Genetics CH 16

An inducer acts as a ; Usually a

"switch" to turn the gene on; chemical substance in the nutrient medium

A separate regulatory gene

(with its own promoter) encodes a regulatory protein

Lac Operon

1. -35 region consensus 2. -10 consensus 3. lacP - promoter 4. Transcription start site 5. lac Repressor 6. operator bound by lac repressor 7. lacZ gene

Genes controlled at many levels

1. Alternation DNA or chromatin structure 2. Level of transcription 3. mRNA processing 4. Regulation of mRNA stability 5. Level of translation 6. Modifications after translation

conjugation f' cell and f-cell

1. Crossing over takes place in HFr chromosome 2. When F factor excises from bacterial chromosome, it may carry some bacterial genes (lac) with it 3. During conjugation the F factor with lac is transferred to F- 4. Produces a partial diploid with 2 copies of the lac gene

What happens at the cellular level when lactose is available?

1. Permease actively transports lactose into the cell 2. where the enzyme B-galactosidase breaks it into galactose and glucose 3. B-galactosidase also converts lactose into the related compound allolactose 4. and converts allolactose into galactose and glucose

Trp Operon in E. coli

A negative repressible operon Five structural genes trpE, trpD, trpC, trpB, and trpA—five enzymes together convert chorismate to tryptophan.

Regulator gene:

DNA sequence encoding products that affect the operon function but are not part of the operon

Regulatory elements

DNA sequences that are not transcribed but play a role in regulating other nucleotide sequences

Conjugation

Depends on plasmid Not a reciprocal exchange of genetic material

E. coli—F factor

F+ contains F factor; F- does not contain F factor Contains origin of replication Genes for conjugation--pili

Attenuation in the trp Operon of E. coli; When tryptophan is high,; When tryptophan is low

Four regions of the long 5′ UTR (leader) region of trpE mRNA; region 1 binds to region 2, which leads to the binding of region 3 and region 4, terminating transcription prematurely; region 2 binds to region 3, which prevents the binding of region 3 and region 4, and transcription continues.

Inducible genes

Gene that can be turned on or off depending on the environment

Plasmids

Small circular DNA molecules separate from chromosome Carry genes that are not absolutely required for bacterial function Antibiotic resistance genes

Negative repressible operons

The control at the operator site is negative. But such transcription is usually on and needs to be turned off, so the transcription is repressible.

Negative inducible operons:

The control at the operator site is negative. Molecule binding is to the operator, inhibiting transcription. Such operons are usually off and need to be turned on, so the transcription is inducible.

Some Operons Regulate

Transcription Through Attenuation, the Premature Termination of Transcription

Repressible operons

Transcription is normally on and needs to be turned off.

Inducible operons

Transcription is usually off and needs to be turned on.

an operon is

a group of structural genes plus sequences that control transcription

The operator is usually where

a regulatory protein binds

The binding of the cAMP-CAP complex to DNA produces

a sharp bend in DNA that activates transcription.

Corepressor

a small molecule that binds to the repressor and makes it capable of binding to the operator to turn off transcription.

cAMP:

adenosine-3′, 5′-cyclic monophosphate The concentration of cAMP is inversely proportional to the level of available glucose.

Lac mutations: Structural gene mutations

affect the structure of the enzymes but not the regulation of their synthesis

Attenuation:

affects the continuation of transcription, not its initiation. This action terminates the transcription before it reaches the structural genes.

Regulatory protein might

bind to operator site to regulate the transcription of mRNA

Mutations in lacO are (a) The partial diploid lacI + lacO + lacZ −/ lacI + lacOc lacZ+ is (b) The partial diploid lacI + lacO + lacZ+/lacI + lacOc lacZ− is

constitutive and cis acting. constitutive, producing β-galactosidase in the presence and absence of lactose. inducible (produces β-galactosidase only when lactose is present), demonstrating that the lacO gene is cis acting.

The enzymes required for glucose utilization are

constitutively expressed

Constitutive expression

continuously expressed under normal cellular conditions

Regulatory genes

encoding products that interact with other sequences and affect the transcription and translation of these sequences

Structural genes

encoding proteins

lacI+lacP−lacZ+/lacI+lacP+lacZ−

fails to produce functional β-galactosidase.

The partial diploid lacI s lacZ +/lacI + lacZ +

fails to produce β-galactosidase in the presence and absence of lactose because the lacIs gene encodes a superrepressor.

Lac mutations: Partial diploid

full bacterial chromosome + an extra piece of DNA on F plasmid

- lacZ+lacY−/lacZ−lacY+ produces

fully functional β-galactosidase and permease.

lacI+lacO+Z−/lacI+lacOclacZ+ produce

fully functional β-galactosidase constitutively.

lacI+lacZ−/lacI−lacZ+ produces

fully functional β-galactosidase.

If the nutrient medium contains lactose instead of glucose as the carbon source,

genes that express enzymes to utilize lactose are turned on

Lactose enzyme genes are _______ , and allolactose is the ________ ; These enzyme genes form the

inducible; inducer; lac operon

Negative control

inhibit gene expression

lacI+ is dominant over; A single copy of lacI+ brings about

lacI− and is trans-acting.; normal regulation of lac operon.

Lac mutations: Regulator-gene mutations

lacI− leads to constitutive transcription of three structural genes.

Lac mutations: Operator mutations

lacOc: C = constitutive; lacOc is dominant over lacO+, which is cis acting.

Lac mutations: Promoter mutations

lacP−: cis acting

Proteins (enzymes) are the product

of mRNA catalyst reactions in a biochemical pathway

Episomes

plasmids that are capable or replicating freely and are able to integrate into the bacterial chromosomes F (fertility) factor

Operon:

promoter + additional sequences that control transcription (operator) + structural genes

Attenuator—

secondary structure that forms in the 5' UTR region of some operons and causes premature termination of transcription

Antiterminator—

secondary structure that forms in the 5' UTR that inhibits the termination of transcription

E. coli normally grow in a; The carbon source is usually ;

simple medium containing salts, a nitrogen source, and a carbon source; glucose

Inducer

small molecule that turns on the transcription.

Positive control

stimulate gene expression

Two different secondary structures can be formed by

the 5′ UTR of the mRNA transcript of the trp operon.

The inducer influences ; via _____ DNA sequences called

the transcription of the inducible gene(s); cis controlling; operators which are found adjacent to the coding sequence of the gene(s)

Catabolite repression:

using glucose when available and repressing the metabolite of other sugars. This is a positive control mechanism. The positive effect is activated by catabolite activator protein (CAP). cAMP binds to CAP; together CAP-cAMP complex binds to a site slightly upstream from the lac gene promoter.

The partial diploid lacI + lacZ −/lacI − lacZ + produces

β-galactosidase only in the presence of lactose because the lacI gene is trans dominant.; KNOW STRUCTURE