1. Chapter 18 Campbell and Reece
OPERONS
Chapter 18
Campbell and Reece

2. E. Coli synthesizes tryptophan
5 genes clustered together
each reaction in pathway catalyzed by specific enzyme
one promoter (site where RNA polymerase can attach) serves all 5 genes

3. E. Coli synthesizes tryptophan
transcription of these 5 genes  1 very long mRNA that codes for 5 polypeptides that make up the 5 enzymes in tryptophan pathway
mRNA is punctuated with stops & starts

4. E. Coli synthesizes tryptophan
advantage of grouping genes: can use 1 “switch” controls cluster of functionally related genes: they are “coordinately controlled”
“switch” is a segment of DNA called an operator

5. Operator positioned w/in or between promoter & genes
controls access of RNA polymerase to genes

6. Operon
includes: promoter, operator, & genes

7. How does the “switch” work?
just by itself: trp operon is in “on” position
to switch it off: trp repressor must bind to operator which blocks attachment of RNA polymerase
repressor proteins are specific for each operator
repressors are protein product of a regulatory gene called trpR
located some distance from trp operon
has own promoter

8. Regulatory Genes continuously expressed @ low rate
trp repressor is allosteric (2 alternate shapes: active & inactive)
trp repressor made in its inactive form with low affinity for trp operator
repressor binding is reversible
operator vacillates between 2 states
1 w/out repressor bound
1 with repressor bound
duration of each state depends on # of active repressor molecules around

9. Regulatory Genes
trp repressor is allosteric (2 alternate shapes: active & inactive)
trp repressor made in its inactive form with low affinity for trp operator
when tryptophan binds to trp allosteric site  protein changes to active form shape  can attach to operator turning operon off

10. Tryptophan functions as corepressor:
small molecule that cooperates with repressor protein to switch operon off
if cell’s tryptophan levels high more molecules will associate with a repressor protein to switch it off
if cell’s tryptophan levels low  transcription of operon’s genes resume

11. trp operon animation
hill.com/sites/dl/free/ /126997/ animation28.html

12. 2 Types of (-) Gene Regulation
Repressible Operon
transcription usually in “on” position but it can be repressed when specific molecule binds to regulatory protein
ex: trp operon
Inducible Operon
usually in “off”position but can be induced when small molecule interacts with regulatory protein
ex: lac operon

13. Lac operon
lactose: disaccharide in milk available to E. coli in human intestine when person drinks milk
lactose  glucose + galactose
hydrolysis reaction by enzyme β- galactosidase
in absence of lactose E. coli has only a few molecules of β-galactosidase
+ milk  w/in 15 min bacteria has 1,000s x more

15. Lac Operon 3 genes: 1: β-galactosidase
2: enzymes that function in lactose utilization

16. Lac Operon
entire transcription unit is under control of 1 main operator & promoter
lacl: regulatory gene outside of operon codes for allosteric repressor protein that can switch off lac operon by binding to operator
by itself lac repressor active  binds to operator  switching lac operon off

17. Repressor
achieved by specific small molecule called an inducer  inactivates repressor
lac operon: its inducer is allolactose (isomer of lactose: sm amt made when lactose enters cell)
no lactose  no allolactose  lac repressor in active shape  genes of lac operon not being transcribed

19. Inactivation of Repressor
+ lactose  allolactose binds to lac repressor  it changes shape  nullifying repressor’s ability to attach to operator  transcription of lactose- utilizing enzymes

20. Lac Repressor

21. Inducible Enzymes ex: lac operon
their synthesis is induced by a chemical signal (allolactose)
generally, function in catabolic pathways

22. Repressible Enzymes ex: tryptophan synthesis
generally function in anabolic pathways
both repressible & inducible enzymes involves negative control of genes (operons switched off by active form of repressor protein)

23. http://highered. mcgraw- hill
hill.com/sites/ x/student_view0/ chapter13/animation_quiz_- _combination_of_switches__the_lac_ope ron.html
y/mbio/animations/main.asp?chno=ch10a0 1