Chapter 15 regulation of gene expression
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Chapter 15- Regulation of Gene expression. Regulation is important! Regulation makes sense - the bacteria/cell does the logical, intelligent thing- so you can ask “what would I do if I were a microbe?”, and usually you’ll be right. Lots of wood (fuels)/brick analogies, and stories.

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Chapter 15- Regulation of Gene expression

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Chapter 15- Regulation of Gene expression

  • Regulation is important!

  • Regulation makes sense- the bacteria/cell does the logical, intelligent thing- so you can ask “what would I do if I were a microbe?”, and usually you’ll be right.

  • Lots of wood (fuels)/brick analogies, and stories.


Some terms:

  • Inducible- the environmental effect (specific molecule) turns it (the genes in question) on- e.g., ability to use lactose

  • Constitutive- always on

  • repressible- the environmental effect (specific molecule) turns it (the genes in question) off. e.g.- ability to make tryptophan

  • So genes, or the production of a product, can be considered, inducible, constitutive, or repressible.


  • These are separate from how the regulation takes place- it can be regulated by:

  • negative control- regulator turns it off.

  • positive control: regulator turns it on.


Selected Bacterial Operons

OPERON: coordinately regulated genes, usually on the same mRNA in bacteria.

  • By turning genes on and off in response to the environment, the cell works more efficiently, only making gene products needed at that time.

  • Lac operon: catabolism of lactose. 2 enzymes needed:

  • lac permease: brings lactose in

  • β-galactosidase: splits into glucose and galactose.

  • Fuel.


TWO conditions needed to utilize lactose: 1) lactose has to be there; 2) glucose has to be absent- again, it makes sense!


Control system: fig 15-1-7: This is the situation when no glucose is present, and lactose induces the operon. Please study these figures!!!!


Lactose (actually, allolactose- is really much smaller Allostearic interaction

Repressor acts negatively to inhibit transcription.

www.prenhall.com/klug, http://vcell.ndsu.nodak.edu/animations/home.htm


Proof: isolation and behavior of constitutive mutants, such as Oc and lacI-, and dominance/recessive behavior as merodiploids. Table 15.1 Also biochemistry- isolation of repressor protein, binding characteristics to operator, etc.


However, good repressor will work, so an I-/I+ merodiploid will be regulated.


A good operator will NOT work to restore regulation!!


A superrepressor (Is) will bind, shutting off transcription, no matter what- it will be dominant over an I+


CAP protein and positive regulation

  • When glucose is in abundance, the lac operon is not turned on, even when lactose is present! Makes sense!

  • Regulation involved CAP, cAMP, and RNA polymerase

  • CAP: catabolite activator protein: stimulates transcription when cAMP levels are high.


  • cAMP: Fig. 15-9: Formed when glucose levels are low. The enzyme that produces cAMP (adenyl cyclase), is inhibited by allosteric interaction with glucose. Thus as glucose levels fall, the enzyme is more active, cAMP levels rise.

  • CAP + cAMP bind to promoter, turns weak promoter into STRONG promoter!


CAP is kind of like a transcription factor- + acting, stimulating transcription

CAP regulates probably 50 genes- NO sugar or other carbon source would be used, as long as glucose is there!


Tryptophan biosynthesis

  • Trp: amino acid; (= “bricks”); building block.

  • trp operon: genes for making tryptophan.

  • trp is on when tryptophan levels are low; off when levels are high.


MORE allosteric interaction!


  • Quiz on Friday-

  • Lac operon!


Gene regulation in eukaryotes

  • I. Some considerations: (Fig. 15-12)

  • a. lots more DNA; chromatin has histones, which have to be considered. Chromatin can be available for transcription (decondensed) or unavailable (condensed), which makes a BIG difference!

  • b. transcription in the nucleus, translation in the cytoplasm

  • c. processing: caps, tails, splicing. Alternative splicing!

  • d. long half-life of mRNA in our cells! in bacteria, it’s about 3 min; ours is 20 min or longer.

  • e. cell differentiation: not only environment, but cell type influences gene expression. (i.e., no hairy livers!)


Lots of transcription factors!

  • The regulatory regions can be VERY complex!! Fig. 15-18


What do transcription factors really look like?

There are recurring patterns, called motifs – the beta barrel is one of those others…


Here’s a very simple eukaryotic regulation path. Steroids, unlike other hormones, take a long time to work an do so by affecting transcription. The hormone-receptor scale is WAY off.


Ways to make multiple proteins form the same gene


Just to amaze you…


The quiz (ignore)

  • Lac operon!!! Q’s 4 & 5 in the homework- stuff like that

  • Examples of allostearic interactions

  • Simple statements about CAP, TRP


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