June 26, 2013
Like a barfly before his liver gives out, the yeast S. cerevisiae can tolerate incredibly high levels of ethanol. But unlike the town drunk, S. cerevisiae uses this skill to its own advantage.
In the wild, this yeast ferments sugars to flood the local environment with alcohol. The end result is that it does just fine but any nearby microorganisms are killed.
Humans also take advantage of this unique property of S. cerevisiae. Not only does it allow us to brew beer and ferment grapes into wine without worrying too much about bacterial contamination, but it also helps us generate ethanol as a biofuel. What a cool and useful little beast!
Given how important this property is for both yeast and us, it is perhaps surprising how little we know about how S. cerevisiae pulls this off. A new study out in PLOS Genetics by Pais and coworkers sets out to rectify this situation.
The study yielded a number of interesting findings. It might seem obvious that an organism that can make a lot of ethanol should able to grow in the high-ethanol environment that it created. However, by looking at these characteristics in 68 different S. cerevisiae strains, the authors found that the ability to produce ethanol was at least partially separate at the genetic level from the ability to thrive in it. Pais and coworkers called the first process “high ethanol accumulation capacity” and the second “tolerance of cell proliferation to high ethanol levels.”
Second, they identified DNA differences in three different genes – ADE1, URA3, and KIN3 – that all work together to give certain strains of yeast their high ethanol accumulation capacity. The most interesting of these three is KIN3.
Kin3p is a protein kinase that has a role in DNA repair. Since ethanol is a known mutagen, it may be that the DNA differences in the KIN3 gene make its protein better at DNA repair, rescuing the cell from the DNA damage from high ethanol levels.
The authors found these genes as part of a larger study to identify at the genetic level why some strains of S. cerevisiae did better than others in ethanol. They focused on two strains, CBS1585 and BY710. CBS1585 is a sake yeast strain that can tolerate and grow in high levels of ethanol while BY710 is a laboratory strain that doesn’t do well with either (although still better than most any other beast out there in nature!).
They created diploids using these two strains, sporulated them into haploids, and then screened these haploids for their ability to deal with high levels of alcohol. As would be predicted from their survey of the 68 strains, the haploids could be grouped into three distinct but overlapping pools:
1) High ethanol accumulation capacity in the absence of cell proliferation
2) Cell proliferation at high ethanol levels
3) Poor tolerance and growth in high ethanol
The authors then used pooled-segregant whole-genome sequence analysis to identify the DNA regions critical to the first two functions. Basically this is just what it sounds like. They isolated DNA from the three pools and looked for differences in pools 1 and 2 that weren’t in 3. (OK that is dangerously simplified, but that is the gist of it.)
This is how they identified ADE1, URA3, and KIN3 as important in high ethanol accumulation capacity. We will have to wait for them to pinpoint the important genes in the regions they identified for pool 2 to begin to understand why some strains can proliferate in the presence of high alcohol concentrations.
Once they have identified all of the genes that make certain yeast strains so good at dealing with alcohol, we may be able to engineer a yeast that can make more ethanol for less money. We can make a great little alcohol producer even better!
by D. Barry Starr, Ph.D., Director of Outreach Activities, Stanford Genetics
Categories: Research Spotlight