New & Noteworthy

Stellar Strain Yields Increased SAM Production

March 10, 2012

Yeast geneticists often go the extra mile to get their mutant. But Huang and coworkers went hundreds of extra miles to get theirs. Hundreds of miles straight up, that is.

Yeast sent into space were better able to make SAM down here on Earth. Wikimedia Commons.

In a recent study published online in the Journal of Applied Microbiology, Huang and coworkers identified and improved upon a strain of Saccharmoyces cerevisiae that makes increased amounts of S-adenosyl-L-methionine or SAM. This is an important chemical in many pharmacological and medical uses and is primarily made via microbiological synthesis. Increased production would be an obvious boon to researchers and the pharmaceutical industry.

What makes this study interesting is that the researchers obtained their initial strain from outer space. They shot cultures up into space on a satellite where the poor yeast had to endure the harsh environment there for 18 days. Researchers then collected the samples when the satellite returned to Earth.

Out of six hundred random clones from the flight, researchers found 43 that made at least 10% more SAM than their wild type counterpart. A second round of selection yielded strain H5M147 which made 84% more SAM than the wild type strain.

Unfortunately the researchers were not able to (or did not report in this study) why the strain made extra SAM. They used a technique called AFLP that allowed them to see that there were differences in the new and host strain’s genome, but it did not allow them to pinpoint what those differences were nor which ones were significant. That will have to wait for a future study.

They did manage to ramp up SAM production even further in this strain though. First they added an extra copy of a key player in SAM production, the MAT2 gene. Researchers have tried to coax other strains of yeast to make more SAM by adding MAT2 but to no avail. This space strain apparently has genetic mutations that allow extra MAT2 to increase SAM production. The new strain with the integrated version of MAT2 was called H5MR38.

Finally the researchers tinkered with culture conditions to optimize SAM production in H5MR38 further. They found that using sucrose as the carbon source and adding peptone to yeast extract and urea yielded the most SAM.

In the end, Huang and coworkers managed to get 7.76 grams of SAM per liter of culture after 84 hours. This compares to the previous high in Sacchromyces cerevisiae of 5.7 gram per liter after 120 hours and 3.6 grams per liter from a strain of Pichia pastoris in 100 hours. Clearly their space-derived yeast strain is an improvement over anything else identified so far.

Huang and coworkers aren’t the only ones putting yeast into space

by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics