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Regulation Possiblilites:

Regulation Possiblilites:. Regulate transcription Regulate translation Regulate activity. The lac operon. E-coli uses three enzymes to take up and metabolize lactose. The genes that code for these three enzymes are clustered on a single operon – the lac Operon. What’s lactose??.

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Regulation Possiblilites:

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  1. Regulation Possiblilites: Regulate transcription Regulate translation Regulate activity

  2. The lac operon • E-coli uses three enzymes to take up and metabolize lactose. • The genes that code for these three enzymes are clustered on a single operon – the lac Operon. What’s lactose??

  3. Figure 31-2 Genetic map of the E. coli lac operon. Page 1218

  4. The lac repressor gene • Prior to these three genes is an operator region that is responsible for turning these genes on and off. • When there is not lactose, the gene for the lac repressor switches off the operon by binding to the operator region. • A bacterium’s prime source of food is glucose. • So if glucose and lactose are around, the bacterium wants to turn off lactose metabolism in favor of glucose metabolism.

  5. Isopropyl thio - -D-galactoside

  6. Figure 31-25 The base sequence of the lac operator. Page 1239

  7. Lac repressor binding to DNA animation http://molvis.sdsc.edu/atlas/morphs/lacrep/index.htm

  8. Figure 31-28a X-Ray structures of CAP–cAMP complexes. (a) CAP–cAMP in complex with a palindromic 30-bp duplex DNA. Page 1241

  9. Figure 31-36 X-Ray structure of the lac repressor subunit. Page 1248

  10. Figure 31-37aX-ray structure of the lac repressor-DNA complex. Page 1249

  11. Induction. • Allolactose is an isomer formed from lactose that derepresses the operon by inactivating the repressor, • Thus turning on the enzymes for lactose metabolism.

  12. The lac operon in action. • When lactose is present, it acts as an inducer of the operon (turns it on). • It enters the cell and binds to the Lac repressor, causing a shape change that so the repressor falls off. • Now the RNA polymerase is free to move along the DNA and RNA can be made from the three genes. • Lactose can now be metabolized (broken down).

  13. When the inducer (lactose) is removed • The repressor returns to its original shape and binds to the DNA, so that RNA polymerase can no longer get past the promoter. No RNA and no protein is made. • Note that RNA polymerase can still bind to the promoter though it is unable to move past it. That means that when the cell is ready to use the operon, RNA polymerase is already there and waiting to begin transcription.

  14. Lac movie Lac and trp

  15. Thelac repressor bound to operator sequences and the CAP-cAMP in complex with its30 bp binding site. The TATA box and -35 region of the promoter are also indicated.

  16. Catabolite repression happens when glucose (a catabolite) levels are high. • Then cyclic AMP is inhibited from forming. • When glucose levels drop, more cAMP forms. • cAMP binds to a protein called CAP (catabolite activator protein), which is then activated to bind to the CAP binding site. • This activates transcription, perhaps by increasing the affinity of the site for RNA polymerase. • This phenomenon is called catabolite repression,

  17. Suggested readings on regulation/dna bp Voet pp 1237-1253 Problems 2, 4 Here’s a quiz on the lac operon: http://www.bio.davidson.edu/courses/movies.html

  18. Figure 31-39 A genetic map of the E. coli trp operon indicating the enzymes it specifies and the reactions they catalyze. Page 1251

  19. Figure 31-40 The base sequence of the trp operator. The nearly palindromic sequence is boxed and its –10 region is overscored. Page 1251

  20. Figure 31-41 The alternative secondary structures of trpL mRNA. Page 1252

  21. Figure 31-42a Attenuation in the trp operon. (a) When tryptophanyl–tRNATrp is abundant, the ribosome translates trpL mRNA. Page 1253

  22. Figure 31-42b Attenuation in the trp operon. (b) When tryptophanyl–tRNATrp is scarce, the ribosome stalls on the tandem Trp codons of segment 1.

  23. Table 31-3 Amino Acid Sequences of Some Leader Peptides in Operons Subject to Attentuation.

  24. Figure 31-43The structure of the 5¢ cap of eukaryotic mRNAs. Page 1255

  25. Figure 31-46 An electron micrograph and its interpretive drawing of a hybrid between the antisense strand of the chicken ovalbumin gene and its corresponding mRNA. Page 1257

  26. Figure 31-47 The sequence of steps in the production of mature eukaryotic mRNA as shown for the chicken ovalbumin gene. Page 1258

  27. Figure 31-48 The consensus sequence at the exon–intron junctions of vertebrate pre-mRNAs. Page 1258

  28. Figure 31-49 The sequence of transesterification reactions that splice together the exons of eukaryotic pre-mRNAs. Page 1259

  29. Table 31-4 Types of Introns. Page 1259

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