Bacteria, such as Escherichia coli, divide and double every 20minutes. They grow geometrically until they reach a certain density.During this logarithmic phase, they build up sizable deposits ofbiomass. If unchecked, a single microorganism could quicklypopulate the universe.
Instead, E. coli prudently slows down to stationary phase, in which itstops multiplying. Thus immobilized, the bacterium is naked to itsenemies _ other bacteria, including neighboring E. coli. Whereuponit quickly synthesizes and secretes a small-molecule antibiotic,microcin B17, to ward off, or kill off, the competition.
Streptomyces is another bacterium that turns on its antibiotic geneswhen it goes from logarithmic to stationary phase.
Just as he who lets loose poison gas against his foe needs a gas-maskof his own, that microbe, besides releasing microcin B17, also makesan immunity protein _ microcin G _ to protect itself against its ownantibacterial weapon. It's somewhat comparable, said biologicalchemist Christopher Walsh, to the immune defenses a pregnantwoman raises to shield herself from the foreign antigens in her ownfetus.
Microcin G's mechanism, is still unknown, he observed.
Walsh formerly chaired the department of biological chemistry andmolecular pharmacology at Harvard Medical School, in Boston. Heis the senior author of a paper in the current issue of Science, datedNov. 15, 1996. Its title: "From peptide precursors to oxazole andthiazole-containing peptide antibiotics: Microcin B17 synthase."
Thiazole and oxazole, Walsh explained to BioWorld Today, "are thefive-membered aromatic rings that are the product of cyclization ofdipeptide moieties in the microcin precursor molecule. They are nowplanar in conformation, and called heteroaromatic."
He added: "That's the moiety in the anti-cancer antibiotic,bleomycin, that intercalates into DNA, and targets its gyrase."
DNA Gyrase: Antibiotics' Target Of Choice
The enzyme DNA gyrase is the bacterial equivalent of topoisomeraseII in higher forms of life. It's involved in the decatenation(unchaining) of circular DNA after replication, and serve as targetsfor a number of important anti-cancer and anti-bacterial agents.
"Among these," Walsh pointed out, "are the orally active quinolones,such as ciprofloxacin, which have DNA gyrase as their target. Theyare clearly a big antibacterial item."
Bleomycin is one of five antibiotic-like molecules that sharemicrocin's heterocycle pattern. One of these is pristinamycin, ofwhich a derivative is currently in advanced clinical testing by Rhone-Poulenc Group, in France, Walsh said, "for combating vancomycin-resistant Gram-positive bacterial infection.
"E. coli," he observed, "is not generally thought of as an antibioticproducer."
In their Science article, Walsh and his co-authors reported purifyingthe enzyme complex that cleaves E. coli's 43-amino-acid microcinB17 antibiotic from its 69-moiety precursor molecule. "Then twogenes encode two proteins, which transfer the antibiotic out of theproducing cell, get it out into the neighborhood, if you will.
"E. coli's microcins," Walsh observed, "turned out to be unusual inboth structure and target. While at least one group has worked onexamining its structure," he added, "nobody has tried to produceeither the native molecule or variants of it."
The work reported in Science, he continued, "allows one tounderstand how this particular linear peptide is converted tosomething with eight heterocycles, which now is a DNA gyraseinhibitor."
As for potential therapeutic applications of microcins, Walshobserved: "To the extent that this research sheds light on howpeptides are enzymatically converted into oxazoles and thiazoles, tothe extent that those are in a DNA intercalation unit, then I think onecould hope to adopt a combinatorial approach to biosynthesis ofmicrocins for therapeutic purposes."
In this clinical vein, he pointed out: "A growing number of peptide-based natural products contain thiazole and oxazole heterocyclicrings, and exhibit significant antifungal, antibiotic, antitumor andantiviral activities." He added: "Whether those are the activepharmacophors [therapeutic drug precursors] has not really beenresolved, but I'm suspicious that they are.
"Any time that you focus attention on heterocycles, or figure outways to create diversity in a peptide background, who knows where itwill take you."
So far, it has taken Walsh and his collaborators to confirming their invitro findings in vivo _ putting the microcin genes back into livingE. coli cultures. Their article in an impending issue of MolecularMicrobiology is titled: "The leader peptide is essential for the post-translational modification of the DNA gyrase inhibitor microcinB17." n
-- David N. Leff Science Editor
(c) 1997 American Health Consultants. All rights reserved.