Discovery Could Lead to New Broad-spectrum Antibiotics

Suzanne Walker, PhD, professor of microbiology and molecular genetics at Harvard Medical School, studies bacterial cell wall biosynthesis, one of a few validated broad-spectrum targets for antibiotics. Walker reports in the March 23 online issue of Chemistry and Biology that her lab has identified the biosynthetic genes for moenomycin, the only known natural product that directly inhibits an important family of cell wall biosynthetic enzymes, the peptidoglycan glycosyltransferases. These enzymes make the carbohydrate chains of peptidoglycan.

Moenomycin is used in animals as a growth promoter but it is not used in humans because it has poor physical properties. With their discovery of the biosynthetic genes, the Walker research team has been able to develop a bacterial expression system to make derivatives of moenomycin. Using those derivatives, they have begun to manipulate the structure of moenomycin in order to figure out what parts of the molecule are critical for activity, and what parts can be altered to make a better antibiotic for human use.

The effort to develop better moenomycin-based antibiotics will benefit from two crystal structures of peptidoglycan glycosyltransferases Ð one from the Walker lab and one from Natalie Strynadka's group in Canada showing the location of moenomycin in the active site. These enzymes are considered crucial in antibiotic research and the crystal structures provide the tools to exploit them.

The moenomycin biosynthetic pathway is unusual in terms of both its genetic organization and the novelty of some of the biosynthetic genes in the pathway.

"Because moenomycin does not look like antibiotics whose biosynthetic pathways are understood, we could not find the biosynthetic genes using standard approaches," Walker said. She contracted with the Broad Institute of MIT and Harvard to sequence and assemble the genome of the producing organism.

"Once we had the genome, we searched for genes meeting criteria suggesting that they might be involved in moenomycin biosynthesis," she said. "Once we had a candidate gene cluster, we individually deleted some of the genes in the cluster and then analyzed cell extracts for moenomycin intermediates."

Final confirmation that the Walker team had identified the gene cluster came from experiments showing they could make moenomycin in another organism by transferring the candidate genes into that organism.

"It shows us a new way in which microorganisms can make carbohydrate-based antibiotics," said Walker. "By searching other genomes for similar clusters of genes, we have identified organisms that contain sets of genes that may make related compounds. These other gene clusters could not be assigned a function until the moenomycin genes were identified.

"We are beginning to look in these organisms for new bioactive metabolites that are related to moenomycin, and we hope this work will lead to the discovery of new antibiotics," she said.

The study was funded primarily by Harvard Medical School, with some support from the National Institutes of Health.

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