Week 10 - Exp 8

Week 10 - Exp 8 - BIOL 519 Week #10 - lac Operon and...

Info iconThis preview shows pages 1–3. Sign up to view the full content.

View Full Document Right Arrow Icon
BIOL 519 Week #10 - lac Operon and Indicator Plates 1 BACKGROUND: Experiment 8: lac Operon and Indicator Plates The lac operon of E. coli was the first operon (group of genes that are transcribed together as a unit) whose regulation was understood (based on work performed by Jacob and Monod in the 1950’s and 1960’s). Regulation means the mechanisms used by the cell to determine when to synthesize the proteins encoded in the operon (by transcription and translation), and when not to. The lac operon is important in bacterial genetics both for this historical reason, and because it has been adapted as a very useful tool in the study of regulation of other operons (see below). We will first discuss the regulation of the operon, and then the applications of this operon to a wider variety of bacterial genetic questions. The operon itself - lacZYA I P O Z Y A lacZ encodes the enzyme β -galactosidase - which breaks down lactose to glucose and galactose lacY encodes the lac permease - a transport protein to take up lactose into the cell lacA encodes Transacetylase - acetylates some toxic galactosides to detoxify them lacP - As always, there is a Promoter (P) upstream of the lacZYA operon (see the DNA sequence on a latter page) lacI - Another gene, lacI is encoded in a separate operon (this other operon includes only lacI ) The Promoter for the lacI gene is to the left. lacI encodes the Lac repressor protein. lacO - the Lac operator - lacO does not encode a protein - it is a DNA sequence that serves as the binding site for the Lac repressor protein. Negative Regulation of the lac operon Since Lactose can serve as a carbon and energy source for E. coli and the lac operon encodes the enzymes to take up and degrade lactose, the cell only needs to express the lac operon if lactose is available. LacI is the repressor protein of the lacZYA operon In the absence of lactose, LacI binds to lacO (lac operator) to block transcription (RNAP can not bind to the promoter if LacI is bound at lacO since the promoter and operator sites overlap).
Background image of page 1

Info iconThis preview has intentionally blurred sections. Sign up to view the full version.

View Full DocumentRight Arrow Icon
BIOL 519 Week #10 - lac Operon and Indicator Plates 2 When lactose is present in the cell, a derivative of lactose (allolactose – formed as a side reaction by LacZ) will bind to LacI . This results in a conformational change in the LacI protein such that it can't bind to lacO . If lacI is not bound at lacO , then lacZYA expression will no longer be repressed. The
Background image of page 2
Image of page 3
This is the end of the preview. Sign up to access the rest of the document.

Page1 / 7

Week 10 - Exp 8 - BIOL 519 Week #10 - lac Operon and...

This preview shows document pages 1 - 3. Sign up to view the full document.

View Full Document Right Arrow Icon
Ask a homework question - tutors are online