Biology 12

Chapter 8 Protein Synthesis • MHR 273

Figure 8.26A summary of the main steps in the protein
synthesis pathway in a prokaryotic cell. Each step in the
pathway provides an opportunity for turning off the
expression of a gene.

A cell can conserve more energy and resources if
the control point is earlier rather than later in the
sequence of protein synthesis steps. As you have
seen, the processes of transcription and translation
can take place simultaneously on the same mRNA
molecule in prokaryotes, which means that post-
transcriptional control is not likely to be very
effective. Also, while prokaryotic cells contain
mechanisms for breaking down polypeptides, these
mechanisms appear to play a very small role in
regulating gene expression. Thus, the most
important pathways for the regulation of gene
expression in prokaryotes are those that affect
the rate of transcription of mRNA from DNA.

The Operon Model

The bacterium Escherichia colihas a gene that codes

for an enzyme known as beta-galactosidase (or

beta-g), which E. colineeds to metabolize lactose. If

a colony of E. coliis grown on a medium containing

no lactose, the cells will not manufacture beta-g.

Within minutes after lactose is added to the medium,

however, the E. colicells will produce large

quantities of the enzyme.

In 1961, the research team of François Jacob and

Jacques Monod proposed a model to describe how

changes in the environment affect gene expression

in prokaryotes. From their research, Jacob and

Monod determined that the gene for the beta-g

enzyme is located in a transcription unit they called

an operon. An operonis a stretch of DNA that

contains a set of one or more genes involved in

a particular metabolic pathway, along with a

regulatory sequence called an operator. The

operatoris a DNA sequence located within the

promoter sequences. It functions as a control

element, governing whether or not RNA

polymerase can bind to the promoter sequences

to begin transcription of the genes. Figure 8.27

illustrates the structure of the lactose utilization

operon, or lacoperon, in E. coli.

Negative Gene Regulation in the lacOperon

When no lactose is available for the E. colicell,

a protein called a repressorbinds to the operator

region. The presence of the repressor makes it

impossible for RNA polymerase to bind to the

promoter. As a result, the genes of the lacoperon

are not transcribed. This is an example of negative

gene regulation— that is, a situation in which a

protein molecule interacts directly with the genome

to turn off gene expression.

DNA

mRNA

transcriptional control

post-transcriptional control

post-translational

control

polypeptide

functional

protein

promoter operator

control region coding region

genes for lac enzymes

PlacO Z

Y

ac

lac operon region

Figure 8.27The lacoperon of the E. colichromosome
contains a promoter sequence (known as Plac) that governs
the transcription of all three genes involved in lactose
metabolism. Transcription of the lacoperon genes produces
a single mRNA strand. During translation, internal stop and

start codons within this mRNA strand break the polypeptide

product into the three separate lacenzymes. The operator

determines whether or not RNA polymerase can reach the

genes in order to begin transcription.