The yeast, Saccharomyces cerevisiae is very good at fermenting the ethanol from hexose sugars. That and its high tolerance to the ethanol it produces have led it to become one of the most utilized characters in Industrial scale bioethanol production. But it could be better.
You see, there is another sugar that exists in abundance in cellulosic biomasses. That sugar, xylose, is in fact so abundant that it is second only to glucose quantity. But S. cerevisiae just can't handle xylose. This is where we turn to another yeast, Pichia stipitis. P. Stipitis excels and turning xylose into ethanol, however it has its own short comings. This apparent upstart fails miserably when it comes to ethanol tolerance, a detrimental flaw that renders it nonviable as a large scale producer. If only there was some way to bring these two together...
Enter the duo of Wei Zhang and Anli Geng from the school of life and chemical technology of Ngee Ann Polytechnic, in Singapore. These two have contrived of a way to instill the xylose eating skills of P. stipitis into the hearty bioethanol standard bearer S. cerevisiae through genomic shuffling.
Improved ethanol production by a xylose-fermenting recombinant yeast strain constructed through a modified genome shuffling method
The team looked at traditional way used in teasing S. cerevisiae into fermenting xylose, generally metabolic engineering, and thought there had to be a better way. Metabolic engineering requires the expression of multiple genes through mutagenesis and then post-evolutionary engineering. The process has to be done throughout the complex genomic pathways, and as such takes a lot of work and a lot of time. Genomic shuffling on the other hand enables the team to make changes throughout the entire genome at the same time.
Now, before you ask, "Well, then why don't we always use whole genome engineering?" know that it has its downsides. Genomic shuffling depends very heavily on protoplast fusion methods, which have stability and efficiency problems. This team's goal is to modify the method and quickly produce a strain of S. cerevisiae combined with the P. stipitis genome through direct genome isolation and transformation.
They started off by extracting the whole genome of P. stipitis, and inserting it into S. cerevisiae by electroporation. They then grew the amalgam strain in conditions that S. cerevisiae would not tolerate and obtained eight hybrid strains, which they evaluatated for ethanol production in a xylose containing broth. They picked the crem de le crem strain, F1-8, for a second round of genome reshuffling.
In this second go round; F1-8 had an extracted genome of S. cerevisiae transferred into it. The new strain was then screened on YNBXE selective plates and three positive colonies were obtained, with the strain ScF2 being the most competent ethanol producer. To see how this technique worked compared to traditional protoplast fusion methods normally used they also constructed hybrid strains of F1-8 an S. cerevisiae via that technique, they all died on the YNBXE.
Then the team compared xylose fermentation of F1-8 and ScF2 with their parent strains. ScF2 showed an improved ethanol production over both F1-8 and P. stipitis. This causes the scientists to believe that their modified genomic shuffling method could help efficiently create yeast strains with enhanced ability for turning xylose into ethanol, as it did in the ScF2 strain.
While ScF2 showed a medley of skill, including the fermentation of both glucose and xylose as well tolerance to sugars and ethanol, the researchers speculate that utilizing the new method in conjunction with rational metabolic engineering and directed evolution could lead to more improvement of the strain.
Wei Zhang, & Anli Geng (2012). Improved ethanol production by a xylose-fermenting recombinant yeast strain constructed through a modified genome shuffling method Biotechnology for Biofuels DOI: 10.1186/1754-6834-5-46
Photo Credit: WikiMedia contributor Masur