Abstract:
Exemplary methods for increasing TAG production in an algal cell during imbalanced growth conditions are provided. Some methods comprise knocking out an AOX gene, wherein the AOX gene produces an amino acid sequence having substantial similarity to the amino acid sequence of SEQ. ID. NO. 2. In further methods, the algal cell may be of genus  Nannochloropsis . The AOX gene may be replaced by a construct having a nucleotide sequence having substantial similarity to SEQ ID. NOS. 3 through 5 (inclusive), wherein each of the sequences are next to or in close proximity to one another in a linear fashion. In some methods, the AOX gene may be replaced via homologous recombination. As a result, lipid production by the selected recombinant algal cell may be increased over that produced by a wild-type algal cell.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application claims the benefit and priority of U.S. Provisional Patent Application Ser. No. 61/227,026 filed on Jul. 20, 2009, titled “Manipulation of an Alternative Respiratory Pathway in Photo-Autotrophs,” the disclosure of which is hereby incorporated by reference. 
     The present application by reference incorporates in entirety the following applications: U.S. Non-Provisional patent application Ser. No. 12/581,812 filed on Oct. 19, 2009, titled “Homologous Recombination in an Algal Nuclear Genome,” U.S. Non-Provisional patent application Ser. No. 12/480,635 filed on Jun. 8, 2009, issued on Nov. 27, 2012 as U.S. Pat. No. 8,318,482 and titled “VCP-Based Vectors for Algal Cell Transformation,” and U.S. Non-Provisional patent application Ser. No. 12/480,611 filed on Jun. 8, 2009, issued on Feb. 21, 2012 as U.S. Pat. No. 8,119,859 and titled “Transformation of Algal Cells.” 
    
    
     REFERENCE TO SEQUENCE LISTINGS 
     The present application is filed with sequence listing(s) attached hereto and incorporated by reference. 
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     This invention relates to molecular biology, and more specifically, to the expression of exogenous DNA elements in algal cells and/or the mutation of algal cells. 
     SUMMARY OF THE INVENTION 
     Exemplary methods for increasing TAG production in an algal cell during imbalanced growth conditions are provided. Some methods comprise knocking out an AOX gene, wherein the AOX gene produces an amino acid sequence having substantial similarity to the amino acid sequence of SEQ. ID. NO. 2. In further methods, the algal cell may be of genus  Nannochloropsis . The AOX gene may be replaced by a construct having a nucleotide sequence having substantial similarity to SEQ ID. NOS. 3 through 5 (inclusive), wherein each of the sequences are next to or in close proximity to one another in a linear fashion. In some methods, the AOX gene may be replaced via homologous recombination. Further, the AOX gene may have a nucleotide sequence with substantial similarity to SEQ ID. NO. 1. Additionally, the recombinant algal cell may be grown in the presence of an antibiotic in which the selected recombinant algal cell is resistant. A marker gene may confer resistance to zeocine. As a result, lipid production by the selected recombinant algal cell may be increased over that produced by a wild-type algal cell. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an exemplary CLUSTALW alignment for three amino acids produced by various AOX genes, as found in three different types of algae. 
         FIG. 2  shows an exemplary gene structure and expression diagram for the wild-type  Nannochloropsis  (W2) AOX gene sequence, as found in the  Nannochloropsis  genome. 
         FIG. 3  shows exemplary TargetP1.1 prediction data for the wild-type  Nannochloropsis  (W2) AOX gene sequence. 
         FIG. 4  shows a chart reflecting exemplary expression levels of the wild-type  Nannochloropsis  (W2) AOX gene sequence in the presence and absence of nutrients, labeled as (Nutrients+) and (Nutrients−), respectively. 
         FIG. 5  shows a chart reflecting exemplary environmental oxygen concentrations in the presence of wild-type  Nannochloropsis  under varying light conditions over time. 
         FIG. 6  shows an exemplary W2 AOX knockout construct and integration site. 
         FIG. 7  shows a chart with actual exemplary data comparing the accumulation of fatty acid methyl esters (FAMES) during nutrient starvation of wild-type  Nannochloropsis  (WT) and mutant  Nannochloropsis  (A11). 
         FIG. 8  SEQ. ID. NO. 1 is an exemplary nucleotide sequence for the AOX gene sequence. 
         FIG. 9  SEQ. ID. NO. 2 is an exemplary amino acid sequence for the amino acid produced by SEQ. ID. NO. 1. 
         FIG. 10  SEQ. ID. NO. 3 is an exemplary nucleotide sequence for a VCP promoter gene. 
         FIG. 11  SEQ. ID. NO. 4 is an exemplary nucleotide sequence for a ble marker gene (confers resistance to zeocine). 
         FIG. 12  SEQ. ID. NO. 5 is an exemplary nucleotide sequence for a 3′ UTR gene. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Whole chain electron transport in mitochondrial respiration typically involves four major protein complexes located in the inner mitochondrial membrane. During respiratory electron flow through these four complexes, electron donors, such as nicotinamide adenine dinucleotide phosphate (NADP), and succinate, become oxidized, while molecular oxygen ultimately becomes reduced by the terminal protein complex in the pathway known as cytochrome c oxidase. Respiratory electron flow is coupled to proton pumping across the mitochondrial inner membrane, generating a significant proton motive force (PMF). The PMF is subsequently released through the adenosine triphosphate (ATP) synthase, leading to the generation of energy rich molecular ATP. 
     A second form of respiratory electron flow, known as the alternative respiratory pathway, employs an oxidase known as the alternative oxidase (AOX), which is generally located upstream to the cytochrome c oxidase in the respiratory pathway. The alternative respiratory pathway terminates electron flow with the reduction of molecular oxygen upstream of complex III, therefore bypassing this complex as well as the terminal cytochrome c oxidase. Since complex III and the cytochrome c oxidase pump protons during electron transport, alternative respiration leads to the formation of significantly less ATP per electron donor oxidized, relative to whole chain respiration. In this respect, alternative respiration is a wasteful process in terms of maximizing energy conversion from cellular reductant. For oxygenic photo-autotrophs, it may be necessary to employ such a wasteful pathway during imbalanced growth, when more reduced carbon is generated through photosynthesis than can be used for growth and cell division. Examples of imbalanced growth include excessive irradiance and low nutrient status. The accumulation of excess reduced carbon during imbalanced growth may also be alleviated through the generation of triacylglycerides (TAGs). 
     According to various exemplary embodiments herein, the loss of reductant through an alternative respiratory pathway that generally exists in wild type strains of various photo-autotrophs, such as marine algae, is restricted and/or eliminated by generating an oxidase deletion strain. 
       FIG. 1  shows an exemplary CLUSTALW alignment for three amino acids produced by various AOX genes, as found in three different types of algae. CLUSTALW is a general purpose multiple sequence alignment program for DNA or proteins. Shown in  FIG. 1  is an amino acid sequence alignment for two amino acids produced by two well-characterized AOX genes, as found in  Chlamydomonas reinhardtii  and  Arabidopsis thaliana , with the  Chlamydomonas reinhardtii  having the AOX1 gene sequence, and the  Arabidopsis thaliana  having the AOX1A gene sequence. Also shown in  FIG. 1  is the amino acid sequence produced by wild-type  Nannochloropsis  (W2), having the AOX gene sequence. SEQ. ID. NO. 1 is an exemplary nucleotide sequence for the AOX gene sequence. SEQ. ID. NO. 2 is an exemplary amino acid sequence for the amino acid produced by SEQ. ID. NO. 1. The close similarity of the two well-characterized amino acid sequences to the amino acid sequence produced by the wild-type  Nannochloropsis  AOX gene sequence confirms the identity of the  Nannochloropsis  AOX gene sequence. 
       FIG. 2  shows an exemplary gene structure and expression diagram for the wild-type  Nannochloropsis  (W2) AOX gene sequence, as found in the  Nannochloropsis  genome. In  FIG. 2 , the exemplary gene structure and expression diagram is comprised of seven exons, including a 5′ untranslated region (5′UTR), a 3′ untranslated region (3′UTR), various introns between the exons, and a sequence gap. In total, the wild-type  Nannochloropsis  (W2) AOX gene sequence of  FIG. 2  represents approximately 5,301 base pairs (bp). The exemplary gene structure of  FIG. 2  exists in the genome of the wild-type  Nannochloropsis  (W2). Prior to translation, the introns are spliced out, to form the exemplary AOX gene sequence comprising the 5′ UTR, exon1 through exon7 (inclusive), and the 3′ UTR. 
       FIG. 3  shows exemplary TargetP1.1 prediction data for the wild-type  Nannochloropsis  (W2) AOX gene sequence. TargetP1.1 is software that predicts the subcellular location of eukaryotic protein sequences. The assignment is based on the predicted presence of the following N-terminal presequences: chloroplast transit peptide (cTP), mitochondrial targeting peptide (mTP), and/or secretory pathway signal peptide (SP). For each input sequence (such as the AOX gene sequence), the following may be printed: Name (sequence name truncated to 20 characters), Len (sequence length), and/or Loc (prediction of localization, based on the scores). The Loc codes may be: C: chloroplast, i.e. the sequence contains cTP, a chloroplast transit peptide; M: mitochondrion, i.e. the sequence contains mTP, a mitochondrial targeting peptide; S: secretory pathway, i.e. the sequences contains SP, a signal peptide; and/or other: any other location. RC indicates reliability class, from 1 to 5, where 1 indicates the strongest prediction. As shown in  FIG. 3 , the wild-type  Nannochloropsis  (W2) AOX gene sequence appears to be targeted to the mitochondrion. 
       FIG. 4  shows a chart reflecting exemplary expression levels of the wild-type  Nannochloropsis  (W2) AOX gene sequence in the presence and absence of nutrients, labeled as (Nutrients+) and (Nutrients−), respectively. According to various exemplary embodiments, the wild-type  Nannochloropsis  (W2) AOX gene sequence is expressed at reduced rates during balanced growth (e.g. balanced in terms of nutrients and/or illumination), and is up-regulated during imbalanced growth (e.g. following nutrient starvation and/or high illumination). During imbalanced growth, a portion of environmental carbon (e.g. CO 2 ) is consumed by wild-type  Nannochloropsis  and is retained in the form of triacylglyceride (TAG). At the same time,  Nannochloropsis  is unable to retain other environmental carbon in the form of TAG and instead releases it in the form of carbon dioxide (CO 2 ) by way of the alternative respiratory pathway, facilitated by the AOX gene sequence. 
       FIG. 5  shows a chart reflecting exemplary environmental oxygen concentrations in the presence of wild-type  Nannochloropsis  under varying light conditions over time. In the chart of  FIG. 5 , periods of darkness are shaded; periods of light are not shaded. Exposure of  Nannochloropsis  to excessive illumination results in stimulation of respiration, a significant proportion of which may be eliminated using the AOX specific inhibitor salicylhydroxamic acid (SHAM). Since AOX activity is generally a pathway that eliminates cellular reductant in a wasteful manner, eliminating AOX activity may lead to an increased accumulation of reduced carbon in the form of either cellular components, resulting in higher rates of productivity, or as storage components, resulting in increased rates of TAG accumulation. As shown in the chart of  FIG. 5 , the introduction of 1 millimolar (mM) SHAM results in a decrease in the rate of oxygen consumption, evidencing the influence of the increased activity of the AOX protein on the alternative respiratory pathway of  Nannochloropsis.    
       FIG. 6  shows an exemplary W2 AOX knockout construct and integration site. In order to permanently eliminate AOX activity in marine algae, provide improved growth, and/or facilitate TAG accumulation (e.g. in outdoor pond systems), the inventors generated the exemplary W2 AOX knockout construct as shown and described in connection with  FIG. 6  to create a mutant  Nannochloropsis . Utilizing homologous recombination as described in U.S. Non-Provisional patent application Ser. No. 12/581,812 filed on Oct. 19, 2009, titled “Homologous Recombination in an Algal Nuclear Genome,” which is hereby incorporated by reference, the inventors replaced the AOX gene sequence of the wild-type  Nannochloropsis  (W2) with the promoter, ble and 3′ UTR genes as illustrated in the W2 AOX knockout construct shown in  FIG. 6 . Exemplary promoters, ble and 3′ UTR genes that may be suitable for such purposes are described in U.S. Non-Provisional patent application Ser. No. 12/480,635 filed on Jun. 8, 2009, titled “VCP-Based Vectors for Algal Cell Transformation,” which is hereby incorporated by reference. SEQ. ID. NO. 3 is an exemplary nucleotide sequence for a VCP promoter gene, SEQ. ID. NO. 4 is an exemplary nucleotide sequence for a ble marker gene (confers resistance to zeocine), and SEQ. ID. NO. 5 is an exemplary nucleotide sequence for a 3′ UTR gene. SEQ. ID. NOS. 3-5, as a unit, via homologous recombination, may replace the wild-type AOX gene as found in the genome of wild-type  Nannochloropsis.    
       FIG. 7  shows a chart with actual exemplary data comparing the accumulation of fatty acid methyl esters (FAMES) during nutrient starvation of wild-type  Nannochloropsis  (WT) and mutant  Nannochloropsis  (A11). TAGs are a subset of FAMES. The mutant  Nannochloropsis  was constructed per  FIG. 6  and the associated description herein. As shown in  FIG. 7 , the mutant  Nannochloropsis  (A11) accumulates higher amounts of FAMES as a percentage of total mass at all times than the wild-type  Nannochloropsis  (WT). 
     While various embodiments have been described herein, it should be understood that they have been presented by way of example only, and not limitation. Thus, the breadth and scope of a preferred embodiment should not be limited by any of the herein-described exemplary embodiments.