School Papers

The for energy. If glucose is not available



The Lac Operon and the PaJaMo experiment


Bacteria can readily utilize glucose as a
source for energy. If glucose is not available bacteria can use other sources
of energy such as lactose. The process of lactose digestion was discovered by
Jacob and Monod in 1959, where they called this
action the lac operon. The lac operon is one of the most studied operons and it
is known that it is repressible, inducible system. The metabolism of
lactose was found to be a complicated procedure controlled by three genes
namely lac Z, lac Y, and lac A.  The Lac
Z gene encodes an enzyme called ?-galactosidase whose function is to digest
lactose into glucose and galactose.  The
function of the Lac Y gene is to produce a protein,
which is embedded into the bacterial membrane that allows the galactose to pass from
the media to the cell. The function of the Lac A is not well understood.  A single mRNA and one single promoter control
those genes. This whole bundle of genes is called an operon. Beside those genes
there are other factors responsible for the function of the Lac operon, those
factors are a glucose sensor better known as catabolite activator protein (CAP),
and a lactose sensor which is known as the lactose suppressor, and an inducer. The inducer is formed as a result of lactose
metabolism, where lactose is transformed into allolactose by the action of ?-galactosidase, which is part of the lac operon. Allolactose
will bind to the repressors and allow them to fall off the operator, thus force
the activation of the lac operon

. This operon will be inducible
in the presence of lactose and will be suppressed in the presence of glucose. To
be able to understand this phenomena  Pardee, Jacob, and Monod devised an
experiment to introduce mutation through bacterial conjugation.  This allows the Bacteria to exchange
genetic material where one bacterium (F+) will donate a small circular DNA
called plasmid to another bacterium lacking this plasmid (F-). Pardee, Jacob, and Monod were able to further
study the lac operon by mixing a wild type bacteria with a mutant lacking ?-galactosidase and
also lacking the repressor genes.. One
of the reasons that F+ plasmid can become stable in the F- host is that it has
an insertion sequence which has two repeats on either end and contains genes which
can insert itself into the F- bacterial chromosome, thus becomes part of the
genome. This is called Hfr strain (high frequency of recombination). Hfr can
start to replicate and transfer DNA of the F plasmid and pulls host chromosome
and transfer genetic material. This allows the transfer of genes from one cell
to another. Utilizing this knowledge, they started an experiment with a bacterial strain that has
an inactive repressor lacI- and an inactive ?-galactosidase strain (lacZ-) and conjugated it with another
stain that was lacZ+ and lacI+.  In the
first set of experiences they found that there was an increase of ?-galactosidase production but only for two hours where it was shut off. When
they added an inducer at two hours it would continue to produce. However when
did the reverse strain beta-galactosidase was never produced unless they added
an inducer. Because the second systems cell had active beta-galactosidase
present and an active repressor at the start it would not produce ?-galactosidase (because it was getting repressed since the start)
unless there was an inducer.