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Gregory Wagner

Lec32 BIOL207 2014-03-31 CHAPTER 12 A. Lac operon a. Operon = group of protein coding sequences under control of a common promoter (very common in prokaryotes) b. E. coli prefers to use glucose as an energy source but will use lactose if glucose is scarce and lactose is present c. Lac operon contains protein coding genes: i. lacZ (beta-galactosidase) cuts lactose (Lac) into glucose (Glu) and galactose (Gal) ii. lacY permease to transport lactose into the cell iii. lacA function still not certain d. Lac operon contains regulatory sequences in non -coding DNA i. O = operator ii. P = promoter iii. CBS = CAP binding site e. Other important proteins (not part of Lac operon) i. lacI = lac repressor is a protein (trans-factor) that regulates expression of other genes by binding to (cis-elements) DNA sequences ii. CAP= cAMP binding protein (also a trans -factor) f. Lac operon regulation i. For transcription of any gene to occur, RNApol must bind to its promoter ii. When no Lac is in environment lacI is able to bind to O , thus preventing RNApol from binding to P iii. When Lac is present, it binds to lacI, and changes shape of lacI so that lacI can no longer bind to O (allosteric regulation) iv. BUT if both Lac and Glc are available, no need to use Lac, so E. coli has another layer of Lac operon regulation based on Glc availability 1. When Glc is low, cAMP is high, cAMP binds to CAP 2. cAMP changes shape of CAP and allows CAP to bind to CBS 3. when CAP is bound CBS, rate of tran scription by RNApol is increased 4. so maximum Lac operon transcription occurs when Lac
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