Sourcing Marine Natural Products

METABOLIC ENGINEERING

The biosynthetic genes for certain bioactive peptides have been isolated from bacteria living symbiotically in sea squirts and expressed in Escherichia coli, according to a new study. The work offers a sustainable way to supply marine natural products for human use.

Many marine natural products are potentially therapeutic. The true source of these bioactive compounds, however, has been hotly debated, says William Fenical of Scripps Institution of Oceanography. On the basis of structures, scientists have suspected that bioactive compounds from marine invertebrates are made by the symbiotic bacteria they harbor.

Last year, two groups identified bacterial symbiont genes likely to be responsible for polyketides in a marine sponge (Proc. Natl. Acad. Sci. USA 2004, 101, 16222) and for bryostatins in a marine bryozoan (Chem. Biol. 2004, 11, 1543). The researchers did not settle the issue of source because they did not show that the genes lead to the natural products. As Jon Clardy of Harvard Medical School puts it, "Looking at a gene sequence doesn't put compounds in a bottle."

Now, for the cytotoxic cyclic peptides called patellamides, Eric W. Schmidt at the University of Utah; Jacques Ravel at the Institute for Genomic Research, Rockville, Md.; and coworkers have taken the next step to pinpoint the source. After identifying the biosynthetic genes from the genome of Prochloron didemni, which is the bacterial symbiont of the sea squirt Lissoclinum patella, they cloned the genes and inserted them in E. coli, which produced the expected compounds (Proc. Natl. Acad. Sci. USA 2005, 102, 7315).

The work is the first "to establish that the small molecule is really made by the symbiotic microbe," Clardy says. Major efforts have been undertaken to produce marine natural products synthetically, given that supplies are limited and difficult to sustain, he explains. "Now, we're inching closer to a good supply source by culturing and not by synthesis," he adds.

Fenical--as well as David J. Newman of the National Cancer Institute--tells C&EN that scientists in Australia and the U.K. disclosed similar results last November at a meeting of the Society for Industrial Microbiology in San Diego. Team member Marcel Jaspars of the University of Aberdeen, Scotland, says they used shotgun cloning to express the patellamide genes in E. coli, whereas Schmidt and colleagues used a deliberate approach.

"Our next step was to be the sequencing of the producing clones, which would have led us to the same conclusions" as in the PNAS paper, Jaspars says. "We have not yet been successful in publishing our work," he adds.

Meanwhile, Schmidt's lab is continuing to improve the harvest of patellamides from E. coli to support a slew of experiments Schmidt is eager to do. At the top of his list, he says, is testing the flexibility of the biosynthetic pathway.