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Chapter 14 gene regulation inbacteria
Gene regulation = level of gene expression.
Constitutive genes: unregulated genes, constantly expressed. Proteins that are constantly necessary.
Gene regulation is necessary for:
Metabolism
Response to environmental stress
Cell division
Differentiation and Development
Fitness of cell
Important to know is that repressors and activators bind to the DNA.
Inducers = small effector molecules that increase transcription. Bind to activators and repressors
genes that are regulated this way inducible.
Corepressors = small effector molecules bind to repressors and cause them to bind to DNA.
repressible
Inhibitors = bind to activators and prevent them from binding to DNA repressible
Francois Jacob and Jacques Monod = lactose metabolism, regulation of lac operon. Enzyme
adaptation.
Operon concept = regulatory unit consisting of a few structural genes under control: one promoter
and one terminator.
Polycistronic mRNA = an RNA that contains the sequence of two or more genes.
Catabolism = breakdown of a substance, operons, inducible
Anabolism = biosynthesis of a substance, operons, repressible
Lactose operons genes:
- CAP site = Catabolite Activator Protein
- Lac Z encodes B-galactosidase -> enzyme cuts lactose into glucose and galactose. Sometimes
allolactose.
- Lac Y encodes lactose permease -> membrane protein necessary for transport lactose into
cytoplasm.
- Lac A encodes transacetylase -> modifies nonmetabolizable lactose to prevent toxic build up by
diffusing out of the cell.
- Lac I does not belong in lac operon, has own promoter. Produces lac repressor.
Allolactose is an inducer, binding to the lac repressor, resulting in expression of lac operon gene.
Merozygote = partial diploid
Cis mutant = constant expression, repressor can not bind to operator
Trans mutant =
Is (Super repressed) = constitutive repressor
Iq ( quantity) = produces more repressor than the normal gene
lacO- or lacOc = made sure of constitutive expression. Repressor wont bind to gene.
Catabolite repression = glucose as catabolite. High levels of glucose means low levels of cAMP. cAMP
,(comes from ATP) binds to CAP protein, 2 subunits. CAP with cAMP bind then to CAP site in DNA.
Diauxic growth = the sequential use of two sugars by a bacterium
No lactose, glucose: the repressor is binding to DNA due to no lactose. Slow very slow transcription.
Lactose, no glucose: high cAMP, high allolactose, high cAMP high transcription rate
Lactose, glucose: CAP doesn’t bind, but no repressor. Transcription rate is low.
No lactose or glucose: high cAMP, so CAP is binding to CAP site. However, no lactose means
repressor binding to DNA causing no transcription rate.
Three operator binding sites:
- O1 slightly downstream from promotor
- O2 located further downstream
- O3 upstream from promotor
- only O1 and O2, or O1 and O3 go well. O2 and O3 not.
14.3 The expression of the trp operon
Repressible operon =
example is the tryptophan operon (trp) biosynthesis of the amino acid tryptophan.
Genes involved: trp E,D,C,B and A encode for enzymes.
trpR and trpL are involved in regulation, trpR trp repressor protein, has its own promotor.
trpL leader peptide,
attenuation
When tryptophan levels are low,
the repressor (tpr) cannot bind to
DNA.
When tryptophan levels are high,
it bind to repressor changing its
structure allowing it to bind to
repressor site.
, Attenuator sequence =
Transcription usually begins, but is terminated before the entire mRNA is made. Because of
attenuator sequence. This process inhibits the expression of the genes after the attenuator
sequence is recognized by RNA polymerase.
trpL gene = codes for 14 amino acids. 1-2 2-3 and 3-4.
No translation, 1-2 and 3-4
Low tryptophan levels, 2-3 stem loop forms
High tryptophan levels, 3-4 stem loop form. Stop codon, stops quickly.
!The histone macroH2A is associated with the barr body (inactive X chromosome in females)!
Antisense RNA inhibit translation. At low osmolarity, the ompF is produced, high osmolarity not.
3-5% of bacterial genes are regulated by riboswitches.
Riboswitches transcription:
TPP levels low: secondary structure called antiterminator (stem loop)
TPP levels high: TPP binds to RNA as it is being made. Terminator stem loop forms. Negative
feedback, translation high means more TPP translation low means low TPP.
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