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Introduction
There are a variety of types of microcins encoded
by bacterial plasmids. Microcins are
naturally occurring antibiotics that inhibit the growth of
bacteria. When bacteria are under the stress of low nutrients,
they express microcin along with
membrane binding “pumps” that export the microcin out of the
cell. Microcin is toxic not only to
surrounding bacteria but also to the
bacteria that produce it, so it is
important that during the expression
of microcin they also produce these
proteins that quickly export it out of the
cell. The inhibition
of surrounding bacteria gives the microcin producing
bacteria more opportunity to acquire
sufficient nutrients
to survive.
The microcin J25 (MccJ25) inhibits
bacterial RNA polymerase. The 21 residue microcin J25 forms a
very unique “lassoed tail” structure. The chain of amino acids
binds to itself forming an 8 amino acid ring and a tail, looped
and held through the ring. The free terminal amine group
condenses to form a peptide bond with the carboxyl group of the
Glu 8 (blue), 8 residues from the amine
terminus. The tail amino acids (the
13 amino acids not included in the ring structure)
loop through the ring and are held in place. The large Phe19
(green) and Tyr20 (red) residues are facing opposite directions
on the tail; one is above the ring and
one below. Their size physically
hold the tail from sliding either direction through the ring.
Other residues (Val6 (orange), Pro7 (yellow), Glu8 (blue)) on
the ring are also thought to stabilize the
lasso structure by interacting with the Phe19 (green) and
Tyr20 (red) on the tail.
It is unknown how microcin J25 is oriented when it inhibits
bacterial RNA polymerase, but it is known that it blocks the
secondary channel. The secondary channel is where the
nucleotides enter the active cite and combine into a strand of
RNA. When this channel is blocked, it is impossible to
continue synthesis
of RNA. The synthesis of RNA by RNA polymerase is crucial
for survival because it is a key stage of
protein expression. Without transcription,
bacteria will no longer be able to produce
proteins to support their functions and growth.
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JMol scripts and text by Sam Williams
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