Genetics Test 3

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Flow of information from DNA to final active protein

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Transcriptional Control

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This form of control occurs when regulatory proteins affect RNA polymerases ability to bind to a promoter and initiate transcription. The process of making mRNA’s only for proteins the cell needs. If genes for uneeded proteins are not transcirbed into mRNA, in the next pathway, then ribosomes cannot make these proteins. Transcriptional control is particularly important due to its efficiency, it saves the most energy for the cell because it controls gene expression before the cell expends many resources.

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Translational Control

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This form of control occurs either through regulation of the mRNAs life span or ability to be translated. Prevention of production of the mRNAs for uneeded proteins. Translational control allows more rapid changes than transcriptional control in the amounts of different proteins because the mRNA has already been made and is available for translation.

This form of control occurs either through regulation of the mRNAs life span or ability to be translated. Prevention of production of the mRNAs for uneeded proteins. Translational control allows more rapid changes than transcriptional control in the amounts of different proteins because the mRNA has already been made and is available for translation.

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Post Translational Control

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After translation, many proteins have to be activated by chemical modification, such as the addition of a phosphate group, in order to function. These modifications can be controlled to regulate gene expression. Post-translational control provides the most rapid response of all three mechanisms because only one step is needed to activate or inactivate an existing protein.

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constitutively

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transcribed all the time

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Galactoside Permease

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transports lactose into the cell

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Beta-Galactosidase

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breaks down lactose inside the cell

E. Coli produces a high level of B-Galactosidase only when lactose is present in the environment.

lactose itself regulates the gene for B-galactosidase, as an inducer

Significant amounts of B-galctosidase are produced only when lactose is present and glucose is absent.

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inducer

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small molecule that triggers transcriptoin on a specific gene

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Back 9

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Two ways that transcription of any gene can be regulated

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negative control

positive control

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Negative Control

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occurs when a regulatory protein called a repressor binds to DNA and shuts down transcription.

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postive control

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occurs when a regulatory protein called an activator binds to DNA and triggers transcription

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the gene that encodes B-galactosidase

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lacZ

muatant allele: lacZ -

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gene that encodes galactoside permease

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lacY

mutant allele: lacY -

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lacZ-

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cells cannot cleave lactose, even in the presence of inducer (lactose)

No B-galactosidase; gene for B-galactosidase is defective.

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lacY -

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cells cannot accumulate lactose

no membrane protein (glactoside permease) to import lactose; gene for galactoside permease is defective.

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lacI -

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cells can cleave lactose even if lactose is absent as an inducer.

constitutive (constant) expresssion of lacZ and lacY; gene for regulatory protein that shuts down lacZ and lacY is defective.

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lacI

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gene for regulatory protein that shuts down lacZ and lacY

the normal product of the lacI gene prevents the transcription of lacZ and lacY when lactose is absent. Negative regulator because lactose triggers the production of B-galactosidase.

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constitutive mutants

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cells that are abnormal because they produce a product at all times instead of regulating expression of the product

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lac I gene

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the lacI gene produces a repressor protein that binds directly to DNA and overlaps the promoter for the lacZ and lacY genes. Binding of the repressor blocks RNA polymerase, either by preventing inititiation or by interfering with the binding of RNA polymerase to the promoter.

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lacZ and lacY

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Genes involved in lactose metabolism are under negative control

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a) lactose absent inside the cell; repressor active

b) lactose present in cell; repressor inactive

c) lactose present or absent inside cell; mutant repressor gene; transcription occurs

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lactose absent inside the cell

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the repressor is active

the repressor binds to DNA

Transcription is blocked

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lactose present inside the cell

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repressor is inactive

lactose (the inducer) binds to repressor

repressor releases from DNA

transcription occurs

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lactose present or absent inside mutant

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mutatnt repressor gene

transcription still occurs

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lactose (allolactose)

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lactose acts as an inducer by causing the repressor to release from DNA and ending negative control.

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polycistronic

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an mRNA that codes for two or more different polypeptides

Manu bacterial mRNA's are polycistronic but almost all eukaryotic mRNAs code for a single polypeptide.

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operon

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a set of coordinatley regulated bacterial genes that are transcribed together into one polycistronic mRNA.

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lac operon

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the group of genes involved in lactose metabolism

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lacA

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gene that codes for the enzyme transacetylase

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transacetylase

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this enzyme catalyzes reactions that allow certain types of sugars to be exported from the cell when they are too abundant and could harm the cell.

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the lac operon is a set of coordinately regulated genes

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this view emphasizes the arrangment of genes and regulatory sequences and is not drawn to scale.

The lacI gene is not part of the lac operon, but it is close to the operon genes and is included in this figure because of its importance in lac operon control

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lacZ, lacY and lacA genes are adjacent

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The lacZ,lacY, and lacA genes are adjacent and are transcribed into one mRNA initiated fromthe single promoter of the lac operon. This is known as cotranscription, and it results in thecoordinated expression of the three genes.

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The repressor protein encoded by lacI binds to a specific sequence in DNA

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The repressor protein encoded by lacI binds to a specific sequence in DNA and preventstranscription of the lac operon genes (lacZ,lacY, and lacA). Jacob and Monod proposed that lacIis expressed constitutively; that the repressor binds to a DNA sequence in the lac operon calledthe operator; and that repressor bound to the operator prevents the RNA polymeraseholoenzyme from binding to the promoter of the lac operon.

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The inducer (lactose) binds to the repressor

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The inducer (lactose) binds to the repressor. When it does, the repressor changes shape. Theshape change caused the repressor to come off the DNA. recall that this form of contorl overprotein function is allosteric regulation (Ch.8, Section 8.4). In allosteric regulation, a smallmolecule binds to a protein and causes it to change its shape and activity. When the inducerbinds to the repressor, the repressor can no longer bind to DNA, and transcription can proceed.

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catabolite activator protein CAP

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transcribed and translated constitutively

always available to regulate transcription

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CAP

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binds to a regulatory sequence of DNA just upstream of the promoter, CAP binding site

CAP bound to the regulatory sequence in DNApromotes the association of the RNA polymerase holoenzyme with the promoter through a stabilizinginteraction between CAP and the holoenzyme.

CAP must be bound to cyclic AMP cAMP in order to bind to DNA

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Glucose regulation of cAMP

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When glucose levels outside the cell are high cAMP synthesis is inhibited, and without cAMP, CAP does not bind to the CAP binding site in DNA.

In this case, transcription is not stimulated.

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Glucose regulation of cAMP

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when the glucose level outside the cell is low, cAMP synthesis within the cell ramps up, the cAMP level increases, and CAP forms a complex with cAMP.

CAP associated with cAMP then binds to the CAP binding site upstream of the promoter and stabilizes the interaction of RNA polymerase with the lac operon promoter. Transcription now begins much more frequently.

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Inducer exclusion

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when glucose inhibits the transport of sugars other than glucose into thecell

when glucose is abundant in the environment, the transport of lactoseinto the cell by galactoside permease is inhibited.Because lactose does notaccumulate in the cytoplasm, the repressor remains bound to the operator. Negative control is in place.

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Inducer exclusion

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when the glucose level outside the cell is low, galctoside permease is active. If lactose ispresent, it is transported into the cell and induced lac operon expression.

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the trp operon (tryptophan)

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The trp repressor binds to its operator only when it is bound by its regulator, tryptophan.

When the tryptophan level drops, the repressor no longer binds the operator. The trp operon genes arenow transcribed, the mRNA is translated, and the tryptophan level is restored. This regulation is a formof negative feedback control, a form of control in which the final product of a pathway inhibits theproduction of the product.

the small molecule regulator that binds to the repressor allosterically is called the co-repressor.

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trp operon

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global gene regulation

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the coordinated regulation of many genes

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regulon

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a set of separate genes and operons that contain the same regulatory sequences and are controlled by a single type of regulatory protein.

Like operons, regulons can be under negative control by a repressor protein or under postive contorl byan activator protein.

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SOS response regulon

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allows bacterial cells to surviveand repair extensive damage to DNA

is under negative control

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SOS response

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damage toDNA sets off an SOS signal that induces the transcription of more than 40 genes that code for enzymesneeded for DNA repair, recombination, and specialized DNA polymerases that can use damaged DNAas a template. These proteins work to allow the cell first to survive the DNA damage and ultimately torepair it.

A key part of the regulatory logic of the SOS system is that it is under the control of one of its owngenes. The gene, called lexA, codes for the LexA protein that represses transcription of SOS regulongenes while the cell is healthy. DNA damage sets off a signal for LexA to be cleaved and thereforeinactivated. Inactivation of LexA releases the brake on transcription of all SOS genes, not only thoserequired for DNA repair, but lexA itself. While repair proteins are being produced and going to work,the DNA damage that signals LexA cleavage diminishes and levels of active LexA are replenished. Bythe time damaged DNA is repaired, the SOS regulon is restored to its starting point by active LexA.The entire system is ready to spring into action again should DNA be significantly damaged.

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SOS regulon uninduced state

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genes of the regulon silenced by a common repressor

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SOS regulon induced state

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Repressor inactivated; regulon genes expressed