose

• when glucose and lactose are high the lac operon is turned on but not producing much protein as glucose is the preferred substrate