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Lecture 2

MBB 222 Lecture Notes - Lecture 2: Catabolite Repression, Camp Receptor Protein, Lac Operon


Department
Molec Biol & Biochem
Course Code
MBB 222
Professor
Edgar Young
Lecture
2

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Unit 2 Complexes
Regulated Genes 1155-1158, 1178-1179
Expression
o Process by which information of a gene is used in the synthesis of a functional gene
product
Constitutive vs regulated
o Unvarying gene expression is constitutive gene regulation
o Regulated gene expression is based on the rise and fall to molecular signals.
Housekeeping genes
o Genes that are required at all times for the enzymes of metabolic pathways and are
expressed at a constant level.
Basal expression rate
Positive vs negative regulation
o Negative regulation is regulation by means of a repressor protein that blocks
transcription. The action of a repressor binding to the operator is determined by a
molecule or signal called an effector which can make the repressor bind to DNA and
prevent polymerase activity, or it can remove the repressor from the DNA and allow
transcription to occur unhindered.
o Positive regulation is the regulation by means of an activator that enhances the activity
of polymerase at a promoter. Activator-binding sites are usually adjacent to promoter
sites.
Inducible/ induction vs repressible/ repression
o Gene products that increase in concentration under particular molecular circumstances
are referred to as inducible, in a process of increasing their expression is induction.
o Conversely, gene products that decrease in concentration in response to molecular
circumstances are referred to as repressible, and the process is called repression.
Transcriptional activator vs transcriptional repressor
o Repressors are transcriptional factors that repress the activity of RNAP, or impede
access of RNA polymerase to the promoter. Bind to binding sites called the operator,
which are generally near the promoter.
o Activators enhance the activity of RNA polymerase-promoter interaction.
Specificity factor
o Alter the specificity of RNA polymerase for a given promoter or a set of promoters
Promoter vs operator
o The promoter is the sequence required for the binding of polymerase to initiate
transcription.
o The operator is the binding-site on DNA for a repressor.
Effector (small molecular cofactor)
o Effectors are essentially molecular signals that bind to the active site in a repressor or
activator to either induce or repress transcription.
Allostery (conformational change)
Trans-acting vs cis-acting (diffusible factor vs operator)
Enhancer/ upstream activator sequence (UAS)
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o The regulatory sequences generally bound by transcription activators are usually called
enhancers in eukaryotes, and upstream activator sites in yeast. May be thousands of
base pairs upstream from the transcription start site, or even downstream. With the
correct binding of regulatory proteins, the enhancer increases transcription at nearby
promoters.
Basal transcription factor vs activator
Coactivator proteins
o Act as intermediaries between transcription activators and the Pol II complex
Mediator
o Major eukaryotic coactivator
o Binds to the CTD of Pol II
o Required for both basal and regulated transcription at promoters and stimulates
phosphorylation of the CTD by TFIIH.
Architectural regulator (protein) and DNA looping
o The distance between the binding sites for activators and repressors and promoters are
bridged by looping out the DNA in between. This process is made possible with the help
of architectural regulator proteins. They bind the intervening sites and facilitate the
looping of DNA.
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E.coli lac operon 1159-1160, 1165-1166
Glycosides
Galactose vs glucose vs arabinose
Glucoside vs galactosidase
Isomerization
Transglycosylation
Lactose vs allolactose
o Lactose is made up of galatoside and glucose
o Allolactose is an isomer of lactose, which is used as an inducer to cause a
conformational change in the repressor by binding to a specific site.
Thiogalactoside
IPTG
o IPTG is molecule that is an inducer that cannot be metabolized
o This allows researchers to understand the physiological function of lactose as a carbon
source, separate from its function in the regulation of gene expression.
Gene vs gene product
Lac operon
o An operon required for the transport and metabolism of lactose in E.coli. An operon
consists of an operator, promoter, and other genes.
Lacz gene vs beta-galactosidase
o LacZ gene is expressed and creates an enzyme called beta-galactosidase.
o Beta-galactosidase is responsible for breaking down lactose in galactose and glucose.
Lacy gene vs beta-galactoside permease
o LacY gene is expressed as beta-galactoside permease
o Beta-galactoside permease is responsible for the uptake of lactose into the cell
Laca gene vs beta-galactoside transacetylase
o LacA gene makes beta-galactoside transacetylase
o Its function is unknown.
Lac operator
o The lac operator is not a single region, instead it has three promoters that are separated
by other sequences, like a promoter or other genes.
O1 vs O2 vs O3 lac operators
o O1 and O2 are binded together by the repressor by looping out the LacZ gene
o O1 an O3 are binded together by the repressor by looping out the promoter region.
lacI gene vs lac repressor
o The LacI gene expresses as the repressor.
camp receptor protein (CRP/catabolite activator protein/CAP)
o activator protein, sometimes called CAP.
o CRP is a homodimer, with binding sites for DNA and cAMP.
inducer
o an inducer is a molecule that has an effect on gene regulation. It can bind to the
repressor or activator. They function by disabling repressors.
cyclic AMP
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