Operons

Big picture

Prokaryotic control of genome expression 2 levels of control

Change in # of enzyme molecules produced Change in activity of enzymes present

Ex: feedback inhibition Genes turned on/off based on metabolic needs:

operons Operator + promoter + group of genes Often include several enzymes w/ in a pathway

* Regulator gene - a gene that codes for a repressor protein molecule. The repressor molecule binds to the operator and prevents RNA polymerase from binding to the promoter.

* Promoter - a short sequence of DNA where RNA polymerase first attaches when a gene is to be transcribed.

* Operator - a short sequence of DNA where the repressor binds, preventing RNA polymerase from attaching to the promoter.

* In addition, the part of chromosome (in eukaryotic cells) containing the gene must first decompact to form a "chromosome puffs" before the beginning of transcription.

2 Types of operons

Inducible: Default = off Inducer turns transciption on ex: an enzymatic pathway that allows the cell

to break down lactose Repressible:

Default = on Corepressor turns transcription off Ex: an enzymatic pathway that allows the cell

to synthesize tryptophan

The Lac Operon

Bacteria can use lactose if lactose is present and if there’s not enough glucose

B-galactosidase breaks down lactose Lactose present = transcription is on

Allolactose (inducer) inactivates the repressor Inactive repressor falls off of the operatorRNA polymerase binds

LAC OPERON

http://www.csam.montclair.edu/%7Esmalley/LacOperon.mov

The trp operon

Cell can get tryptophan from environment or by synthesizing it Will only synthesize tryptophan if there isn’t

any in the environment Tryptohan in the environment = no transcription

Tryptophan activates repressorTryptophan is a corepressorRepressor is from regulatory gene (trpR)

Repressor binds to operatorBlocks RNA polymerase

Positive gene regulation

Ex: lac operon transcription also depends on [glucose]

CRP: activates transcription (positive regulation) glucose cAMP cAMP activates CRP

Explain the events that would occur if. . . A cell was in an environment with plenty of

glucose but no lactose, and lactose was added

A cell was in an environment with tryptophan, then the tryptophan disappeared

A cell was in an environment with lactose and plenty of glucose, then the glucose disappeared