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Microbial Models II:
Transposons and Regulation of
Gene Expression
12 November, 2003
Text Chapter 18
Transposable elements (transposons) can move from one location
to another in the genome.
Insertion sequences, the simplest transposons, contain only a
transposase gene flanked by two inverted repeats.
In transposition, transposase
cuts DNA at the target site.
Then, it catalyzes the
movement of the transposon
to the new site.
This movement can cause
mutations in it moves the
transposon into the coding
sequence or regulatory
regions of a gene.
Composite transposons can carry genes from location to location if the
genes are located between two insertion sequences.
Regulation of Gene
Expression
Biochemical
pathways are often
regulated by
feedback inhibition.
Usually, the synthesis
of the enzymes in the
pathway is also
regulated. This
regulation is at the
level of transcription.
The trp operon is a biosynthetic operon - it codes for the enzymes
that make the amino acid tryptophan. This pathway should be kept in
the “off” state when tryptophan is present. When tryptophan is
absent, the bacteria need to make it from scratch. Now the pathway
needs to be turned on.
The lac operon is a catabolic operon - it codes for the enzymes that
burn the sugar lactose for fuel. This pathway should be kept in the
“off” state when lactose is absent. When lactose is present, the
bacteria can burn it for fuel. Now the pathway needs to be turned on.
Furthermore, the
bacterium does not burn
lactose for fuel if its
preferred carbon source
(glucose) is available. The
decision about the
presence or absence of
glucose is independent of
the decision about the
presence or absence of
lactose, and is made by the
CAP protein and the
nucleotide cAMP.
The cell only expresses the
lac operon when lactose is
present and glucose is
absent.