Subscribe rss-microbe
Home Current Topics Amplified Genes Help Yeast Strains Cope with Industrial-Scale Demands
Amplified Genes Help Yeast Strains Cope with Industrial-Scale Demands Print E-mail
Yeast strains, used on an industrial scale in Brazil for several decades to make ethanol from sugarcane, selectively amplify some genes encoding sugar metabolism and others conferring tolerance to stress, according to U.S. and Brazilian researchers. These adaptive changes at the chromosomal level not only enable these strains of Saccharomyces cerevisiae to thrive under relatively harsh conditions during batch fermentations, but they also could point the way to designing even more efficient strains to enhance ethanol fuel production. The genes in question congregate at the ends of chromosomes where they rearrange as the cells adapt to fluctuating conditions in the fermenters.

Although ethanol producers in Brazil began using S. cerevisiae to convert sugarcane into fuel about 30 years ago, they made little effort to investigate the genetic properties of those strains until recently, according to Gavin Sherlock of Stanford University in Stanford, Calif. Recently, he and his collaborators at Stanford and in Brazil began evaluating five industrially important strains of S. cerevisiae, designated BG-1, CAT-1, PE-2, SA-1, and VR-1, which collectively produce billions of gallons of ethanol each year. Compared to a standard laboratory strain of yeast, S288C, the five industrial strains have amplified copies of SNO and SNZ genes, which are involved in making vitamin B6 and thiamine as well as in conferring tolerance to oxidative stress.

Vitamin B6 eventually is converted to thiamine pyrophosphate, an essential cofactor in metabolic reactions leading from sugars to ethanol. Although high levels of the intermediate thiamine can inhibit yeast growth, increasing the copy numbers of the SNO and SNZ yeast genes enables yeast strains to overcome that effect and to grow robustly in media with high sugar and thiamine levels. "This suggests a natural adaptation specifically selected for in Brazilian fuel ethanol yeast strains," Sherlock says. "Copy number variation has been an underappreciated form of genetic variation." Details appear in the December 2009 Genome Research (19:2271- 2278).

Meanwhile, molecular geneticist Lucas Argueso at Duke University Medical Center in Durham, N.C., and his Brazilian collaborators focused on one of those industrial strains, namely PE-2, and a derivative isolate, designated JAY270, whose genomic sequence was determined. Although most JAY270 genes align with those of the reference strain S288C, JAY270 contains extra gene copies, all of which cluster near the ends of its 16 chromosomes.

Here again, those additional genes confer tolerance to stress, according to Argueso, who refers to these genes as "optional tool kits that a specific strain needs to survive in its environment." Because these genes are inserted at the ends of chromosomes, they are less likely to cause problems when yeast cells carrying them undergo meiosis, he points out, noting that rearrangements at the center of such chromosomes can render such cells sterile. "It's the best of both worlds," he says, arguing that these yeast strains benefit from modified genomes but without much added risk of becoming sterile.

"Yeast strains used to produce biofuels must tolerate ethanol stress so they don't die from their own product," Argueso says. Thus, he points to MPR1 and MPR2, two of the additional genes in JAY270 but not S288C, that protect yeast from oxidative stress and ethanol toxicity. Similar to what Sherlock and his collaborators report, the SNO and SNZ genes are found in higher copy numbers within JAY270, according to Argueso. Details appear in the December 2009 issue of Genome Research (19: 2258-2270).

With those findings in mind, Argueso and his coworkers are further manipulating JAY270 genes to learn whether it can perform even more efficiently during industrial-scale fermentations. They will also test whether these or similar genetic changes occur when yeast strains are fed 5-carbon sugars such as xylose from cellulosic feedstocks instead of 6-carbon sugars from sugarcane and corn. Although yeast strains can metabolize 5-carbon sugars in bench-top fermentations, they do not perform well and tend not to survive under harsher industrial-scale conditions. "Perhaps we can introduce modifications that allow our industry-adapted strains to break down 5-carbon sugars to ethanol," Argueso says.

Carol Potera
Carol Potera is a freelance writer in Great Falls, Mont.