Revision as of 16:37, 17 October 2018

Demonstrate project

The goal of our project is two-folded. First we want to degrade cellulose to glucose using Saccharomyces cerevisiae expressing an artificial cellulosome. Secondly we want to produce styrene from a S. cerevisiae strain grown on glucose.
We have designed and constructed S. cerevisiae strains that should be able to meet these goals. In order to demonstrate that these strains meet the goals of our project, a number of experiments were performed

• Cellulose degradation

  First off, we bought commercial cellulases BglI, CbhI, and EgA. The former 2 are identical to the cellulases we want to express, and EgA is an isozyme. These are tested on different cellulose sources to compare activity. We also purified EgII, which we want to express, ourselves, and tested it as well.

  

  

  Secondly, we transformed S. cerevisiae with an artificial cellulosome containing 3 different cellulases and a scaffold. The cellulosome activity was tested by growing the strains on cellobiose and cellulose.

  

  	OD600 at T=0h	OD600 at T=12h	D600 at T=36h
  YPH499++	0.51	0.55	1.36
  negative control	0.1	0.06	0.1

  Table 1. Growth on phosphorylated cellulose using YPH499 containing PAL2 and the cellulosome (YPH499++). OD600 was measured at t=0, t=12h and t=36h. Initial OD600 value of YPH499++ is high due to absorbance from the phosphorylated cellulose.

  Figure 1 shows that CbhI can hydrolyze phosphorylated cellulose and (ball-milled) ReCell, and figure 2 shows the same for EgII. The growth on cellobiose as shown in figure 3 demonstrates that BglI is active, as this is the only cellulase present that can hydrolyze cellobiose into glucose required for growth. These results demonstrate that the 3 cellulases we plan on using each function as expected. Table 1 shows that the S. cerevisiae strain containing the cellulosome can grow on phosphorylated cellulose. This confirms that we made the goal of constructing a strain that can degrade cellulose and grow on the created glucose.

• Styrene production

  To produce styrene in S. cerevisiae, the PAL2 gene from Arabidopsis thaliana is expressed. The Pal2 enzyme catalyzes the reaction of phenylalanine to trans-cinnamate. Natively present Fdc1 then converts trans-cinnamate into styrene. PAL2-containing strains are cultured on glucose medium, and HPLC is performed.

  

  As shown in figure 4, both trans-cinnamate and styrene are present in the PAL2-containing strains, but not in the control strain. This suggests that Pal2 indeed converts phenylalanine to trans-cinnamate, which can be natively converted to styrene. It thus demonstrates that we meet our second goal, of creating styrene using S. cerevisiae.

• Consolidated bioprocessing

  After demonstrating that both original goals of the project have been met, we decided to take it further. The PAL2 gene is expressed in a S. cerevisiae strain that is also expressing the artificial cellulosome. This strain is then cultured with cellobiose.

  

  

  As shown in figure 5, the trans-cinnamate levels are higher in the strain grown on cellobiose compared to the strain grown without cellobiose. Figure 6 shows styrene production in the strain grown on cellobiose, whereas nothing is visible in the control. This demonstrates that the consolidated strain is able to convert cellobiose into trans-cinnamate and styrene.

In conclusion, we have demonstrated that we can grow our S. cerevisiae strains on cellulose, and that it can produce styrene from glucose and cellobiose. This leads to the reasonable assumption that this strain is able to break down and grow on cellulose and meanwhile produce styrene.