A new study has suggested that a gene from Arabidopsis, the model plant, shows a potential to be used as a tool to engineer yeast to increase ethanol production through robust flocculation.
In the industrial conversion of sugar to ethanol with the help of yeast, flocculation is an easy and economical method to separate yeast cells from fermentation product. It is a crucial step in the production of alcoholic beverages and bio-ethanol in which timing is very important. Thus industry is on the lookout for methods of more efficient flocculation.
While premature flocculation hampers complete fermentation and results in poorly attenuated sweet wort that may lead to severe off-flavors, late or poor flocculation requires yeast cells to be removed by laborious and expensive methods like fining, filtration or centrifugation, which are laborious and expensive procedures.
A team of scientists led by Dr Jitender Thakur at the National Institute of Plant Genome Research (NIPGR) working on a IYBA grant by the Department of Biotechnology showed that that over-expression of AtMed15, a gene encoding a Mediator subunit of Arabidopsis, in yeast can cause robust and constitutive flocculation without affecting the growth.
Mediator, a multi-protein complex involved in transcription of class II genes, was first discovered in yeast and then characterized in many metazoans revealing a striking structural conservation of the complex. However, sequences of Mediator subunits are not well conserved raising a question on the functional conservation of these individual subunits. Expression of AtMed15 in yeast results into up-regulation of a set of flocculin genes like Flo1, Flo5, Flo9 and Flo11 that are required for the process of flocculation.
The cells adhered strongly on the surface of the agar media and showed robust flocculation in the liquid media without affecting the growth. The AtMed15-induced adhesion and flocculation were observed in different carbon sources. Calcium-assisted cell wall-bound mannan-binding proteins were found to be involved in this flocculation, which was unaffected by wide fluctuation of pH or temperatures revealing its constitutive robust nature.
The team consisting of Ph.D. student Mr. Pradeep Dahiya, a postdoctoral Research Associate Dr. Divya S. Bhat and group leader Dr. Jitendra K. Thakur claims that this is the first report where a gene from other kingdom has been used to induce flocculation in yeast.
The study published in the Journal Scientific Reports (6: 27967, DOI: 10.1038/srep27967) suggests that these characteristics can make AtMed15-driven flocculation suitable for exploitation in bioreactors as the cells are self-immobilized in flocs, which can withstand severe fluctuations in parameters like pH and temperature.
Expression of few flocculation related Flo genes was up-regulated in these cells. Interestingly, there was significant increase in ethanol production by the yeast expressing AtMed15 that can be exploited for use in bio-ethanol production and brewery and beverage industries.