Wee work-around lets microbes thrive

Crafty, thieving cells manage to synthesize vital molecules

A group of salt-loving microbes has cobbled together a method for synthesizing cellular building blocks, using a hodgepodge of machinery that would make MacGyver proud. Much of the machinery in the newly discovered assembly line was acquired from distantly related bacteria, probably through wholesale genetic theft that occurred long ago, researchers report in the Jan. 21 Science . A lot of cellular day-to-day living entails breaking down compounds into molecules for fuel. But food is also converted into building blocks that cells use to create larger structures, such as fatty acids or carbohydrates. Before such construction can commence, compounds often are pared down to intermediate molecules that can be shunted down specific assembly lines, depending on what’s needed. A key such molecule is acetyl coenzyme A, which ultimately helps cells create additional useful building blocks if the right enzymes are working the assembly line. Plants, some fungi and some bacteria use a process called the glyoxylate cycle to achieve this, and for 50 years scientists thought this was the only way it could be done. But a few years ago, researchers discovered that some microbes have an alternate technique. The newly discovered third route is a further testament to the ingenuity of life, says study leader Ivan Berg of Universität Freiburg in Germany. “Sixty years ago, people thought in terms of the biochemical unity of life,” says Berg. “Now it’s obvious that we have many pathways for many processes.” The new pathway isn’t tidy. It takes nine steps for the salt-loving microbes to convert acetyl-CoA into an intermediate molecule that the glyoxylate cycle makes in one step. And enzymes that typically work other assembly lines appear to have been called upon for this alternate pathway, dubbed the methylaspartate cycle by Berg and his colleagues. In fact, some of the enzymes involved in the new cycle look so much like enzymes used by distantly related bacteria that the researchers say the salt-loving microbes stole the genes for these enzymes sometime in the evolutionary past. This theft may have been crucial in the establishment of this microbial group, which uses oxygen but evolved from critters that subsist in oxygen-free environments. “This exciting result showcases how different organisms communicate in ecosystems and pass information and metabolic capabilities onto each other,” says microbial biochemist Scott Ensign of Utah State University in Logan. The scientists have confirmed that the new cycle is present in two microbes, Haloarcula marismortui and Natrialba magadii , but preliminary analysis suggests the cycle is used by about half the species in Haloarchaea, a branch of the Archaea family tree whose members thrive in extreme conditions. Many of the microbes subsist in salty lakes on rare, ephemeral blooms of nutrients. In the lean times between blooms, the methylaspartate cycle might help these microbes survive by tapping stored nitrogen, allowing them to continue to construct things like proteins. That’s a neat trick, Ensign says.

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