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Gene Expression Regulation: The lac Operon

Gene Expression Regulation: The lac Operon. Control Mechanisms. There are 42 000 genes that code for proteins in humans However, not all proteins are required at all times. E.g. Insulin is only required in a cell when glucose levels are high.

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Gene Expression Regulation: The lac Operon

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  1. Gene Expression Regulation:The lac Operon

  2. Control Mechanisms • There are 42 000 genes that code for proteins in humans • However, not all proteins are required at all times. • E.g. Insulin is only required in a cell when glucose levels are high. • It would be inefficient and wasteful for a cell to transcribe, translate the insulin gene when glucose levels are low • Regulation is therefore vital to an organism’s survival • Regulation – the turning on or off of specific genes depending on the requirements of an organism

  3. Why Turn Genes On and Off? • Cell Specialization • each cell of a multicellular eukaryote expresses only a small fraction of its genes • Development • different genes needed at different points in life cycle of an organism • afterwards need to be turned off permanently • Responding to organism’s needs • cells of multicellular organisms must continually turn certain genes on & off in response to signals from their external & internal environment

  4. Signal NUCLEUS Chromatin Chromatin modification: DNA unpacking involving histone acetylation and DNA demethlation DNA Gene available for transcription Gene Transcription Exon RNA Primary transcript Intron RNA processing Tail mRNA in nucleus Cap Transport to cytoplasm CYTOPLASM mRNA in cytoplasm Degradation of mRNA Translation Polypetide Cleavage Chemical modification Transport to cellular destination Active protein Degradation of protein Degraded protein Figure 19.3 • The control of gene expression can occur at any step in the pathway from gene to functional protein • The focus of today’s lesson will be on the regulation of gene expression at the transcriptional level.

  5. Gene Expression Gene expression for all genes falls into one of two categories. (prokaryotes and eukaryotes) • constitutive expression – genes which are always turned on • known as housekeeping genes • induced expression – genes which are only turned on as needed

  6. lac Operon • lac - lactose • operon – several genes in a sequence all controlled by a single promoter (mainly in prokaryotes; some eukaryotes). It also includes an operator which is the “on” and “off” switch. promoter gene 1 gene 2 gene 3 operator

  7. lac Operon Cells mainly use glucose as a source of energy. The lac operon is only turned on when glucose is absent, but lactose is present. lac Operon Animation

  8. lac Operon Details Operon codes for 3 enzymes – found in E. coli • beta-galactosidase (lacZ gene) • enzyme which breaks down lactose • permease(lacY gene) • protein transporter which brings lactose into cell • transacetylase(lacA gene) • adds acetyl group to galactose

  9. Repressor Protein repressor protein (lacI gene) • transcribed by a different gene from the lac operon • binds to the operator portion in the presence of glucose • prevents RNA polymerase from transcribing genes when bound to operator • Lactose is not needed as an energy source because glucose is present When do you want the repressor to bind / not bind to the operon?

  10. Promoter Regulatorygene Operator DNA lacl lacZ NoRNAmade 3’ RNApolymerase mRNA 5’ Activerepressor Protein (a) Lactose absent, repressor active, operon off. The lac repressor is innately active, and inthe absence of lactose it switches off the operon by binding to the operator. Figure 18.22a When lactose is absent: • enzymes are not needed to metabolise lactose • repressor binds to the operator to inhibit transcription

  11. lac operon DNA lacl lacz lacY lacA RNApolymerase 3’ mRNA 5’ mRNA 5' mRNA 5’ β-Galactosidase Permease Transacetylase Protein Inactiverepressor Allolactose(inducer) (b) Lactose present, repressor inactive, operon on. Allolactose, an isomer of lactose, derepresses the operon by inactivating the repressor. In this way, the enzymes for lactose utilization are induced. Figure 18.22b When lactose is present: • Lactose (or allactose) binds to the repressor protein • lactose-repressor complex cannot bind to the operator • transcription can proceed

  12. Effector Molecules Since lactose is the molecule that determines when the operon is turned on or off, it is known as an effector molecule. • effector molecule – any molecule that can regulate the activity of a protein • inducer – effector molecule that binds repressor protein to cause it to fall off operator

  13. Regulatorygene Promoter lac operon Operator DNA lacl lacZ DNA lacl lacz lacY lacA RNApolymerase 3’ NoRNAmade mRNA 5’ mRNA 5' mRNA 5’ 3’ RNApolymerase mRNA 5’ Protein Inactiverepressor Allolactose(inducer) Activerepressor Protein (b) Lactose present, repressor inactive, operon on. Lactose absent, repressor active, operon off. (a)

  14. lac Operon Animation lac Operon Animation

  15. Gene Expression Regulation:trpOperon

  16. trp Operon • trp – tryptophan The genes of the trp operon are used to make the amino acid tryptophan. It is turned off when enough tryptophan is in the cell. Tryptophan is the effector molecule.

  17. trp Operon Operon codes for 5 genes – found in E. coli Five polypeptides combine to make three enzymes. • each enzyme participates in a step to make tryptophan

  18. Repressor Protein repressor protein (trpR) • transcribed as a different gene from trp operon • binds to operator when tryptophan is present • prevents RNA polymerase from transcribing genes when bound to operator When do you want the repressor to bind / not bind to the trp operon?

  19. trp operon Promoter Promoter Genes of operon RNA polymerase trpR trpD trpC trpB trpE trpA DNA Operator Regulatory gene 3’ mRNA 5’ mRNA 5’ C E D B A Polypeptides that make up enzymes for tryptophan synthesis Inactiverepressor Protein (a) Tryptophan absent, repressor inactive, operon on. RNA polymerase attaches to the DNA at the promoter and transcribes the operon’s genes. When tryptophan needs to be made: • enzymes are required to make tryptophan • repressor is NOT bound to operator • transcription can proceed

  20. DNA No RNA made mRNA Protein Active repressor Tryptophan (corepressor) Tryptophan present, repressor active, operon off. As tryptophan accumulates, it inhibits its own production by activating the repressor protein. (b) When cell has enough tryptophan: • tryptophan binds to repressor • repressor can now bind operator to prevent transcription RNA Polymerase

  21. Effector Molecule Since tryptophan is the molecule that determines when the operon is turned on or off, it is known as an effector molecule. corepressor – effector molecule that binds repressor protein to cause it to bind to the operator

  22. trp operon DNA Promoter No RNA made Promoter Genes of operon trpR trpD trpC trpB trpE trpA DNA Operator mRNA 3’ 5’ mRNA 5’ C E D B A Polypeptides that make up enzymes for tryptophan synthesis Inactiverepressor Protein Protein Active repressor Tryptophan (corepressor) (a) Tryptophan absent, repressor inactive, operon on. Tryptophan present, repressor active, operon off. (b)

  23. http://www.youtube.com/watch?v=8aAYtMa3GFU

  24. Classwork/Homework • Section 5.5 Questions pg. 258 #1-6

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