BIOL 330 Study Guide - Quiz Guide: Lac Repressor, Lac Operon, Repressor
Transcription Regulations (part 1):
• Terms:
o Coupled transcription/translation: phenomena in bacteria where translation of the
mRNA occurs simultaneously with its transcription
o Operon: unit of bacteria gene expression and regulation, including structural genes and
control elements in DNA recognized by regulator gene products
▪ “on/off” switch for gene expression
o Trans-acting: product that can function on any copy of its target DNA
▪ Diffusible protein or RNA
o Cis-acting: affect activity only of sequences on its own molecule of DNA or RNA
▪ Usually does not code for protein
o Regulatory genes: codes for a product that controls the expression of other genes
o Structural genes: encodes any RNA or protein product other than a regulator
o Negative regulation: repressor protein binds to an operator to prevent a gene from
being expression
▪ Turns off transcription
o Positive regulation: transcription factor is required to bind to promoter in order to
enable RNA polymerase to initiate transcription
o Inducible regulation: gene is regulated by presence of an inducer (a substrate)
o Repressible regulation: gene is regulated by the product of its enzyme pathway
(corepressor)
o Polycistronic mRNA: many genes that functions collectively (in the same pathway) to
code for proteins
o Constitutive expression: state in which a gene is expressed continuously
o Gratuitous inducer: molecule structurally similar to another molecule that induces
transcription and can also induce transcription in absence of official inducer
• Gene regulation can be:
o Negative inducible
o Positive inducible
o Negative repressible
o Negative repressible
• Lac operon is a negative inducible: repressor (lac repressor) binds to operator to prevent
transcription (which is usually on)
o Addition of specific β-galactoside sugars induces transcription of all three genes of the
lac operon
o Lac mRNA is extremely unstable and induction is easily reversible
• The lac repressor is controlled by a small molecule inducer
o Has two binding sites; one for the operator DNA and one for the inducer
o True inducer: allolactose
o Gratuitous inducer: β-galactosidases
o Inducer binds to repressor to lower its affinity to the promoter, hence allowing
transcription
• Types of mutations:
o 1.) cis-acting constitutive mutation:
▪ Causes constitutive expression in all three lac structural gene (operator
mutation)
▪ Only affect genes on contiguous stretch of DNA
▪ Prevents expression of lac ZYA and are uninducible (promoter mutation)
▪ Cis-dominant: site or mutation that affects the properties only of its own
molecule of DNA
o 2.) trans-acting mutation:
▪ Affects expression of all lac ZYA clusters (lacl mutation)
▪ Causes constitutive expression and are recessive (mutation that eliminates lacl
function)
▪ Stop binding with operator (mutations in DNA-binding sites)
• Operator competes with low affinity sites to bind repressor
o Proteins that have a high affinity for a specific DNA also has a low affinity for other DNA
sequences
o Repressor binds to an operator moving from a low affinity site
• Catabolic repression: ability of glucose to prevent expression of a number of genes
o Catabolic repressor protein: activator protein that binds to a target sequence at a
promoter to enable transcription
o Example: cAMP
o Low levels of glucose makes cAMP
o Reduced cAMP=inhibits transcription of operon that uses CRP activity
• trp operon is a repressible operon with three transcription units:
o negatively controlled by amino acid tryptophan (its product)
o tryptophan: activates an inactive repressor
o a repressor (or activator) will act on all loci that have a copy of its target operator
sequence
• attenuation: regulation of bacterial operons by termination of transcription at a site located
before the first structural gene
o attenuator is located between the promoter and the first gene of the trp cluster
▪ absence of trp-tRNA suppresses termination
▪ in presence of trp-tRNA, attenuator forms hairpin hairpin structure that causes
termination
Transcription regulations (part 2):
• terms:
o histones: eukaryotic proteins found in nuclei that packages and order DNA into
nucleosomes
o cofactors: bind to activators to increase transcription rate
o antirepressor: positive regulator that functions in opening chromatin
o architectural protein: protein that can alter the shape of DNA once attached to it and
control whether bond proteins can contact each other
• eukaryotic gene expressions are usually controlled at the initiation level of transcription which
involves the opening of chromatins
• how is a gene turned on?
