Control of Prokaryotic Bacterial Genes AP Biology 2007

Control of Prokaryotic (Bacterial) Genes AP Biology 2007 -2008

Bacterial metabolism Bacteria need to respond quickly to changes in their environment u STOP u GO AP Biology if they have enough of a product, need to stop production why? waste of energy to produce more how? stop production of enzymes for synthesis if they find new food/energy source, need to utilize it quickly why? metabolism, growth, reproduction how? start production of enzymes for digestion

Remember Regulating Metabolism? Feedback inhibition u u product acts as an allosteric inhibitor of 1 st enzyme in tryptophan pathway but this is wasteful production of enzymes Oh, I remember this from our Metabolism Unit! AP Biology - = inhibition -

Different way to Regulate Metabolism Gene regulation u AP Biology instead of blocking enzyme function, block transcription of genes for all enzymes in tryptophan pathway saves energy by not wasting it on unnecessary protein synthesis Now, that’s a good idea from a lowly bacterium! - = inhibition -

Gene regulation in bacteria Cells vary amount of specific enzymes by regulating gene transcription u turn genes on or turn genes off turn genes OFF example if bacterium has enough tryptophan then it STOP doesn’t need to make enzymes used to build tryptophan turn genes ON example if bacterium encounters new sugar (energy GO source), like lactose, then it needs to start making enzymes used to digest lactose AP Biology

Bacteria group genes together Operon u genes grouped together with related functions u promoter = RNA polymerase binding site u AP Biology example: all enzymes in a metabolic pathway single promoter controls transcription of all genes in operon transcribed as one unit & a single m. RNA is made operator = DNA binding site of repressor protein

So how can these genes be turned off? Repressor protein binds to DNA at operator site u blocking RNA polymerase u blocks transcription u AP Biology

Operon model Operon: operator, promoter & genes they control serve as a model for gene regulation RNA polymerase RNA TATA repressor polymerase gene 1 gene 2 gene 3 gene 4 1 2 3 4 enzyme 1 enzyme 2 enzyme 3 enzyme 4 m. RNA promoter DNA operator Repressor protein turns off gene by blocking RNA polymerase binding site. AP Biology repressor = repressor protein

Repressible operon: tryptophan Synthesis pathway model When excess tryptophan is present, it binds to tryp repressor protein & triggers repressor to bind to DNA RNA polymerase u RNA trp TATArepressor polymerase blocks (represses) transcription gene 1 gene 2 gene 3 gene 4 1 2 3 4 enzyme 1 enzyme 2 enzyme 3 enzyme 4 m. RNA promoter DNA trp operator trp repressor protein trp trp trp conformational change in AP Biology repressor protein! trp repressor tryptophan trp tryptophan – repressor protein complex

Tryptophan operon What happens when tryptophan is present? Don’t need to make tryptophan-building enzymes Tryptophan is allosteric regulator of repressor protein AP Biology

Inducible operon: lactose lac RNA polymerase lac Digestive pathway model lac When lactose is present, binds to lac repressor protein & triggers repressor to release DNA lac lac RNA lac TATA repressor polymerase u induces transcription gene 1 gene 2 gene 3 gene 4 1 2 3 4 enzyme 1 enzyme 2 enzyme 3 enzyme 4 m. RNA promoter operator repressor lac conformational change in AP Biology repressor protein! lac repressor DNA repressor protein lactose – repressor protein complex

Lactose operon What happens when lactose is present? Need to make lactose-digesting enzymes Lactose is allosteric regulator of repressor protein AP Biology

1961 | 1965 Jacob & Monod: lac Operon Francois Jacob & Jacques Monod first to describe operon system u coined the phrase “operon” u AP Biology Jacques Monod Francois Jacob

Operon summary Repressible operon u usually functions in anabolic pathways u synthesizing end products when end product is present in excess, cell allocates resources to other uses Inducible operon u usually functions in catabolic pathways, u produce enzymes only when nutrient is available AP Biology digesting nutrients to simpler molecules cell avoids making proteins that have nothing to do, cell allocates resources to other uses

Don’t be repressed! How can I induce you to ask Questions? AP Biology 2007 -2008

Control of Eukaryotic Genes AP Biology 2007 -2008

The BIG Questions… How are genes turned on & off in eukaryotes? How do cells with the same genes differentiate to perform completely different, specialized functions? AP Biology

Evolution of gene regulation Prokaryotes single-celled u evolved to grow & divide rapidly u must respond quickly to changes in external environment u exploit transient resources Gene regulation u turn genes on & off rapidly u AP Biology flexibility & reversibility adjust levels of enzymes for synthesis & digestion

Evolution of gene regulation Eukaryotes multicellular u evolved to maintain constant internal conditions while facing changing external conditions u u homeostasis regulate body as a whole growth & development w long term processes specialization w turn on & off large number of genes AP Biology must coordinate the body as a whole rather than serve the needs of individual cells

Points of control The control of gene expression can occur at any step in the pathway from gene to functional protein 1. packing/unpacking DNA 2. transcription 3. m. RNA processing 4. m. RNA transport 5. translation 6. protein processing 7. protein degradation AP Biology

1. DNA packing How do you fit all that DNA into nucleus? u DNA coiling & folding double helix nucleosomes chromatin fiber looped domains chromosome from DNA double helix to AP Biology condensed chromosome

Nucleosomes 8 histone molecules “Beads on a string” 1 st level of DNA packing u histone proteins u 8 protein molecules positively charged amino acids bind tightly to negatively charged DNA AP Biology DNA packing movie

DNA packing as gene control Degree of packing of DNA regulates transcription u tightly wrapped around histones no transcription genes turned off § heterochromatin darker DNA (H) = tightly packed § euchromatin lighter DNA (E) = loosely packed H AP Biology E

DNA methylation Methylation of DNA blocks transcription factors no transcription genes turned off u attachment of methyl groups (–CH 3) to cytosine u u nearly permanent inactivation of genes AP Biology C = cytosine ex. inactivated mammalian X chromosome = Barr body

Histone acetylation Acetylation of histones unwinds DNA u loosely wrapped around histones u attachment of acetyl groups (–COCH 3) to histones AP Biology enables transcription genes turned on conformational change in histone proteins transcription factors have easier access to genes

2. Transcription initiation Control regions on DNA u promoter nearby control sequence on DNA binding of RNA polymerase & transcription factors “base” rate of transcription u enhancer distant control sequences on DNA binding of activator proteins “enhanced” rate (high level) of transcription AP Biology

Model for Enhancer action Enhancer DNA sequences u Activator proteins u distant control sequences bind to enhancer sequence & stimulates transcription Silencer proteins u bind to enhancer sequence & block gene transcription AP Biology Turning on Gene movie

Transcription complex Activator Proteins • regulatory proteins bind to DNA at Enhancer Sites distant enhancer sites • increase the rate of transcription regulatory sites on DNA distant from gene Enhancer Activator Coactivator A E F B TFIID H RNA polymerase II Coding r egion T A Core promoter and initiation complex Initiation Complex at Promoter Site binding site of RNA polymerase AP Biology

3. Post-transcriptional control Alternative RNA splicing u AP Biology variable processing of exons creates a family of proteins

4. Regulation of m. RNA degradation Life span of m. RNA determines amount of protein synthesis u m. RNA can last from hours to weeks AP Biology RNA processing movie

RNA interference NEW Small interfering RNAs (si. RNA) u short segments of RNA (21 -28 bases) bind to m. RNA create sections of double-stranded m. RNA “death” tag for m. RNA w triggers degradation of m. RNA u cause gene “silencing” post-transcriptional control turns off gene = no protein produced si. RNA AP Biology !

Action of si. RNA Hot … new Hot t in b opic iolog y dicer enzyme m. RNA for translation si. RNA double-stranded mi. RNA + si. RNA breakdown enzyme (RISC) m. RNA degraded AP Biology functionally turns gene off

RNA interference 1990 s | 2006 “for their discovery of RNA interference — gene silencing by double-stranded RNA” Andrew Fire AP Biology Stanford Craig Mello U Mass

5. Control of translation Block initiation of translation stage u regulatory proteins attach to 5' end of m. RNA prevent attachment of ribosomal subunits & initiator t. RNA block translation of m. RNA to protein AP Biology Control of translation movie

6 -7. Protein processing & degradation Protein processing u folding, cleaving, adding sugar groups, targeting for transport Protein degradation ubiquitin tagging u proteasome degradation u AP Biology Protein processing movie

1980 s | 2004 Ubiquitin “Death tag” mark unwanted proteins with a label u 76 amino acid polypeptide, ubiquitin u labeled proteins are broken down rapidly in "waste disposers" u proteasomes Aaron Ciechanover AP Biology Israel Avram Hershko Israel Irwin Rose UC Riverside

Proteasome Protein-degrading “machine” cell’s waste disposer u breaks down any proteins into 7 -9 amino acid fragments u cellular recycling AP Biology play Nobel animation

6 7 Gene Regulation protein processing & degradation 1 & 2. transcription - DNA packing - transcription factors 5 4 initiation of translation m. RNA processing 3 & 4. post-transcription - m. RNA processing - splicing - 5’ cap & poly-A tail - breakdown by si. RNA 5. translation - block start of translation 1 2 initiation of transcription AP Biology m. RNA splicing 3 6 & 7. post-translation - protein processing - protein degradation 4 m. RNA protection

Turn your Question Genes on! AP Biology 2007 -2008

6 Gene Regulation 7 1 & 2. _________________ - ____________ 5 4 1 2 3 & 4. _________ - ____________________ 5. _________ - ____________________ AP Biology 3 4 6 & 7. _________________ - ____________