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Chap 18

BIOL 135

Professor Ott

For this lecture:

• Will go over:
  • Regulation of gene expression in prokaryotes
  • Regulation of gene expression in eukaryotes
• Please refer to Sec. 18.1-18.2 of textbook 2

How can this multicellular organism (Anableps anableps, or

“cuatro ojos”) see above and below the water?

• Upper half of their eye is well-suited for aerial vision and lower half for aquatic vision
• But all eye cells contain same genes
• This is eukaryotic organism Figure 18.1a 4

If all eye cells contain the same genes, how do they function

differently?

• Differential gene expression = expression of different genes - So cells can function differently
• By controlling transcription
  • Common for both prokaryotes and eukaryotes
• Genes are turned on and off in response to signals from their external and internal environments in both prokaryotes and eukaryotes - Internal environments = presence of food molecule - External environments = ex. presence of dangerous antibiotic Figure 18.1b 5

Bacteria can regulate transcription

• Natural selection has favored bacteria that express only genes that encode products needed by cell - Making proteins requires energy
• Cell can regulate production of enzymes by gene regulation - By transcription!
• One mechanism for this type of regulation of groups of genes is called operon model (in prokaryotes) Figure 18.2 7

What is an operon?

• Cluster of functionally related genes can be coordinately controlled by a single “on-off switch” Figure 18.UN03 8

What switches off an operon?

• Protein repressor
• Repressor prevents gene transcription by binding to operator and blocking RNA polymerase - Repressor is product of separate regulatory gene, located away from operon - Repressor can be in an active or inactive form, depending on presence of corepressor
• Corepressor = molecule that cooperates with repressor protein to switch operon off Figure 18.3 10

Seeing an operon in action

• For ex.: E. coli has trp operon
  • Can synthesize amino acid tryptophan when it has insufficient tryptophan
  • Tryptophan makes up cellular proteins
• By default, trp operon is on and genes for tryptophan synthesis are transcribed (a) Figure 18.3 11

Repressible vs inducible operons

• Repressible operon is one that is usually on: binding of a repressor to operator shuts off transcription - trp operon is repressible operon
• Inducible operon is one that is usually off: molecule called an inducer inactivates repressor and turns on transcription - lac operon is inducible operon Figure 18.UN04 and UN05 13

What is the lac operon?

• Found in E. coli
• lac operon contains genes that code for enzymes used in hydrolysis and metabolism of lactose (break down lactose) - Colloquially known as lactase, think of lactose intolerance
• Regulatory gene ( lacI ) is located outside operon and encodes repressor protein that can switch off operon Figure 18.4 14

trp and lac operons: inducible vs repressible

• Inducible enzymes usually function in catabolic pathways (break things down) - Their synthesis is induced by chemical signal (allolactose in lac operon)
• Repressible enzymes usually function in anabolic pathways (build things up) - Their synthesis is repressed by high levels of end product (tryptophan represses trp operon) 16

What are constitutive operons?

• Operons that are always on
• Usually housekeeping genes
• Genes that are constantly expressed to make products for cell
• To maintain essential functions 17

What is an example of positive gene regulation?

• Some operons are also subject to positive control - Like lac operon - E. coli prefers glucose over lactose - But when glucose is scarce and lactose is present, lac operon is needed to produce enzymes for lactose breakdown Figure 18.5 19

What is an example of positive gene regulation?

• Cyclic AMP receptor protein (CRP) is activator of transcription - CRP helps regulate other operons involved in catabolic pathways
• When glucose is scarce, CRP is activated by binding with cyclic AMP (cAMP) (a) - Activated CRP attaches to promoter of lac operon and increases affinity of RNA polymerase - Accelerating transcription Figure 18.5 20