BIOL 112 Chapter Notes - Chapter 17: Tryptophan, Trp Operon, Lac Repressor

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6 Apr 2012
Chapter 17 Notes
Control of Gene Regulation - Bacteria
17.1 Gene Regulation and Information Flow
DNAmRNAProteinActivated Protein
1. Transcriptional Control: DNAX-> mRNA (Slowest, Most effective-no use of energy)
2. Translational Control: mRNAX-> protein
3. Post-translational Control: proteinX->activated protein (Fastest, lots of energy needed)
- Constitutively: all the time, referring to genes that are transcribed all the time
17.2 Identifying the Genes Involved in Lactose Metabolism
- LacI: regulatory protein to allow a set of genes to be transcribed
- 3 genes (lacZ,lazY,lacA) code for three proteins (LacZ,LacY,LacA)
lacZ codes for LacZ : a beta-glactosidase- this enzyme cleaves lactose into glucose and glactose
lacY codes for LacY: a permease that transports lactose across the cell membrane
lacA codes for LacA: transacetylase (LacA is not involed in lactose untilization, function unknown)
17.3 Mechanisms of Negative Control: Discovery of the Repressor
- negative control: regulatory protein binds to DNA (upstream the promoter) and shuts down
transcription, it must be taken away for transcription to occur
- lacI gene = repressor as it exerts negative control over lacZ and lacY to inhibit transcription
The lac Operon
- Operon: a region of prokaryotic DNA that codes for serious of functionally related genes and is
transcribed from a single promoter into a polycistronic mRNA
- When there is no lactose present there is no need for the proteins to catabolize it so the LacI binds to
DNA, over lapping the lac operon and prevents transcription
- When there is lactose present, lactose binds to the LacI which changes its shape, decreasing its affinity
for the lac operson and makes it release its bind to the DNA = transcription can occur
* Note the bond between lactose and LacI is not permanent, when lactose is gone, LacI binds again
trp operon
- same as lac operon, but repressor binds to operator DNA and exerts negative control ONLY when it is
complexed with tryptophan, then tryptophan is absent, repressor does not bind and operon is
expressed constitutively (exact opposite of lac operon)
17.4 Mechanisms of Positive Control: Catabolite Repression
- positive control: also called induction- occurs with regulatory protein must bind to the DNA (upstream
the promoter) in order for transcription to begin
- MalT is a positive regulator, when it binds to DNA (near promoter region of malPQ operon), it does
NOT overlap the operator and is constitutively transcribed at low levels
- Without MalT the RNA polymerase has a very poor binding and little transcription occurs, altose is
present and it binds to MalT it, MalT changes shape so it has higher affinity for operon, then RNA
polymerase binds to MalT and the promoter and transcribes operon
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