Chlamydia bacterium yields surprise genome.

The bacterium responsible for chlamydia, the most common sexually transmitted bacterial disease in the United States and the leading cause of preventable blindness worldwide, has now surrendered the genetic secrets that may one day bring about its destruction. In the Oct. 23 Science, the nearly 900 genes of Chlamydia trachomatis make their debut thanks to a research group led by Richard S. Stephens of the University of California, Berkeley.

Not surprisingly, scientists are thrilled at the unveiling of the genome for this microbe, which lives only inside other cells.

"It's going to tremendously facilitate, as well as stimulate, research," says Thomas P. Hatch of the University of Tennessee in Memphis.

"It's like we were working in a room with the lights turned out, and now someone has turned them on," agrees Robert C. Brunham of the University of Manitoba in Winnipeg. "One of the most striking things to me is that we had concocted [the view] that chlamydia was a very strange microorganism, but the genome information shows us that it's very much like any other bacterium."

For example, its genome contains all the genes needed to synthesize ATP, the energy-storage molecule used by most organisms. Because scientists had previously regarded C. trachomatis as an "energy parasite" that steals ATP from its host cell, finding that the bacterium can probably generate the molecule has startled researchers.

The genome may also resolve the so-called penicillin paradox. This antibiotic which targets a common bacterial cell wall component known as peptidoglycan, works on C. trachomatis even though the bacterium wasn't thought to use the molecule. The new results, however, reveal that all the genes required to make peptidoglycans are available.

Many of the misconceptions about C. trachomatis have arisen because scientists have not been able to grow and study it in isolation. "Chlamydia biology and biochemistry are shrouded in mystery," says Hatch.

"It was always very difficult to decide what was going on in chlamydia and what was going on in the host cell," adds Brunham.

While having the chlamydia genome won't necessarily give scientists the ability to grow the bacterium, the knowledge has brought new insights. Scientists had previously identified just a single surface protein for the bacterium. The genome reveals several more.

"There's a whole new family of surface proteins we didn't know about, which definitely could be very important in developing a vaccine," says Brunham. He has been experimenting with DNA vaccines, which rely on the idea that the gene for a protein, rather than the protein itself, can stimulate an immune response (SN: 5/11/96, p. 302).

Scientists are puzzled by an absence of some genes thought essential to bacteria. In particular, a gene called FstZ drives cell division in all other known, bacterialike microbes. It's unclear how C. trachomatis can reproduce without it.

From an evolutionary perspective, as well, the new genome is a treasure chest. The bacterium possesses at least 20 genes that it has apparently borrowed from host cells over the years; other bacterial genomes have only 3 or 4. The appropriated genes most closely resemble plant genes, suggesting that C. trachomatis originally infected an ancient plantlike organism. The sheer number of stolen genes also implies that the bacterium has made a home inside cells for eons, note Stephens and his colleagues.

"A case can be made that chlamydia may have gone intracellular a long, long time ago. ... [It] may have indeed done it before multicellular organisms existed," says Hatch.

Researchers should gain an even better perspective on the evolution of C. trachomatis once they complete the genome of Chlamydia pneumonia, a related bacterium that causes pneumonia and may trigger atherosclerosis (SN: 6/1A./97, p. 375).