SBI 3U7
Protein Synthesis
Control Mechanisms
Levels of Control of Gene Expression in Eukaryotic Cell
 Eukaryotes lack an universal regulatory mechanisms to control the expression of genes coding
proteins
Type of Control
Description
Transcriptional
 In nucleus
 Regulates which genes are transcribed
 Controls the rate at which transcription occurs
Posttranscriptional
 In nucleus after DNA was transcribed and primary transcript was
formed
 Introns are removed from the primary transcript and exons are
spliced together in mRNA transcript
Translational
 In cytoplasm before protein product
 Controls how often and how rapidly mRNA transcripts will be
translated into proteins
 This control affects the length of time it takes for mRNA to be
activated and the speed at which cytoplasmic enzymes destroy
mRNA
Posttranslational
 In cytoplasm after protein product
 Proteins become functional only after they pass the cell membrane.
 Different control mechanisms affect the rate at which protein
becomes active and the time that it remains functional.
Prokaryotes
The Operon Model
 Bacteria do not require the same enzymes all the time. They produce enzymes needed at the
moment.
 In 1961, French microbiologists Francis Jacob and Jacques Monod proposed the operon model
to explain regulation of gene expression in prokaryotes; they received a Nobel prize for this.
 The Operon model: includes several genes that code for enzymes in the same metabolic
pathway and are located in sequence on the chromosomes
 It includes the following:
A regulator gene
A promoter
An operator
Structural genes
The lac Operon
Located outside the operon
Codes for a repressor protein molecule
Is a sequence of DNA where RNA polymerase attaches when a gene is
transcribed
Is a short sequence of DNA where the repressor binds, preventing RNA
polymerase from attaching to the promoter
Code for enzymes of a metabolic pathway, and are transcribed as a unit
SBI 3U7
Protein Synthesis
Lactose
 Disaccharide found in milk
 Consists of two sugars: glucose and galactose
 E.coli bacterial cells found in the intestinal lining can
use the energy supplied by lactose for growth. To be
able to use this energy E.coli must split lactose into
glucose and galactose.
 E. coli produces an enzyme called -galactosidase
o By using a negative regulation control the transcription and translation of the
-galactosidase gene (in the bacterial cell) produce the enzyme only when necessary.
Regulatory Gene Promoter
Operon
Structural genes
lacZ
mRNA
protein
Encodes the
enzyme
-galactosidase
lacY
Encodes the
enzyme
-galactosidase
permease
lacA
Encodes the
enzyme
transacetylase
Repressor
-blocks/activates the transcription of the
-galactosidase gene by binding/no
binding to the lactose operator
(signal molecule or
inducer)
Lactose is present
Repressor
Protein
Lactose is absent
lactose
glucose +
galactose
facilitates
entry of
lactose in cell
accessory
in lactose
metabolism
SBI 3U7
Protein Synthesis
Repressor
Protein
The trp Operon
 Jacob and Monod found some operons in E. coli that exist in the “on” condition rather than the
“off” condition.
 E.Coli bacteria use the amino acid tryptophan to produce protein.
 This prokaryotic cell produces five polypeptides to yield three enzymes to synthesize the amino
acid tryptophan.
 In the trp operon, the regulator (upstream) codes for a repressor protein that has a binding site to
tryptophan
 When tryptophan is in abundance, it will bind to the repressor protein allowing the repressor to
bind to the operator. This will inhibit transcription.
 Since you need trp to activate the repressor, Tryptophan is called a Corepressor.
Tryptophan
is absent
Homework: p. 258 #1-6