# Patent

## Title
Alkane exporter and its use

## Abstract

Recombinant cell expressing at least one heterologous alkane exporter protein comprising an ATP binding cassette (ABC), wherein the ABC comprises of an amino acid consensus sequence as set forth in SEQ ID No. 1. The use and method of producing or increasing resistance to biofuels with the same.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application claims the benefit of priority of U.S. Provisional Patent Application No. 61/661,602 filed Jun. 19, 2012, the contents of which being hereby incorporated by reference in its entirety for all purposes.


STATEMENT REGARDING SEQUENCE LISTING
The Sequence Listing associated with this application is provided in text format in lieu of a paper copy, and is hereby incorporated by reference into the specification. The name of the text file containing the Sequence Listing is 690148.471USPC_SEQUENCE_LISTING.txt. The text file is 42 KB, was created on Nov. 16, 2015, and is being submitted electronically via EFS-Web.
FIELD OF THE INVENTION
Yeast recombinant systems having improved Alkane tolerance.
BACKGROUND OF THE INVENTION
The development of renewable biofuels, such as bio-ethanol [1], butanol [2], bio-diesel [3-5] and jetfuels [6], helps to address energy security and climate change concerns. For economically industrial production of biofuels, titers and yield of biofuels synthesis must be sufficiently high. However biofuels are frequently toxic to cells, thereby placing a limit on the yield. Hence, biofuel toxicity is an important issue that needs to be addressed. There are several strategies for addressing biofuel toxicity in microorganisms. Alper et al. [7] employed a global transcription machinery engineering (gTME) approach to improve ethanol tolerance. Stanley et al. [8] used an adaptive evolution engineering method to select stable ethanol tolerant mutants of Saccharomyces cerevisiae, whereas Hou et al. [9] developed novel genome shuffling method to improve biofuel tolerance.
ATP binding cassette transporters (ABC transporters) are transmembrane ion channels found in all organisms. ABC transporters often share common domain architecture with two transmembrane domains (TMD) and two nucleotide binding domains (NBD) that hydrolise ATP or other nucleotides. Pleitropic drug resistance 5 (PDR5) is the most extensively studied ABC transporter from S. cerevisiae. 
Yarrowia lipolytica, a non-conventional oleaginous yeast that efficiently assimilates and utilizes hydrophobic substrates such as alkanes, fatty acids and lipids, was recently used as a model system to study mechanisms of assimilation and degradation of hydrophobic substrates (HS) [12-14]. The characterization of Y. lipolytica mutant, ΔABC1 (YALI0E14729g), with a defective phenotype for hexadecane (C16) utilization, suggested that ABC1 may be involved in import or export of long chain alkanes [15]. Similarly, Y. lipolytica mutant ΔABC2 (YALI0C20265g) showed a decreased cell growth on decane [16]. In addition, genome exploration revealed two homologues, ABC3 (YALIB02544g) and ABC4 (YALIB012980g), which may be also involved in alkane transportation.
Toward the aim of improving alkane tolerance in yeast, classical strain engineering strategies, including mutagenesis and adaptive evolutionary engineering together with genome shuffling and genomic library [17], have been widely used. However, it takes about 6 months to generate ethanol-tolerant mutant by employing the adaptive evolutionary engineering method [8]. Some strategies, are extremely laborious to generate positive mutants. For example, the procedure of yeast genome shuffling, includes EMS treatment, sporulation, spore purification, adequate cross and mutant selection [9]. In addition, for the genomic library approach, more than 10,000 transformants have to form genomic library [17] and be screened. Further, this approach has little room for errors as it needs very high ligation and transformation efficiency. It would require personnel with very good molecular biology techniques. For improvement strategies, such as the random mutagenesis and selection, there is no guarantee about the positive mutant being effective and stable.
SUMMARY
A first aspect of the invention includes a recombinant cell expressing at least one heterologous alkane exporter protein comprising an ATP binding cassette (ABC), wherein the ABC comprises of an amino acid consensus sequence as set forth in SEQ ID No. 1.
A further aspect of the invention includes the use of the recombinant cell for biofuel production.
Another aspect of the invention includes a method for the production of a biofuel comprising cultivating the recombinant cell under conditions that allow (i) the expression of the at least one heterologous alkane exporter protein; and (ii) the production of a biofuel.
Another aspect of the invention includes a method of increasing resistance towards biofuel toxicity in a cell comprising: (a) introducing a nucleic acid molecule encoding for a heterologous alkane exporter protein comprising an ATP binding cassette (ABC), wherein the ABC comprises or consists of an amino acid consensus sequence as set forth in SEQ ID No. 1; and (b) cultivating the cell under conditions that allow expression of the heterologous alkane exporter protein.
Other aspects of the invention would be apparent to a person skilled in the art with reference to the following drawings and description of various non-limiting embodiments.



BRIEF DESCRIPTION OF THE DRAWINGS
The drawings are not necessarily drawn to scale, emphasis instead generally being placed upon illustrating the principles of various embodiments. In the following description, various embodiments of the invention are described with reference to the following drawings.
FIG. 1. Intracellular alkane accumulation by ABC2 and ABC3. S. cerevisiae BY4741 with and without ABC2/3 were cultured under exposure of 0.5% alkane or 20% undecane. After 48 h incubation, intracellular alkane levels were measured. Intracellular alkane levels were normalized to that of control carrying empty plasmid. Data shown are the mean±SD of four biological replicates.
FIG. 2. Alkane susceptibility test for S. cerevisiae. The alkane susceptibility was tested in cells expressing ABC2/3 or with empty plasmid. (A) Alkane susceptibility test on agar plate. Serial dilutions of cells expressing ABC2/3 were spotted on agar plates with alkanes (decane, undecane) as vapour phase. Plates were incubated at 28° C. for 2 days. (B) Alkane susceptibility test in liquid culture. Overnight cell culture was diluted into induction medium (final OD600=0.4) with different alkanes (decane & undecane) concentrations. The plates were incubated for 48 hr at 28° C. The OD600 value of each sample was determined and plotted against its corresponding alkane concentration. Symbols for strains are: control sample with empty plasmid (open circle), cells expressing ABC2 (filled circle), cell expressing ABC3 (filled triangle).
FIG. 3. mRNA transcripts levels of ABC2 and ABC3 in Y. lipolytica. Quantitative RT-PCR analysis of ABC2 (A) and ABC3 (B) in Y. lipolytica cells corresponding to treatment with octane, nonane, decane, undecane and dodecane (C8-C12). Each value of qRT-PCR was normalized to β-actin expression levels and expressed as the fold change relative to the levels detected in control samples, which were cells without alkane treatment and set equal to 1. Error bars represent the SD of triplicate.
FIG. 4. Expression and subcellular localization of ABC2 and ABC3. (A) Expression of ABC2 and ABC3. The expression of ABC2/3 carrying 6×His tag were confirmed by western blot analysis. The arrow shows the right band size. The positions of molecular mass markers are indicated at left. (B) Subcellular localization of ABC2 and ABC3 determined by fluorescence microscopy. Yeast cells carrying plasmid encoding ABC2/3-EGFP fusion proteins were cultured and harvested. Phase contrast figures and fluorescence figures are shown.
FIG. 5. Transporter sequence comparison and alkane susceptibility assay of ABC2 and ABC3 mutants. (A) Multiple sequence alignment of NBD2 of ABC transporters. ABC2 [GenBank: CAG82364] (SEQ ID NO: 6); ABC3 [GenBank: CAG82646] (SEQ ID NO: 7); PDR5 [GenBank: CAA99359] (SEQ ID NO: 8); and PDR15 [GenBank: AAB64846] (SEQ ID NO: 9). Starting amino acid numbers are shown in each line. Sequences were aligned in ClustalW, and the shading was created by Mobyle Pasteur Boxshade. Single letter abbreviations for amino acid residues are as follows: A, Ala; C, Cys; D, Asp; E, Glu; F, Phe; G, Gly; H, His; I, Ile; K, Lys; L, Leu; M, Met; N, Asn; P, Pro; Q, Gln; R, Arg; S, Ser; T, Thr; V, Val; W, Trp; and Y, Tyr. (B) Alkane susceptibility assay with ABC2 and ABC3 mutants.



DETAILED DESCRIPTION
We focused on harnessing pleiotropic drug resistance (PDR) family of the ATP-binding cassette (ABC) in yeast, as a direct mechanism for reducing biofuel toxicity.
Accordingly, a first aspect of the invention comprises a recombinant cell expressing at least one heterologous alkane exporter protein comprising an ATP binding cassette (ABC), wherein the ABC comprises of an amino acid consensus sequence as set forth in SEQ ID No. 1.
The ATP binding cassette (ABC) includes a nucleotide binding domain of the ABC wherein the nucleotide binding domain (NBD) of the ABC comprises or consists of a amino acid consensus sequence set forth in SEQ ID No. 1. The NBD of both ABC2 as set forth in SEQ ID No. 2, and ABC3 as set forth in SEQ ID No. 3 are examples of NBD having the amino acid consensus sequence set forth in SEQ ID No. 1.
The recombinant cell includes an expression system. The expression system as used herein, refers to a modified operon the addition or modification of a nucleic acid sequence needed for gene sequence expression. The construct may include promoters and or enhancers as known in the art. Promoter regions vary from organism to organism, but are well known to persons skilled in the art for different organisms. The nucleic acid expression system can be synthesised de novo for protein expression of alkane exporter comprising an ATP binding cassette (ABC) in a cell or made by any means known in the art.
The term ‘heterologous’ refers to a nucleic acid sequence expressing a protein whereby the nucleic acid sequence is derived from a different organism often the sequence was initially cloned from or derived from a different cell type or a different species from the recipient. Typically the genetic material coding for the protein (the nucleic acid such as complementary DNA) is added to the recipient cell. The genetic material that is transferred typically must be within a format that encourages the recipient cell to express the the nucleic acid as a protein. Suitable expression systems are known in the art.
The term “nucleic acid” as used herein refers to any isolated or synthesised nucleic acid molecule in any possible configuration, such as single stranded, double stranded or a combination thereof. Isolated nucleic acids include for instance DNA molecules, RNA molecules, analogues of the DNA or RNA generated using nucleotide analogues or using nucleic acid chemistry, locked nucleic acid molecules (LNA), peptide nucleic acid molecules (PNA) and tecto-RNA molecules. DNA or RNA may be of genomic or synthetic origin and may be single or double stranded. Such nucleic acid can be e.g. mRNA, cRNA, synthetic RNA, genomic DNA, cDNA, synthetic DNA, a copolymer of DNA and RNA, oligonucleotides, etc. Any nucleic acid capable of expressing the polypeptides of the invention including the nucleotide binding domain of the ABC having an amino acid consensus sequence set forth in SEQ ID No. 1, including ABC2 or ABC3 as set forth in SEQ ID NO. 4 and SEQ ID NO. 5 respectively in a cell would be suitable. Preferably the nucleic acid molecule encoding the heterologous alkane exporter protein comprising an ATP binding cassette (ABC) construct.
In one embodiment the expression system is comprised in a vector.
The term “vector” relates to a single or double-stranded circular nucleic acid molecule that can be transfected into cells and replicated within or independently of a cell genome. A circular double-stranded nucleic acid molecule can be cut and thereby linearized upon treatment with restriction enzymes. An assortment of nucleic acid vectors, restriction enzymes, and the knowledge of the nucleotide sequences cut by restriction enzymes are readily available to those skilled in the art. A nucleic acid molecule encoding an alkane exporter comprising an ATP binding cassette (ABC) can be inserted into a vector by cutting the vector with restriction enzymes and ligating the pieces together. Preferably the vector is a plasmid
The term ‘alkane exporter protein comprising amino acid sequence expressing an ATP binding cassette (ABC)’ capable of exporting alkanes. Preferably the alkane exporter protein relates to an ABC transporter that allows cells to survive in at least 0.75% alkane. Preferably the alkane exporter protein is capable of exporting C6-C16 alkanes, preferably C8-C12 alkanes, more preferably C10-C11 alkanes. In various embodiments the expression of the ABC transporter allows cells to survive in at least 0.75% decane or to survive in 20% undecane. Further, an alkane exporter relates to an ABC transporter that is able to reduce the intracellular alkane level by at least 5 fold when cells are exposed to between 0.5% to 20% alkane compared to cells not expressing the ABC transporter. In various embodiments the expression of the ABC transporter allows cells to reduce the intracellular alkane level by at least 5 fold when cells are exposed to between 0.5% decane or to reduce the intracellular alkane level by at least 30 fold when cells are exposed to between 20% undecane compared to cells not expressing the ABC transporter.
In some embodiments the nucleotide binding domain comprises amino acid sequence set forth in SEQ ID No. 2. In such embodiments preferably the ABC comprises amino acid sequence set forth in SEQ ID No. 4.
In other embodiments the nucleotide binding domain comprises amino acid sequence set forth in SEQ ID No. 3. In such embodiments preferably the ABC comprises amino acid sequence set forth in SEQ ID No. 5.
Preferably the expression system is a cell based expression system. In some embodiments the cell is a eukaryotic cell. Preferably, the cell is a yeast cell. Preferably, the cell is of the genus Saccharomyces. Most preferably the cell is a Saccharomyces cerevisiae cell.
Another aspect of the invention includes the use of the recombinant cell for biofuel production. Preferably the biofuel comprises or consists of C6-C16 alkanes.
Another aspect of the invention includes a method for the production of a biofuel comprising cultivating the recombinant cell under conditions that allow (i) the expression of the at least one heterologous alkane exporter protein; and (ii) the production of a biofuel.
In some embodiments the production of biofuel may be achieved by reducing alkane accumulation in a cell
Another aspect of the invention includes a method of increasing resistance towards biofuel toxicity in a cell comprising: (a) introducing a nucleic acid molecule encoding for a heterologous alkane exporter protein comprising an ATP binding cassette (ABC), wherein the ABC comprises of an amino acid consensus sequence as set forth in SEQ ID No. 1; and (b) cultivating the cell under conditions that allow expression of the heterologous alkane exporter protein.
Preparing a recombinant cell may comprise the steps of: constructing an expression system capable of expressing an alkane exporter comprising an ATP binding cassette (ABC) wherein a nucleotide binding domain (NBD) of the ABC comprises or consists of a amino acid consensus sequence set forth in SEQ ID No. 1; and introducing the construct to the cell.
In various embodiments the expression system is a vector preferably a plasmid as discussed herein for use in the cell.
In some embodiments the recombinant cell comprises an expression system constructed with the nucleotide binding domain (NBD) comprising or consisting of an amino acid consensus sequence set forth in SEQ ID No. 1. In various embodiments the NBD sequence is set forth in SEQ ID No. 2. In such embodiments preferably the ABC comprises amino acid sequence set forth in SEQ ID No. 4.
In other embodiments the recombinant cell comprises an expression system constructed with the nucleotide binding domain (NBD) comprising or consisting of an amino acid consensus sequence set forth in SEQ ID No. 1. In various embodiments the NBD sequence is set forth in SEQ ID No. 3. In such embodiments preferably the ABC comprises amino acid sequence set forth in SEQ ID No. 5.
In some embodiments the cell is a eukaryotic cell. Preferably, the cell is a yeast cell. Preferably, the cell is of the genus Saccharomyces. Most preferably the cell is a Saccharomyces cerevisiae cell.
Preferably biofuel refers to an alkane. Preferably the biofuel is a C6-C16 alkane, preferably C8-C12 alkane, more preferably C10-C11 alkanes. Alkane may refers to a medium length alkane of C6, C7, C8, C9, C10, C11, C12. In various embodiments alkane refers to a decane. In various embodiments alkane refers to an undecane.
EXAMPLES
To determine the potential of ACB transporter as alkane exporters, we screened Y. lipolytica ABC1, ABC2, ABC3 and ABC4 for their transport potential, and have selected ABC2 and ABC3 as our candidates. By heterologous expression of ABC2 and ABC3 in S. cerevisiae, the tolerance of Baker's yeast against decane and undecane has been significantly improved.
Hitherto, there has been no reported characterization of ABC2 and ABC3 transporters of Y. lipolytica in other organisms. With the expression of ABC2 and ABC3 in S. cerevisiae. (FIG. 1), cells expressing ABC2 and ABC3 had  ̃5-fold lower intracellular decane level relative to the control. Further, ABC2 and ABC3 transporters were shown to reduce the intracellular undecane level approximately 30-fold compared to the control sample. The sharp decrease in intracellular alkane levels strongly suggests that ABC2 and ABC3 may function as decane and undecane exporters.
Most notably, this is the first study to characterize the function of ABC2 and ABC3 transporters, and these two transporters are the first characterized eukaryotic medium-chain alkane exporters.
Better Strategy Over Other Methods
Compared with classical tolerance improvement strategies, our use of heterologous expression of transporters has the following distinct advantages:

    
    
        a) It is less time consuming to generate tolerance strains through heterologous expression of transporters, as compared to conventional evolutionary strategies. For example, it takes about 6 months to generate ethanol-tolerant mutant by employing the adaptive evolutionary engineering method [8]. For the heterologous expression of ABC2 and ABC3, it takes less than 3 days to express these transporters in host cells to increase cell tolerance.
        b) Our strategy requires significantly less effort to carry out. The process of heterologous expression of transporters involves only 2 simple techniques, namely transformation and induction.
        c) Our approach has a stronger guarantee for performance. Our heterologous transporters have been confirmed to improve cell tolerance once they are expressed.

For Yield Improvement in Biofuel Production

    
    


In the process of biofuel production such as alkane, product toxicity is the chief concern and it lowers the yield and titers significantly. To overcome this shortcoming, we have demonstrated that heterologous expression of ABC2 and ABC3 can increase the tolerance of S. cerevisiae against decane and undecane. And several studies show improvement in tolerance leads to clear increases in biofuel yield. For example, ethanol production in an engineered strain of S. cerevisiae was improved by 15% when its ethanol and glucose tolerance were improved through global transcriptional machinery engineering [7]. Limonene tolerance in E. coli was improved by heterologously expressing an efflux pump and the corresponding strain showed a 64% improvement in limonene yield [18]. Thus, there is clear evidence that tolerance improvements can increase production. And we believe that the improvement of C10 and C11 alkane tolerance of S. cerevisiae can enhance the possible alkane yield.
Strains and Media
All cells involved in cloning experiments were E. coli TOP10 (Invitrogen) unless otherwise stated. Luria-Bertani (BD) was used as the medium for cloning studies unless otherwise stated. Ampicillin (100 μg/ml) was added to the culture media for antibiotic selection where appropriate.
The yeast strains S. cerevisiae BY4741 (ATCC 201388) and Y. lipolytica CLIB122 (CIRM) were used for function characterization. S. cerevisiae BY4741 were cultured in rich medium (YPD), synthetic minimal medium lacking uracil (SC-U) or induction medium. YPD medium (1% yeast extract, 2% peptone, 2% glucose) was used to routinely maintain wild type strain. SC-U medium (0.67% yeast nitrogen base, 0.192% uracil dropout and 2% raffinose) was used for growing pYES2 transformants. Induction medium (0.67% yeast nitrogen base, 0.192% uracil dropout, 1% raffinose and 2% galactose) was used for protein induction in S. cerevisiae cells. Medium containing 0.67% yeast nitrogen base supplemented with 0.5% casein hydrolysate and 2% glucose was used for growth of Y. lipolytica for qRT-PCR sample preparation. Yeast growth media components were purchased from Sigma-Aldrich.
Alkanes (octane (C8), nonane (C9), decane (C10), undecane (C11) and dodecane (C12)) purchased from Sigma-Aldrich were added to culture medium for protein function analysis where appropriate.
Plasmid Construction
Plasmid pYES2 (Invitrogen) with the GAL1 promoter was used as an expression vector. To clone 6×His-tagged ABC2, genomic DNA of Y. lipolytica CLIB122 was used as a PCR template with two pairs of primers ABC2-F1, ABC2-R1 and ABC2-F2, ABC2-R2. The two PCR products were combined through the Splicing Overlap Extension (SOE) method [19] using primers ABC2-F1 and ABC2-R2. The resulting DNA fragment was digested with Hind III and Not I and cloned into pYES2 cut with the same restriction enzymes, creating pYES2ABC2. Plasmid pYES2ABC3 was constructed as for pYES2ABC2. Site-directed mutagenesis of transporters, ABC2-E988Q, ABC2-H1020A, ABC3-E989Q and ABC3-H1021A were constructed by mutating glutamate to glutamine and histidine to alanine respectively.
Plasmid pYES2ABC2-EGFP, which encodes yeast enhanced green fluorescent protein (EGFP) at the C-terminus of the ABC2 open reading frame, was constructed as follows. We used pYES2ABC2 as a PCR template with primer set ABC2-F1 and ABC2-EGFP-R2. The resulting DNA fragment was digested with Hind III and Not I and cloned into pYES2 cut with the same restriction enzymes, creating pYES2ABC2-1. EGFP was amplified from pKT127 (Euroscarf) [20] using primer sets EGFP-F and EGFP-R, digested with Not I and Sph I and inserted into the same restriction sites of pYES2ABC2-1 to create pYES2ABC2-EGFP. Plasmid pYES2ABC3-EGFP was constructed as for pYES2ABC2-EGFP. For construction of pYES2EGFP, EGFP was amplified by PCR from pKT127 using primers EGFP-control-F and EGFP-R, digested with Not I and Sph I and cloned into pYES2. All restriction and ligation enzymes were purchased from New England Biolabs (NEB).
Quantitative RT-PCR
To assess whether ABC1, ABC2, ABC3 and ABC4 are involved in alkane transport in Y. lipolytica, we analysed the effects of alkanes with different chain length (C8-C12) on the transcription levels of these four ABC transporter genes using quantitative RT-PCR.
Total RNA samples from 24 h alkane treated and untreated Y. lipolytica CLIB122 cells were prepared using RNeasy Mini Kit (Qiagen), followed by cDNA synthesis using H minus Reverse transcriptase kit (Fermentas). Quantitative RT-PCR analysis was performed on a Bio-Rad iQ5 real-time PCR detection system using SsoFast EvaGreen Supermix kit (Bio-Rad). The actin gene (YALI0D08272g) [21] was used as reference gene for Y. lipolytica. Relative mRNA levels were derived using comparative CT method.
Compared with control samples without alkane treatment, the transcription levels of ABC1 and ABC4 did not change much when treated with different alkanes (C8-C12) (data not shown). However, the mRNA levels of ABC2 were significantly increased when Y. lipolytica was treated with octane (C8), nonane (C9), decane (C10) and undecane (C11) (p<0.05), while the mRNA levels of ABC3 were significantly increased toward nonane (C9) and decane (C10) (p<0.05) (FIG. 3). These results strongly suggest that two of the ABC transporters, ABC2 and ABC3, may play a critical role in the transport of alkanes with shorter chain length (C8, C9, and C10). Thus, based on the qRT-PCR results, ABC2 and ABC3 were chosen for further analysis of their alkane transport behaviour.
Expression and Subcellular Localization of ABC2 and ABC3
Western Blot Analysis
To confirm the expression of these two transporters, a 6×His tag was attached to the C terminus of ABC2 and ABC3.
S. cerevisiae cells carrying the plasmids encoding the 6×His-tagged ABC2 and ABC3 were cultured in induction medium and harvested at OD600=1-2 (early exponential phase). The protein extraction method here is based on alkaline lysis [22] and glass bead lysis [23] methods. The following handling process was carried out in the cold room ( ̃4° C.). Cell pellets (around 14 mg) were resuspended in 300 μl cold lysis buffer (0.1 M NaOH, 2% β-mercaptoethanol, and protease inhibitor mixture (Roche Applied Science)). After 5 min, glass beads (425-600 μm, Sigma) were added to the suspension until the suspension was covered. Cells were lysed by vortexing for 2 min. The lysate obtained was clarified by transferring supernatant into a new tube. Protein in the lysate was fully dissolved by adding SDS (final concentration around 2%) and gently stirring for 10 min. After centrifugation, the supernatant was mixed equally with Laemmli sample buffer (Bio-Rad) and separated on a SDS-polyacrylamide gel. The sample gels were used for blotting. Proteins were blotted onto a 0.2 μm nitrocellulose membrane (Bio-Rad) through Trans-Blot Turbo Blotting System (Bio-Rad). 6×His-tagged ABC2 and ABC3 were detected using anti-6×His-tag antibody (HRP) (ab1187, Abcam) and 3,3′,5,5′-Tetramethylbenzidine (TMB) Liquid Substrate (Sigma) system.
Through immunodetection for 6×His-tagged proteins, specific bands could be assigned to ABC2 (165 kDa) and ABC3 (167 kDa) (FIG. 4A). This western blot results confirm the expression of ABC2 and ABC3.
Fluorescence Microscopy
Next, to further analyse the localization of ABC2 and ABC3, each of them was tagged with EGFP at its C terminus.
S. cerevisiae BY4741 cells carrying plasmid pYES2ABC2-EGFP and pYES2ABC3-EGFP were grown to the early logarithmic phase in induction medium, harvested and mounted on the poly-L-lysine-coated slide glass. EGFP fluorescence was analysed with a fluorescent microscope (Zeiss Axio Scope A1).
As shown in FIG. 4B, strong fluorescence was observed on the plasma membrane of cells containing ABC2-EGFP and ABC3-EGFP fusion proteins respectively. These results suggest that ABC2 and ABC3 are located on the plasma membrane of S. cerevisiae cells.
Toxicity Test
Alkane Susceptibility Test on Agar Plate
Alkane susceptibility test on plates was performed according to the methods of Mauersberger et al. [14, 15]. Exponentially growing cells in induction medium were centrifuged and re-suspended with induction medium at OD600=1. Ten microliter aliquots of successive 10-fold dilutions (non diluted, 10-1, 10-2, 10-3) of cells were spotted onto the induction medium plate. Medium chain alkanes were supplied as vapour phase by placing 200 μl alkane on a sterile filter paper in the lid of the petri dish. Plates were incubated at 28° C. for 2 days.
Alkane Susceptibility Test in Liquid Culture
Overnight culture was diluted into 5 ml induction medium in 50 ml glass bottle (Sigma) at an initial OD600 of 0.4. Alkanes were added as different final concentration. Bottles were sealed tightly with butyl rubber stopper (Sigma) and silver aluminum seal (Sigma). Liquid culture was performed at 28° C. with shaking. Growth was monitored by measuring the OD600 at different time point. Cell culture used for time point OD checking was collected from the glass bottle using needle and syringe.
Viability Improvement Over Native S. cerevisiae 
Toxicity test was implemented to study the effect of ABC2 and ABC3 on the tolerance of the cells toward alkanes. The toxic effects of alkanes on S. cerevisiae with ABC2 and ABC3 were measured through alkane susceptibility test on agar plates. FIG. 2A shows that for cells expressing ABC2 and ABC3, the cell tolerance toward decane and undecane was considerably improved. It was observed that the expression of ABC2 would lead higher tolerance toward decane than ABC3.
To further analyze the effect of ABC2 and ABC3 toward decane and undecane, alkane susceptibility tests in liquid culture were conducted. As shown in FIG. 2B, 0.5% decane and 20% undecane would cause a significant decrease in viability of the control cells. With the expression of ABC2, decane tolerance was boosted about 80-fold (20% vs. 0.25% decane) and cells can survive in 20% undecane. For ABC3, decane tolerance is increased slightly by about 3-fold (0.75% vs. 0.25% decane) and cells are resistant to 20% undecane. It is apparent that the expression of ABC2 and ABC3 transporters improved the tolerance toward C10 and C11 alkanes.
The results on agar plates and in liquid medium showed that both ABC2 and ABC3 expressing cells have greatly enhanced tolerance toward decane and undecane compared with native S. cerevisiae. 
Gas Chromatography (GC) Analysis
Intracellular alkane accumulation was analysed with GC-FID after 48 h incubation with 0.5% decane or 20% undecane.
After induction for 48 h with or without addition of alkanes, S. cerevisiae cells transformed with pYES2, pYES2ABC2 and pYES2ABC3 were harvested at 6000 g for 5 min at 4° C. After washing with 50 mM Tris.Cl, cells were equally divided into two parts, one part for alkane GC analysis and the other for determination of total protein concentration. For GC analysis, cell pellets were re-suspended in freshly prepared Chloroform/Methanol (v/v, 2:1). Dodecane was added into cell suspension as an internal standard. Acid-washed glass beads were added until the suspension was covered. Cells were then lysed by mechanical agitation using FastPrep-24 (MPBio) for 6 min at 6 m/s. The crude extract was obtained by pipette. After addition of autoclaved ddH2O, the crude extract was emulsified for 10 min by inversion. After centrifuge, the crude extract was separated into two phases. The bottom phase containing alkane was transferred into a new 1.5 ml microcentrifuge tube and purified as above with HPLC grade chloroform and autoclaved ddH2O until particulate matter was no longer observable. The purified solution was transferred into a clear GC vial for GC analysis. To check the total protein concentration, Cell pellets were re-suspended into 50 mM Tris.Cl and lysed via mechanical agitation with acid-washed glass beads using FastPrep-24 for 6 min at 6 m/s. Protein concentration of obtained crude extract was determined using the Bradford protein assay (Bio-Rad). Intracellular alkane levels were normalized to internal standard and cell lysate protein content.
Novel Use of ABC2 and ABC3 for Tolerance Improvement in S. cerevisiae Toward Decane and Undecane
After confirming the function of ABC2 and ABC3 transporters, which can pump out decane and undecane out of cell, we demonstrated that the expression of ABC2 and ABC3 increased S. cerevisiae tolerance toward decane and undecane through lowering intracellular alkane level.
Proved by the toxicity test, as shown in FIG. 2, we found that both ABC2 and ABC3 expressing cells exhibited enhanced tolerance toward decane and undecane. This is the first report demonstrating that the expression of ABC2 and ABC3 transporters improves the cell tolerance toward decane and undecane.
Glutamate is Required for Energy-Dependent Efflux Pumping of ABC2 and ABC3
Two different models of ATP hydrolysis mechanisms were proposed for ABC transporters before: the “catalytic carboxylate” model and the “catalytic dyad” model. According to the “catalytic carboxylate” model, the highly conserved glutamate residue at the C terminus of the Walker B motif is essential for ATP hydrolysis. However, in the “catalytic dyad” model, interactions between glutamate of the Walker B motif and the histidine of the H-loop are a prerequisite for ATP hydrolysis.
Sequence alignment of ABC2, ABC3, pleitropic drug resistance 5 (PDR5) and pleitropic drug resistance 15 (PDR15), of the pleitropic drug resistance network in yeast, showed that these proteins have high similarities in NBD domains which include Walker A motif, Walker B motif, C-loop and H-loop (FIG. 5A). Similar to the widely studied PDR5 model, critical amino acids such as glutamate in C-terminus of Walker B motif and histidine of H-loop are only present in NBD2 but not in NBD1 for ABC2 and ABC3. Hence, to determine the ATP hydrolysis mechanism of ABC2 and ABC3, the glutamate (E988 for ABC2 and E989 for ABC3) and the histidine (H1020 for ABC2 and H1021 for ABC3) in NBD2 of ABC2 and ABC3 were mutated to glutamine and alanine, respectively. As shown in FIG. 5B, ABC2-E988Q and ABC3-E989Q mutants were highly sensitive against both decane and undecane, while ABC2-H1020A and ABC3-H1021A mutants still showed increased tolerance against undecane and decreased resistance against decane. Therefore, histidine is deemed not as essential as glutamate for these transporters' activity, and ATP is most likely to be hydrolyzed by the catalytic carboxylate mechanism.
By “comprising” it is meant including, but not limited to, whatever follows the word “comprising”. Thus, use of the term “comprising” indicates that the listed elements are required or mandatory, but that other elements are optional and may or may not be present.
By “consisting of” is meant including, and limited to, whatever follows the phrase “consisting of”. Thus, the phrase “consisting of” indicates that the listed elements are required or mandatory, and that no other elements may be present.
The inventions illustratively described herein may suitably be practiced in the absence of any element or elements, limitation or limitations, not specifically disclosed herein. Thus, for example, the terms “comprising”, “including”, “containing”, etc. shall be read expansively and without limitation. Additionally, the terms and expressions employed herein have been used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed. Thus, it should be understood that although the present invention has been specifically disclosed by preferred embodiments and optional features, modification and variation of the inventions embodied therein herein disclosed may be resorted to by those skilled in the art, and that such modifications and variations are considered to be within the scope of this invention.
By “about” in relation to a given numberical value, such as for temperature and period of time, it is meant to include numerical values within 10% of the specified value.
The invention has been described broadly and generically herein. Each of the narrower species and sub-generic groupings falling within the generic disclosure also form part of the invention. This includes the generic description of the invention with a proviso or negative limitation removing any subject matter from the genus, regardless of whether or not the excised material is specifically recited herein.
Other embodiments are within the following claims and non-limiting examples. In addition, where features or aspects of the invention are described in terms of Markush groups, those skilled in the art will recognize that the invention is also thereby described in terms of any individual member or subgroup of members of the Markush group.
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## Claims

1. A recombinant Saccharomyces cerevisiae cell expressing a heterologous ATP binding cassette 2 (ABC2) polypeptide having the amino acid sequence set forth in SEQ ID NO:4, wherein the ABC2 polypeptide is capable of exporting C10-C11 alkanes.

2. The recombinant Saccharomyces cerevisiae cell of claim 1, wherein the cell comprises a plasmid comprising a nucleic acid molecule encoding the heterologous ABC2 polypeptide.

3. A method for the production of a C6-C16 alkane comprising culturing the recombinant Saccharomyces cerevisiae cell of claim 1 under conditions that allow (i) the expression of the heterologous ABC2 polypeptide; and (ii) the production of a C6-C16 alkane.

4. A method of increasing resistance of toxicity to C10-C11 alkanes in a Saccharomyces cerevisiae cell comprising:
a. transforming a Saccharomyces cerevisiae cell with a nucleic acid molecule encoding a heterologous ATP binding cassette 2 (ABC2) polypeptide having the amino acid sequence set forth in SEQ ID NO:4, wherein the ABC2 polypeptide is capable of exporting C10-C11 alkanes; and
b. culturing said transformed Saccharomyces cerevisiae cell under conditions that allow expression of the heterologous ABC2 polypeptide.

5. The method of claim 4 wherein the nucleic acid molecule is within a plasmid.

6. The method of claim 4, wherein said transformed Saccharomyces cerevisiae cell is capable of producing a C6-C16 alkane.

7. The method of claim 6, wherein said transformed Saccharomyces cerevisiae cell is capable of producing a decane.

8. The method of claim 6, wherein said transformed Saccharomyces cerevisiae cell is capable of producing an undecane.