Team:Nevada/Project/Ecoli

Project Introduction

Our project aims to overcome problems in modern oil reserve limitations by utilizing E. coli for the production of biodiesel (C-12 fatty acids) and bioethanol. In the past there have been a number of examples of biofuel production in E. coli, however 30-40% of production cost is based on media costs(Galbe, Sassner, Wingren and Zacchi, 2007). Our project will surmount these high production costs by engineering the cyanobacteria, Synechocystis PCC 6803, to secrete large quantities of glucose that will feed our biofuel-producing E. coli. In addition to cutting costs, Cyanobacteria and E. coli will be co-cultivated in an apparatus that allows for the mutual transfer of carbon to produce biofuels. This consumption of atmospheric carbon dioxide through photosynthesis provides an energy source that is beneficial to the environment. Not only will this project provide an efficient and environmentally friendly means for producing biofuels without the need for a carbon source, but the use of novel bacteria also frees up land and water reserves for food crops. The idea that two different bacterial species can be developed into a cooperative system also holds further industrial implications for medicine production and other endeavors.

E. coli Project

The goal of the E. coli team was to transform E. coli with genes that produce medium chain fatty acids and ethanol, both of which can be used as biofuels. In addition, the group wanted to develop an E. coli glucose insensitive inducible promoter that would not be down-regulated in the presence of high glucose concentrations. In our system glucose levels produced by Cyanobacteria may be unpredictable; therefore having an E. coli inducible promoter that is glucose-insensitive will provide more control over expression of the biofuel genes.

Pathway for Producing Biofuels and Ethanol in E. coli

Fatty Acid Production

Cloning Stratagy
Bay Laurel thioesterase (BTE) is a gene that is naturally found in the bay leaves (Umbellularia californica) used in cooking. The BTE produces medium chain 12-carbon and 14-carbon fatty acids (Voelker et al.,1996). The Nevada team changed the codon sequence for BTE based on the publication by Welch et. al. “Design Parameters to Control Synthetic Gene Expression in Escherichia coli” by changing the codons that are more common in plants to codons more common in E. coli without changing the polypeptide sequence. The synthesized gene also contained an E. coli ribosome binding sequence, a His tag sequence on the 3’ end of the coding sequence and an E. coli double terminator sequence (BBa_B0014) creating the Nevada thioesterase intermediate part (BBa_K558003).

The constitutive promoter sigma 70 was used from the Registry of Standard Biological Parts (BBa_J23101) and was classically cloned in front of the synthesized Bay Laurel thioesterase gene to create the Nevada BTE generator part (BBa_K558007). Many tests were used to determine the expression of the Bay Laurel thioesterase including SDS-PAGE, Western blot, colorimetric assay and gas chromatography. The one assay that was most successful for measuring free fatty acid content in the growth media was purchased through Bioassay Systems (EnzyChrom™ Free Fatty Acid Assay Kit www.bioassaysys.com). T7 Express Iq E. coli with the BTE generator produced up to 125 uM free fatty acid compared to control line which produced 50 uM fatty acid.

Ethanol Production

Pyruvate Decarboxylase and Alcohol Dehydrogenase Cloning Strategy

The Nevada team designed and synthesized an operon without a promoter (intermediate part) containing pyruvate decarboxylase and alcohol dehydrogenase coding regions along with a standard E. coli ribosome binding site and double terminator. The pyruvate decarboxylase (pdc) and alcohol dehydrogenase (adh) coding region sequences were based on the Zymomonas mobilis sequences (Ingram et. al. 1987) and work conducted by UNIPV-Pavia (iGEM 2009 Part K173016; K173017) and Utah State (iGEM 2009 M11041; M11042). The goal was to express this pdc/adh operon under the control of the constitutive promoter sigma70 (iGEM part J23101), or a glucose insensitive inducible promoter, trc (see details below) and test for expression of functional genes.

The sigma 70-pdc/adh generator (k558001) was assembled using classic cloning techniques. It was then transformed into two E. coli competent cell lines, NEB 10-Beta (New England BioLabs) and T7 Express Iq (New England BioLabs). Colonies were screened through restriction digest analysis, DNA sequencing and tested for enzymatic activity.

Pyruvate Decarboxylase Activity

Pyruvate decarboxylase activity was assayed using aldehyde indicator plates (Ingram et. al. 1987) to test for a Schiff Base reaction to detect the presence of acetaldehyde, a substrate for ethanol production. The figure below shows pink colonies after an overnight incubation on Schiff base plates. The streaked colony on the top of the plate is a negative control cell line (white-light pink). The streaked colony on the right are a typical example (pink color) of sigma 70-pdc/adh generator in the NEB 10-Beta cells. The cells on the left (dark pink) and represent what is typically seen for the sigma 70-pdc/adh generator expressed in T7 Express Iq cell lines. From the results shown here and other experiments, we have concluded that the NEB T7 Express Iq cells are a better choice for expression of the 70-pdc/adh generator.

Alcohol Dehydrogenase Activity

Ethanol production was tested using EnzyChrom Ethanol Assay Kit (BioAssay Systems). The figure below shows percent ethanol production in T7 Express Iq cells (NEB) in 2% versus 10% glucose medium over 24 and 48 hours. The sample grown in 2% Glucose over 24 hours yielded 0.02% ethanol, and the sample grown in 10% Glucose over 48 hours yielded 0.018% ethanol. From the results shown here and other experiments, we have concluded that our modified E. coli is in fact producing ethanol. Furthermore, we have also concluded that glucose addition to the growth medium does have an effect on cell growth. It appears that these cells can only tolerate glucose additions up to 2.0% as the sample with a 10.0% glucose addition produced less ethanol.

E. coli Results