Optimizing Yeast For Xylose Fermentation Through Systems Biology

随着化石燃料价格的上涨，以及我们对糟糕的空气质量、温室气体排放和气候变化对健康影响的日益关注，我们越来越多地寻求可再生替代能源，如乙醇。目前，世界上大部分乙醇是由甘蔗或玉米发酵生产的。然而，纤维素发酵具有许多优点，因为纤维素是植物细胞壁的主要成分，在农林废弃物中含量丰富，不与食物来源竞争。不幸的是，要使纤维素乙醇生产成为一个商业上可行的行业，仍然存在许多障碍。一个问题是，我们用来酿酒的酵母已经有上千年的历史了，尽管它在利用谷物和葡萄中的葡萄糖等单糖酿造酒精方面非常有效，但在利用木糖等复杂糖类酿造酒精方面却效率低下。由于纤维素中很大一部分的糖分是木糖，因此优化酵母中的木糖发酵是非常必要的。传统上，为了解决这个问题，已知在木糖利用中起作用的其他物种的基因已经被整合到酵母中；然而，木糖发酵仍然效率低下。与我们目前对调控木糖发酵的基因的了解状况所限制的“定向”方法不同，贝茨实验室最近采取了一种全球方法，我们系统地筛选了酵母中的几乎每一个基因，以确定移除一个基因对木糖发酵是负面还是正面影响。令人惊讶的是，我们发现移除或缺失四个与木糖代谢无关的基因，极大地提高了酵母发酵木糖的能力。这说明了进行无偏见的全球或全基因组筛查的力量。这项建议的目的是扩大我们的全基因组研究，以了解这四个基因的缺失如何导致高效的木糖发酵，并进一步表征调控酵母自然发酵木糖能力的基因。我们的总体目标将是设计这些基因，最大限度地利用木糖生产乙醇，这将有助于加拿大的纤维素生物燃料行业。**********

With the rise of fossil fuel prices and our growing concerned over the health impacts of poor air quality, greenhouse gas emission and climate change, increasingly we are looking to renewable alternative energy sources such as ethanol. Presently the majority of the world's ethanol is produced by the fermentation of sugarcane or corn. However cellulosic fermentation has many advantages as cellulose, the main component of plant cell walls, is plentiful in agricultural and forestry waste and does not compete with food sources. Unfortunately, numerous hurdles remain in making cellulosic ethanol production a commercially viable industry. One problem is the yeast we have been using to make alcohol for 1000s of years, though outstandingly efficient at making alcohol from simple sugars like glucose found in grains and grapes, are inefficient at making alcohol from complex sugars such as xylose. As a significant percentage of the sugar content of cellulose is xylose, it is essential to optimize xylose fermentation in yeast. Traditionally, to address this issue genes with known roles in xylose utilization from other species have been incorporated into yeast; however xylose fermentation is still inefficient. In contrast to taking a "directed" approach, which is limited by our present state of knowledge of the genes regulating xylose fermentation, the Baetz lab has recently taken a global approach where we systematically screened nearly every gene in yeast to see if removing a gene had negative or positive impact on xylose fermentation. Surprisingly, we found that the removal or deletion of four genes, with no known connection to xylose metabolism, dramatically improved yeast's ability to ferment xylose. This illustrates the power of performing un-biased global or genome-wide screens. The aim of this proposal is to extend our genome-wide studies to understand how the deletion of these four genes results in efficient xylose fermentation and to further characterize the genes regulating yeast's natural ability to ferment xylose. Our overall goal will be to engineer these genes to maximize ethanol production from xylose, which will aid the cellulosic biofuels industry in Canada. **********