genetics chapter 14

•Adenyl cyclase is regulated by glucose levels

-High glucose levels inhibit adenyl cyclase -Low glucose levels, adenyl cyclase is active

lacI

-repressor, encoded by the LacI gene -regulatory protein of lac operon -controlled by its own promoter -binds the lac operator, prevents transcription of the lac operon (negative regulatory system)

A polycistronic mRNA contains the sequence(s) of ______.

2+ genes

inducer

Binds to repressor or activator → Inactivates the repressor in an operon.

True or false: Effector molecules influence gene expression by binding to DNA.

False

The Lactose Operon

Found in E. coli cells •Operon containing three genes involved in the metabolism of lactose (a disaccharide sugar) - lacZ, lacY and lacA

Inducible Systems

Genes only transcribed under specific conditions, i.e., presence of nutrient •Presence of a nutrient activates an inducer •Gene expression turned ON •Example: lac operon, when sugar lactose is present, genes that encode enzymes that metabolize lactose are turned on

Inducible Operon

Maximally expressed only when substrate (A) is present Compound A inducer binds protein A, protein A binds promoter RESULT: Transcription occurs

Operator

Region of DNA that controls RNA polymerase's access to a set of genes with related functions. binds regulatory protein

Repressible Operon

Repressed when increased amounts of product (P) are present Product binds Protein R (repressor), Repressor binds promoter RESULT: NO transcription

How does an effector molecule influence gene expression?

The effector molecule binds to a regulatory protein and affects its ability to bind DNA.

An operon is a group of two or more genes under the transcriptional control of ______.

a single promoter

positive regulation

activates transcription -Regulatory protein binds DNA or RNA polymerase directly -Helps RNA polymerase load onto promoter to initiate transcription

Allolactose → inducer

binds the lac repressor when it is bound to the DNA -Causes a change in the shape of the repressor -Repressor can no longer bind the operator -RNA polymerase can access the lac promoter and the lac genes are transcribed

inhibitor molecule

binds to activator → turns the enzyme off by closing active site

What type of genes encode proteins the bacterial cell needs all the time?

constitutive

unregulated genes are also called

constitutive genes

The research of Monod and Jacob that led to the initial understanding of gene regulation stemmed from an interest in ______.

enzyme adaptation

A protein such as a repressor that inhibits transcription is exerting ________ control of transcription.

negative

-

not functioning

In the absence of lactose, the lac operon is turned

off

In presence of lactose, the lac operon is turned

on

O^c mutation

operator can't bind to repressor

The research of Jacob and Monod provided ______.

our initial understanding of gene regulation

The lacY gene encodes lactose ____, a membrane protein.

permease

Transcription regulation by an activator protein is considered to be ______ control.

positive

lacZ

product: B-galactosidase - enzyme that cleaves lactose into two monosaccharides

lacY

product: lac permease - protein that transports lactose into the cell

lacA

product: lac transacetylase - protein thought to protect cell from buildup of toxic products created by action of B-galactosidase on other galactosides

The operon is flanked by a(n) ________ that signals the beginning of transcription and a(n) ______ that signals the end of transcription.

promoter; terminator

negative regulation

represses transcription -Blocks RNA polymerase from binding promoter -Transcriptional initiation does not occur

No Lactose →

repressor bound to operator -> no transcription of operon

LacI-

repressor that can't bind to operator

An inducer could bind to a(n) ______ and prevent it from binding to the DNA, or it could bind to a(n) ______ and cause it to bind to the DNA.

repressor: activator

LacI^s

super repressor, binds permanently to operator

+

wildtype

Repressible Systems

•Gene expression is stopped or slowed when its expression is no longer needed -Amino acid biosynthesis -Operon turned off when that amino acid concentration is high enough in cell •Presence of a repressor turns OFF gene expression •Usually end product of biosynthetic pathway that regulates genes that encode biosynthetic enzymes •Example: trp operon

Operon

•Group of genes that are coordinately regulated and encoded on a single mRNA •Polycistronic •Includes a promoter and a DNA sequence (the operator) that binds the regulatory protein

promoter

bind RNA polymerase → transcription

activators and repressors

bind to DNA close to the promoter and either activate or repress the activity of RNA polymerase

Corepressor molecule

binds to repressor → Takes inactive repressor proteins and makes it an active repressor protein.

Glucose + lactose →

cAMP levels low - can't bind to CAP - can't bind to promoter - no enhancement of transcription

Lactose + no glucose →

cAMP levels rise - binds to CAP - Complex binds to promoter -> increases transcription

Unregulated genes have ______ levels of expression

constant

The enzyme beta-galactosidase cleaves the sugar lactose into _________ and ______________

galactose; glucose

An inducer is a small effector molecule that causes transcription to ______.

increase

Lactose →

inducer binds to repressor and it leaves operator -> transcription is possible

Effector Molecules (small molecule):

inducers, corepressors and inhibitors

The three protein-encoding genes in the lac operon are

lacA, lacY and lacZ

role of B-galactosidase in lac operon

lactose → glucose + galactose. lactose → allolactose

Polycistronic

mRNA codes for more than one protein. Found mainly in Prokaryotes.

An operon encodes a __________ mRNA, an RNA that contains the sequences of two or more genes

polycistronic

In bacteria, the most common way to regulate gene expression is by influencing ______.

the rate of transcription initiation

Lac Operator and Promoter

​​•Operator has three sites •O1, O2, and O3 (when all are bound by repressor, achieve maximal repression) •Promoter has -10 and -35 and CAP (catabolite activator protein) binding sites 3 states: Inducible (normal) Constitutive (always on) Repressed (always off)

Binding of Regulator (Activator or Repressor Protein) to DNA

•Affected by signaling molecule •Binds to allosteric site on regulator, changes its three dimensional shape •Alters ability of regulator to bind DNA -May increase its ability to bind DNA -May destroy its ability to bind DNA