BIO SCI 98 Lecture Notes - Lecture 3: Lac Operon, Lac Repressor, Allolactose
19 Jun 2018
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CHAPTER 20 The Regulation of Gene Expression in Bacteria pp. 694-715
Specificity of Transcription Initiation
1. sigma factor = first level of specificity
a. Whole classes of genes turned on/off depending on the sigma factor expressed
2. Regulators
a. DNA-binding proteins
b. Bind to specific sites near promoters → affect ability/affinity of RNA polymerase
to initiate
c. Repressors inhibit/block polymerase binding
d. Activators have affinity for polymerase
Escherichia coli
- Gram-negative, rod-shaped bacterium
- Found in intestine of warm-blooded organisms
- Part of normal gut flora
- Grows by fission
- Likes to live on glucose
Regulation of lac operon
- Before DNA sequencing existed → scientists isolated
mutant E. coli and assessing growth in lactose
-lacZ: encodes beta-galactosidase
- Breaks lactose into glucose + galactose
- Converts lactose into allolactose
-lacY: encodes lac permease → required to get lactose
into cell
-lacA: encodes beta-galactoside + transacetylase
- Detoxifies toxic sugars transported into cell with
lactose
- One mRNA: 3 ribosome binding sites and 3 proteins made from polycistronic mRNA
- Mutations in lac repressor (lacl) and lac operator (lacO) influence gene expression
- For the lac operon to work, the "OFF" state must be leaky
- lacY: encodes lac permease = transporter required to get lactose into cell
- Leaky "OFF" state allows small amounts of lac permease = lactose uptake
- If this did not happen, lac operon can never be induced
find more resources at oneclass.com
find more resources at oneclass.com
- LacZ: encodes beta-galactosidase = enzyme that converts lactose into allolactose
- Allolactose induces lac operon (binds lacl)
- Leaky "OFF" state lets beta-galactosidase be expressed in small amounts →
formation of allolactose
- Second functional copy of lacl (lac inhibitor) gene can function in trans to regulate both
copies of lac operon
-Merodiploids
- Bacteria are haploid
-Merodiploid has 2 copies of some genes
- Used to work out complementation relationships
- Lac operator
- DNA sequence next to the promoter bound by lac inhibitor (lacI)
- To regulate transcription of lacZ, lacY, and lacA genes
- To regulate genes = lac operator must be continuous with genes (cis)
- Lac inhibitor can diffuse, so work in trans
- Second functional copy of lac operator cannot function in trans to regulate other copy
of lac operon
- Only functions in cis
- In the absence of lactose = lac repressor binds to lac operator to turn off expression of
lac operon
- In presence of lactose = allolactose binds to lac repressor
- Causes conformational change that prevents repressor binding to operator
- Lac gene expression is on
Cis and Trans: Bombers and radios
- Lac repressor can operate in trans and cis
- Lac operator only operates in cis
Three operator sequences
- Each has inverted repeat
- O1 = start site
- O2 = lacZ gene
- O3 = upstream of promoter
- Lac repressor is a tetramer (dimer of dimers) → binds to 2
of the 3 operator sequences
- Binds most tightly to O1
- Binding to O2 and O3 causes DNA looping =
transcription blocked
- Allosteric regulation = binding to an inducer (allolactose)
causes conformational change → reduced affinity for
operator
find more resources at oneclass.com
find more resources at oneclass.com
Document Summary
Chapter 20 the regulation of gene expression in bacteria pp. Before dna sequencing existed scientists isolated mutant e. coli and assessing growth in lactose lacz: encodes beta-galactosidase. Converts lactose into allolactose lacy: encodes lac permease required to get lactose into cell laca: encodes beta-galactoside + transacetylase. Detoxifies toxic sugars transported into cell with lactose. One mrna: 3 ribosome binding sites and 3 proteins made from polycistronic mrna. Mutations in lac repressor (lacl) and lac operator (laco) influence gene expression. For the lac operon to work, the off state must be leaky lacy: encodes lac permease = transporter required to get lactose into cell. Leaky off state allows small amounts of lac permease = lactose uptake. If this did not happen, lac operon can never be induced. Lacz: encodes beta-galactosidase = enzyme that converts lactose into allolactose. Leaky off state lets beta-galactosidase be expressed in small amounts formation of allolactose.
