Cell Bio: Chapter 30- Regulation of Prokaryotic Transcription

Regulation of transcription is regulating ___

Gene expression

What are the two different types of gene expression?

1. Constitutive: gene is always on (cytoskeletal proteins)

2. Regulated: inducible or repressible

What is an operon?

Regulatory unit that contains a group of genes that are transcribed as one from a single promoter

Operons that are induced make what type of proteins?

Proteins that are used to breakdown substrates

What is substrate induction?

Substrate leads to increase in transcription (Lac operon)

Operons that are repressed make what type of proteins?

Proteins used to synthesize molecules

What is end product repression?

Build up synthesized molecule leads to decrease in transcription (Trp operon)

Lac operon

• Codes for proteins necessary to use lactose as a substrate

• Produces polycistronic mRNA: mRNA made from transcribing multiple genes

What proteins does each gene code for in the lac operon?

LacY: galactoside permease: facilitates transports of lactose into the cell

LacA: transacetylase: acetylate the lactose and facilitate its accumulation in the cell

LacZ: Beta galactosidase: cleaves galactoside bond in lactose (glucose + galactose)

If lactose is absent and glucose is present, what happens to the lac operon?

Lac operon is repressed

• LacI: produces repressor proteins which binds to the operator —> inhibits transcription

  • Operator is downstream of promoter

• Most of time repressor is always on operator but sometimes it lets go which allows lactose to bind to repressor (also allows for a small amount of lacmRNA to be made)

If lactose is present and glucose is present, what happens to the lac operon?

Lac operon is derepressed

• Substrate derepression of lac operon: Lactose binds to repressor protein and causes it to detach; results in minimal transcription

• Since glucose is present it indirectly inhibits adenylyl cyclase, which causes low cAMP

• Catabolite activator protein (CAP) is unable to bind to CAP binding site upstream of promoter and boost transcription

If lactose is present and glucose is absent, what happens to the lac operon?

Lac operon is induced

• Absence of glucose increases cAMP levels which causes cAMP to bind to CAP and the complex binds to the CAP binding site

  • This complex makes promoter more readily bound by RNA Pol and results in maximal transcription

Trp operon

• Codes for 5 different proteins necessary to make tryptophan —> polycistronic mRNA

• Regulated by end product repression and attenuation

If the tryptophan level is high what happens to the trp operon?

Trp operon is repressed

• TrpR: makes repressor. It is active when tryptophan binds to it

• End product repression: tryptophan binds to repressor and the complex binds to operator to block transcription

  • Operator is downstream of promoter

If the tryptophan level is low what happens to the trp operon?

Trp operon is derepresed

• Since tryptophan isn’t present the repressor can’t bind to operator so transcription can occur and tryptophan can be produced