Microbial Models II: Transposons and Regulation of Gene Expression 10 November, 2004 Text Chapter 18

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Microbial Models IITransposons and Regulation
of Gene Expression10 November, 2004Text
Chapter 18
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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.
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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.
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Composite transposons can carry genes from
location to location if the genes are located
between two insertion sequences.
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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.
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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.
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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.
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