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Adaptive Systems

Inducible generally enzymes needed for catabolism only turned on if substrate is present substrate is inducer (effector). Repressible generally enzymes involved in anabolism feedback inhibition end-product is repressor (co-repressor). Adaptive Systems. lac operon. trp operon.

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Adaptive Systems

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  1. Inducible generally enzymes needed for catabolism only turned on if substrate is present substrate is inducer (effector) Repressible generally enzymes involved in anabolism feedback inhibition end-product is repressor (co-repressor) Adaptive Systems lac operon trp operon

  2. lac operon • Jacob and Monod • system of 3 structural genes plus regulatory genes • gene products digest lactose • only needed if lactose is present • polycistronic genes

  3. lac operon i p o z y a i = inhibitor gene; produces inhibitor protein p = promoter o = operator gene; allows binding of inhibitor protein z = beta-galactosidase y = permease a = trans-acetylase

  4. lac operon; off i p o z y a RNA polymerase blocked by protein on operator. Transcription stops.

  5. lac operon; induction i p o z y a = lactose lactose binds to the inhibitor protein causing an allosteric change

  6. lac operon; induction i p o z y a Lactose/inhibitor release from operator

  7. lac operon; induction i p o z y a z, y, and a genes are transcribed. lactose-digesting enzymes produced

  8. lac operon; induction i p o z y a As enzymes digest lactose, inhibitor protein is released. Inhibitor binds again to operator TRANSCRIPTION STOPS

  9. Catabolite repression of lac operon • Second level of control • Prevents full transcription of genes unless necessary • glucose preferentially digested • glucose/cyclic AMP levels regulate efficiency of lac promoter

  10. Catabolite repression of lac operon • Glucose a better energy source than lactose • In presence of both glucose and lactose, lac operon on at VERY LOW LEVELS • In absence of glucose, presence of lactose, lac operon fully expressed

  11. Catabolite repression of lac operon • Digestion of glucose converts cAMP to ATP • high glucose = low cAMP • low glucose = high cAMP • cAMP can bind to catabolite activator protein (CAP) • cAMP/CAP complex binds to promoter • increases efficiency of promoter

  12. Catabolite repression of lac operon i p o z y a cAMPATP Glucose CAP

  13. Catabolite repression of lac operon i p o z y a cAMPATP low glucose levels CAP

  14. Catabolite repression of lac operon • cAMP/CAP complex increases efficiency of promoter • Allows transcription at full speed if no glucose present • No cAMP, promoter remains “weak” = low transcription if glucose present

  15. trp operon • Repressible system • 5 polycistronic structural genes plus regulatory genes • Gene products synthesize tryptophan • needed unless tryptophan is available in the environment

  16. trp operon r p o t1 -- t5 r = repressor gene; produces aporepressor protein p = promoter o = operator t1 - t5 = five “tryptophan synthetase” genes

  17. trp operon; tryptophan needed r p o t1 -- t5 RNA polymerase binds; transcription takes place; enzymes produced Aporepressor alone cannot bind to operator

  18. trp operon; repression r p o t1 -- t5 tryptophan

  19. trp operon; repression r p o t1 -- t5 tryptophan binds to aporepressor complex binds to operator; stops transcription

  20. Attenuation of trp operon • second level of regulation to insure genes NOT transcribed if tryptophan is present • 5 trp structural genes very expensive energetically to transcribe/translate • tryptophan synthesis expensive energetically

  21. Attenuation of trp operon • Special sequences in DNA of leader region • leader/attenuator region • pair of inverted repeats • alternate binding possibilities

  22. Attenuation of trp operon r p o t1 -- t5 leader/attenuator region

  23. Attenuation of trp operon 1 2 3 4 RNA transcribed from 1and 2 may form stem-loop RNA transcribed from 3and 4 may form stem-loop RNA transcribed from 2 and 3 may form stem-loop

  24. Attenuation of trp operon -- OR --

  25. Attenuation of trp operon UUUUUU Transcription terminator signal

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