# Patent

## Title
Expression of polypeptides from the nuclear genome of Ostreococcus sp

## Abstract

A method of producing at least one polypeptide from the nuclear genome of Ostreo-coccus sp., the method including introducing at least one recombinant nucleic acid molecule into the nuclear genome of Ostreococcus sp., wherein the recombinant nucleic acid molecule included a first polynucleotide operatively linked to a second polynucleotide, wherein the second polynucleotide encodes at least one polypeptide and wherein the first polynucleotide comprises a promoter sequence allowing expression of the at least one polypeptide in Ostreoccus sp.

## Background
CROSS-REFERENCE TO RELATED APPLICATION
This application is a National Stage entry on International Application PCT/IB2008/000281, filed Feb. 8, 2008, which claims priority to U.S. Patent Application No. 60/900,464, filed Feb. 9, 2007, the disclosure of the prior applications being incorporated in their entirety by reference.


SEQUENCE LISTING
The instant application contains a Sequence Listing which has been submitted via EFS-Web and is hereby incorporated by reference in its entirety. Said ASCII copy, created on Jan. 27, 2010, is named 21305279.txt, and is 23,840 bytes in size.
TECHNICAL FIELD
The technology in this disclosure relates generally to the field of methods for expressing polypeptides from the nuclear genome of green algae, particularly in Ostreococcus sp.
PRIOR ART
The production of recombinant proteins and in particular of monoclonal antibodies represents an important market and is in full expansion. It is evaluated to 10 billions dollars in 2004 and it should exceed 30 billions dollars in 2010. These recombinant proteins are finding use as supplements for healthy individuals or as therapeutic agents for the treatment of pathologic disorder, particularly in cancerology and in infectious diseases.
A primary advantage of using genetic engineering techniques for producing therapeutic agents is that the methods allow for the generation of large amounts of a desired protein. Different methods using different cell types were developed until now. Even where technologies based on bacteria cells were improved, they present limitations for the production of complex proteins such as antibodies. Methods using eukaryotic cells, particularly insect cells and mammalian cells allow for the production of human complex proteins. However, the high cost of these systems constituted the principal limitation of their development.
The production of proteins in transgenic plants currently constitutes an alternative in full rise. These expression systems allow for the production of complex proteins without carrying pathogenic agents like viruses and prions. The main problem with the use of transgenic plants is the risk of contamination relating to the pollen dissemination, especially using human genes or allergens. In fact, the European legislation limits the use of transgenic plants due to such risk. In addition, if such production is cheap, the purification of proteins from the plants can be complex and expansive.
Recently a green algae, i.e. Chlamydomonas reinhardtii, has been used to produce a monoclonal antibody directed to the Herpes virus from its plastidial genome. This algae presents the main advantage to be unicellular and thus to facilitate the purification of recombinant proteins compared with transgenic plants. However, the biased genetic code of Chlamydomonas reinhardtii constitutes the major drawback of this expression system. In fact, it is necessary to genetically modify each codon of a gene comprising A or T at the third position, to obtain a correct expression of a recombinant protein coded by said gene. This required step for the production of protein involves a high production cost for this expression system in Chlamydomonas reinhardtii. Furthermore, expression in the plastid does not allow the glycosylation of glycolproteins, such as antibodies, which is often required for their biological activity or stability.
Thus, a need exists for methods to conveniently produce and purify proteins in large amounts with low cost.
DEPOSIT INFORMATION
An isolated strain of Ostreococcus tauri OTTH95 was deposited pursuant to the Budapest Treaty on the International Recognition of the Deposit of Microorganisms for the Purposes of Patent Procedure in the Culture Collection of Algae and Protozoa (CCAP), at SAMS Research Services Ltd., Oban, Argyll PA37 IQA, Scotland, United Kingdom, on Nov. 14, 2006. The deposit was accepted on Dec. 14, 2006, and designated as CCAP Number 157/1.
SUMMARY
Recombinant proteins can be expressed from Ostreococcus sp. green algae nuclear genome. Accordingly, we provide a method of producing at least one polypeptide in Ostreococcus sp., the method comprising introducing at least one recombinant nucleic acid molecule into Ostreococcus sp., wherein the recombinant nucleic acid molecule comprises a first polynucleotide operatively linked to a second polynucleotide, wherein the second polynucleotide encodes at least one polypeptide and wherein the first polynucleotide comprises a promoter sequence allowing expression of the at least one polypeptide in Ostreococcus sp.
We further provide an expression cassette for expression of at least one polypeptide in Ostreococcus sp., which cassette comprises a promoter sequence operatively linked to and positioned upstream of a restriction enzyme site for insertion of a nucleotide sequence coding for the at least one polypeptide, wherein the promoter sequence allows expression of the at least one polypeptide in Ostreococcus sp.
We still further provide a vector comprising at least one expression cassette according to the invention as described below.
We yet further provide a cell, which can be obtained by the method according to the invention as described below.
We further provide a polypeptide, which can be obtained by the method according to the invention as described below and which comprises an N-glycosylated carbohydrate chain.
We also disclose the use of at least one polypeptide for the preparation of a therapeutic composition.
DETAILED DESCRIPTION
A method of producing at least one polypeptide from the nuclear genome of Ostreococcus sp. is disclosed. The method comprises:

    
    
        (i) introducing at least one recombinant nucleic acid molecule into the nuclear genome of Ostreococcus sp., wherein the recombinant nucleic acid molecule comprises a first polynucleotide operatively linked to a second polynucleotide,

wherein the second polynucleotide encodes at least one polypeptide and

        wherein the first polynucleotide comprises a promoter sequence allowing expression of said at least one polypeptide in Ostreococcus sp.
    
    


The use of Ostreococcus sp. to express a polypeptide or a protein complex according to our methods provides the advantage that a polypeptide or a protein complex can be expressed from the nuclear genome, thus allowing for correct expression of a desired product without genetically modify each codon of a gene, contrary to the Chlamydomonas reinhardtii plastidial expression system.
The term “polypeptide” is used herein to refer to a linear series of amino acid residues connected to one another by peptide bonds between the alpha amino group and carboxy group of contiguous amino acid residues.
As used herein, the term “Ostreococcus sp.” refers to the unicellular green algae sp. of the Prasinophyceae family. According to the invention, the Ostreococcus sp. can be chosen from the group comprising Ostreococcus tauri, available at the Culture Collection of Algae and Protozoa (CCAP) in the United Kingdom (SAMS Research Services Ltd. OBAN, Argyll PA37 IQA, Scotland) with the accession number given by the International Depositary Authority CCAP 157/1, Ostreococcus oceanica, Ostreococcus sp. available at the Roscoff Culture Collection of Marine Phytoplankton (RCC) at Roscoff in France under the references RCC141, RCC143, RCC343, RCC344, RCC356, RCC371, RCC371, RCC393, RCC410, RCC420 and RCC501, preferably Ostreococcus tauri. 
For example, the wild type strain 0TTH0595 of Ostreococcus tauri is available at the Roscoff Culture Collection of Marine Phytoplankton (RCC) at Roscoff in France with the reference RCC 614 or at the Culture Collection of Algae and Protozoa (CCAP) in the United Kingdom (SAMS Research Services Ltd. OBAN, Argyll PA37 IQA, Scotland) with the accession number given by the International Depositary Authority CCAP 157/1. Its entire genome is available under the following EMBL accession numbers: CR954201 (Chrom 1); CR954202 (Chrom 2); CR954203 (Chrom 3); CR954204 (Chrom 4); CR954205 (Chrom 5); CR954206 (Chrom 6); CR954207 (Chrom 7); CR954208 (Chrom 8); CR954209 (Chrom 9); CR954210 (Chrom 10); CR954211 (Chrom 11); CR954212 (Chrom 12); CR954213 (Chrom 13); CR954214 (Chrom 14); CR954215 (Chrom 15); CR954216 (Chrom 16); CR954217 (Chrom 17); CR954218 (Chrom 18); CR954219 (Chrom 19); CR954220 (Chrom 20). The use of Ostreococcus sp. to express a polypeptide according to our method of the invention provides the advantage that its genetic code is not biaised, all codons being present, thus facilitating the expression and production of polypeptide.
A method of introducing at least one recombinant nucleic acid molecule into Ostreococcus sp. can be easily identified by one skilled in the art regarding to their general knowledge. For example, the step (i) of introducing can be performed by electroporation.
The term “recombinant nucleic acid molecule” is used herein to refer to a polynucleotide, which can be resulted from experimental recombination.
The term “polynucleotide” is used herein to mean a sequence of two or more deoxyribonucleotides or ribonucleotides that are linked together by a phosphodiester bond. As such, the term includes RNA and DNA, preferably DNA, which can be a gene or a portion thereof, a cDNA, a synthetic polydeoxyribonucleic acid sequence, or the like, and can be single stranded or double stranded, as well as a DNA/RNA hybrid. Furthermore, the term as used herein include naturally occurring nucleic acid molecules, which can be isolated from a cell, as well as synthetic polynucleotides, which can be prepared, for example, by methods of chemical synthesis or by enzymatic methods such as by the polymerase chain reaction (PCR).
As used herein, the term “operatively linked” means that two or more molecules can be positioned with respect to each other such that they act as a single unit and effect a function attributable to one or both molecules or a combination thereof. For example, a polynucleotide encoding a polypeptide can be operatively linked to a transcriptional or translational regulatory element, in which case the element confers its regulatory effect on the polynucleotide similarly to the way in which the regulatory element would effect a polynucleotide sequence with which it is normally associated with in a cell.
As used herein, the term “promoter sequence” refers to a DNA region comprising a binding site of RNA polymerase as well as at least a binding site of transcription regulatory proteins. A promoter sequence useful for the invention can be easily identified by one skilled in the art with their general knowledge. For example, the promoter sequence useful for the invention can be chosen from the group comprising the Ostreococcus tauri histone H4 promoter sequence (identified by SEQ ID No 1), the Ostreococcus tauri cpx promoter sequence (identified by SEQ ID No 5), the Ostreococcus tauri crd1 promoter sequence (identified by SEQ ID No 4), the Ostreococcus tauri high affinity phosphate transporter (HAPT) promoter (identified by SEQ ID No 3). Particularly, the promoter sequence according to the invention can comprise SEQ ID No 3.
The method can be performed, wherein the second polynucleotide comprises at least one exogenous nucleotide sequence coding at least one polypeptide. As used herein, the term “exogenous nucleotide sequence” refers to a nucleotide sequence, which is not naturally found in the Ostreococcus sp. genome. Furthermore, the term as used herein includes naturally occurring nucleotide sequence, as well as synthetic nucleotide sequence, genomic DNA sequence, cDNA sequence and RNA sequence, preferably DNA sequence. For example, in the method, the at least one exogenous nucleotide sequence can be a marker gene. The term “marker gene” as used herein, refers to a polynucleotide that confers a detectable phenotype. A marker gene can be easily identified by one skilled in the art regarding to their general knowledge. Particularly, a marker gene can be chosen from the group comprising genes inducing resistance to antibiotic like G418 (e.g. KanMx identified by SEQ ID No 6) or nourseothricin acetyltransferase (e.g. Nat1 identified by SEQ ID No 9), and reporter genes like firefly luciferase or renilla genes producing luminescence upon hydrolysis of their substrate luciferin and coelenterazin respectively. Particularly, the at least one exogenous nucleotide sequence according to the invention can be a sequence of therapeutic interest. A sequence of therapeutic interest can be easily identified by one skilled in the art according to its general knowledge. For example, a sequence of therapeutic interest can be a wild type gene, which could be non functional gene in a particular pathology, a negative mutant of a gene, a sequence coding for a functional inhibitor of a gene. Particularly, a sequence of therapeutic interest can be a gene coding for a glycoprotein like an allergen, such as an acarian allergen.
The method of the invention can be practiced, wherein the second polynucleotide encodes a first polypeptide and at least a second polypeptide. Any or all of the encoded polypeptides can be the same or different. As such, the method provides a means to produce functional protein complexes, including, for example, dimers, trimers, and tetramers, wherein the subunits of the complexes can be the same or different. Particularly, the first polypeptide and the at least second polypeptide can correspond to a fusion protein. As used herein, the term “fusion protein” refers to a polypeptide produced by recombinant DNA methods in which a first polypeptide domain is operatively linked to a second polypeptide domain by the peptide bond produced through expression of a single open reading frame to express a single “fused” polypeptide. Particularly, the fusion protein can contains a peptide tag such as a His-6 tag, a “FLAG-epitope”, a biotin or the like, which can facilitate identification or expression of the fusion protein in a cell. Such tags can provide the advantage that they can facilitate isolation of the fusion protein, for example, when it is desired to obtain a purified protein.
Particularly, the first polypeptide can comprise an immunoglobulin heavy chain (H) or a variable region thereof (VH), and the second polypeptide can comprise an immunoglobulin light chain (L) or a variable region thereof (VL). An immunoglobulin heavy chain can associate with an immunoglobulin light chain to form a monovalent antibody in Ostreococcus sp., and two monovalent antibodies can further associate to produce bivalent antibody. As such, the method provides a mean to produce functional protein complexes such as antibodies.
The method can be performed, wherein the second polynucleotide consists of 0.5 to 10 kb, preferably of 0.5 to 5 kb and, more preferably, of 0.5 to 3 kb.
Particularly, the second polynucleotide can comprises a “target signal”. As used herein, the term “target signal” refer to a nucleotide sequence that targets a polypeptide to a particular location regarding to the cell, for example, to the cytosol, nucleus, plasma membrane, endoplamic reticulum, to an extracellular medium (i.e., secretion). Particularly, the second polynucleotide can comprises a secretion signal allowing secretion of the at least one polypeptide in Ostreococcus sp. A secretion signal can be easily identified by one skilled in the art. For example, the secretion signal is chosen from the group comprising the Ostreococcus tauri predicted aqualysin/subtilisin secreted protease sequence peptide (i.e. MRRFLTTVVLTACVSRANAF corresponding to SEQ ID No 17). The use of Ostreococcus sp. to express a polypeptide or protein complex according to the method provides the advantage that Ostreococcus sp. have no cell wall, thus allowing for production of secreted proteins and facilitating the purification of proteins.
The method can further comprise:
(ii) harvesting the at least one polypeptide expressed in Ostreococcus sp.
As used herein, the term “harvesting” means that a polypeptide is isolated from Ostreococcus sp. Particularly, the at least one polypeptide can be substantially purified which means that it is relatively free of proteins, nucleic acids, lipids, carbohydrate or other with which it is naturally associated. Generally, a substantially purified polypeptide constitutes at least about fifty percent, particularly about eighty percent of a sample.
Ostreococcus sp. can be grown in a bioreactor. The bioreactor can be easily identified by one skilled in the art regarding their general knowledge, for example, Labfors-lux (Infors HT).
Particularly, Ostreococcus sp. can be grown in a culture medium comprising at least one compound stimulating the grow of Ostreococcus sp. A compound stimulating the growth of Ostreococcus sp. can be easily identified by one skilled in the art regarding to their general knowledge. For example, the at least one compound stimulating the growth of Ostreococcus sp. can be chosen from the group comprising nitrate, ammonium, phosphate and carbon dioxyde. The use of Ostreococcus sp. to express a polypeptide or a protein complex according to the methods provides the advantage that large populations of Ostreococcus sp. can be grown, thus allowing for production and if desired, isolation of large amounts of a desired product.
Ostreococcus sp. can be grown in a culture medium comprising at least one beta 1,4 galactosyl transferase. The use of Ostreococcus sp. to express a polypeptide or a protein complex according to the methods provides the advantage that a polypeptide or a protein complex can be expressed from the nuclear genome, thus allowing for glycosylation of a desired product, contrary to the Chlamydomonas reinhardtii plastidial expression system.
Moreover, some in silico and immunochemical data suggest that the glycosylation of protein in Ostreococcus sp. differs from that in the higher plants with in particular the absence of immunogenic and allergenic residues like beta 1,2 xylose and alpha 1,3 fucose. Thus, the use of Ostreococcus sp. to express a polypeptide or a protein complex according to methods of the invention provides the advantage that the glycosylation does not provide immunogenic and allergenic residues like beta 1,2 xylose and alpha 1,3 fucose, thus allowing for glycosylation of a desired product without allergenic residues.
We also provide an expression cassette for expression of at least one polypeptide in Ostreococcus sp., wherein the cassette comprises:

    
    
        (a) a promoter sequence operatively linked to and positioned upstream of a cloning site for insertion of a nucleotide sequence coding for said at least one polypeptide,

wherein the promoter sequence allows expression of said at least one polypeptide in Ostreococcus sp.

    
    


As used herein, the term “upstream” refers to the direction opposite to the direction of DNA transcription, and therefore going from 5′ to 3′ on the noncoding strand, or 3′ to 5′ on the RNA transcript.
As used herein, the term “cloning site” is used broadly to refer to any nucleotide or nucleotide sequence that facilitates linkage of a first polynucleotide to a second polynucleotide. For example, a cloning site can be chosen from the group comprising at least one restriction endonuclease recognition site like multiple cloning site, at least one recombinase recognition site like a IoxP site.
A promoter sequence can be easily identified by one skilled in the art with his general knowledge. For example, the promoter sequence can be chosen from the group comprising the Ostreococcus tauri histone H4 promoter sequence (identified by SEQ ID No 1), the Ostreococcus tauri cpx promoter sequence (identified by SEQ ID No 5), the Ostreococcus tauri crd1 promoter sequence (identified by SEQ ID No 4), the Ostreococcus tauri high affinity phosphate transporter (HAPT) promoter (identified by SEQ ID No 3). Particularly, the promoter sequence can comprise SEQ ID No 3.
We also provide a vector comprising at least one expression cassette according to the invention. The vector can be any vector useful for introducing a polynucleotide into a prokaryotic or eukaryotic cell, including a cloning vector or an expression vector. In one embodiment, the vector can further comprise a prokaryotic origin replication. Particularly, the origin of replication can be an E. Coli origin replication. As such, a vector can be passaged and manipulated in a prokaryote host cell as well as in Ostreococcus sp. The vector can further comprise at least one cloning site and/or one regulatory element and/or at least one marker gene and/or at least one target signal.
As used herein, the terms “regulatory element” refers to a nucleotide sequence which regulates the transcription or translation of a polynucleotide to which it is operatively linked.
We also provide a cell, which can be obtained by the method. Particularly, the cell can be an Ostreococcus sp. cell. The Ostreococcus sp. can be chosen from the group comprising Ostreococcus tauri, available at the Culture Collection of Algae and Protozoa (CCAP) in the United Kingdom (SAMS Research Services Ltd. OBAN, Argyll PA37 IQA, Scotland) with the accession number given by the International Depositary Authority CCAP 157/1, Ostreococcus oceanica, Ostreococcus sp., available at the Roscoff Culture Collection of Marine Phytoplankton (RCC) at Roscoff in France under the references RCC141, RCC143, RCC343, RCC344, RCC356, RCC371, RCC371, RCC393, RCC410, RCC420 and RCC501, preferably Ostreococcus tauri. Particularly, the cell according to the invention can be the Ostreococcus tauri cell registered with the RCC under No RCC 614 or available at the Culture Collection of Algae and Protozoa (CCAP) in the United Kingdom (SAMS Research Services Ltd. OBAN, Argyll PA37 IQA, Scotland) with the accession number given by the International Depositary Authority CCAP 157/1.
We also provide a polypeptide, which can be obtained by the method and which can comprises an N-glycosylated carbohydrate chain. In one embodiment, the polypeptide is a single chain antibody. As such, the method provides a mean to produce functional protein complexes such as antibodies, which are N-glycosylated.
We also provide a pharmaceutical composition comprising at least one polypeptide.
We further provide the use of at least one polypeptide for the preparation of a therapeutic composition. Particularly, the therapeutic composition can be for the treatment of a disease chosen from the group comprising cancers, infectious diseases, cardiovascular diseases, neurodegenerative diseases like Alzheimer or Parkinson diseases, genetic diseases like monogenic genetic diseases.
Selected, representative aspects of the disclosure will now be illustrated by the following non-limiting examples.



BRIEF DESCRIPTION OF THE FIGURES
FIG. 1 provides a map of the PotLuc vector, showing relevant restriction sites, the G418 resistance gene (KanMx) under control of the Histone H4 promoter of Ostreococcus tauri followed by the TEF terminator and the luciferase+gene.
FIG. 2A provides a schema of the fusion of the PRR1 gene of Ostreococcus tauri to a luciferase gene in PotLuc, to generate the PotLuc-PRR1 vector. Double heachures indicate regions corresponding to probe used in the Southern blot analysis.
FIG. 2B provides a schema of the fusion of the HAPT (High Affinity Phosphate Transporter) promoter of Ostreococcus tauri to a luciferase gene in PotLuc, to generate the PotLuc-HAPT vector.
FIG. 3 shows the measure of the luciferase luminescence on protein extracts from 34 different Ostreococcus tauri transformants (grey and white boxes). These cells were transformed by a DNA carrying resistance to G418 and the luciferase+gene fused to the whole gene PRR1 of Ostreococcus tauri. In black box, the negative control (no DNA). In grey box, the luminescence, which was two times higher than the negative control. In white box, the luminescence, which was two times less than negative control.
FIG. 4A shows the luciferase activity of three different Ostreococcus tauri stable transformants (PRR1-5, PRR1-7 and PRR1-15). These cells were transformed by a DNA carrying resistance to G418 and the luciferase+gene fused to the whole gene PRR1 of Ostreococcus tauri. These transformants were cultivated in alternation day/night (White box/black box).
FIG. 4B shows the expression of PRR1 mRNA measured with quantitative RT-PCR and regarding to EF1α, control cells.
FIG. 5 shows the detection of the insertion of the gene PRR1 by Southern-Blot in 10 different transformants resistant to G418 and which present a luciferase activity. Lines 1, 5 and 7 correspond to the transformants PRR1-5, PRR1-7 and PRR1-15. Line C corresponds to the negative control which was not transformed. The used probe corresponds to the 3′ region of the PRR1 gene (600 pb) as described in FIG. 2A.
FIG. 6 (A to D) show the optimization of Ostreococcus tauri cells transformation by electroporation by testing different conditions: FIG. 6A shows the effect of the osmoticum. FIG. 6B shows the effect of the pulse duration. FIG. 6C shows the effect of the field strength. FIG. 6D shows the effect of the DNA quantity.
FIG. 7 provides a map of the Potox vector showing relevant restriction sites, the Nourseothricin acetyltransferase resistance gene (Nat1) under control of the Histone H4 promoter of Ostreococcus tauri followed by the Tef terminator, a 6× histidine Tag, a multiple cloning site (MCS) and the phosphate transporter promoter.



EXAMPLES
Example 1
Expression of a Luciferase Fusion Protein in Ostreococcus tauri 
This example demonstrates the expression of a luciferase fusion protein in Ostreococcus tauri. 
Methods
Preparation of Ostreococcus Tauri Competent Cells.
The wild type strain 0TTH0595 of Ostreococcus tauri was used to prepare competent cells. These cells are available at the Roscoff Culture Collection of Marine Phytoplankton (RCCMP) at Roscoff in France with the reference RCC 614 and at the Culture Collection of Algae and Protozoa (CCAP) in the United Kingdom (SAMS Research Services Ltd. OBAN, Argyll PA37 IQA, Scotland) with the accession number given by the International Depositary Authority CCAP 157/1. The whole preparation of Ostreococcus tauri competent cells was carried out in sterile conditions. The cells of Ostreococcus tauri (wild type strain 0TTH0595) were grown in blue light (30 mmol quanta cm−2 second−1) at 20° C. until a density of 30 million cells per ml. The culture medium comprising seawater added with Keller Medium from Sigma-Aldrich (reference: K1630) was filtered on 0.22 μm and autoclaved. After addition of pluronic acid F-68 to 0.1% w/vol final (Sigma Aldrich, reference: P7061), the cells were harvested in conical tubes of 50 ml (Sarstedt), by centrifugation at 8000×g at 4° C. for 8 min. The protocol was then carried out on ice. Salts are washed out from the cells by two washes with 1 ml of 1M sorbitol (10 000 g, 5 min, 4° C.). Cells were resuspended in 50 μl 1M sucrose to a final concentration of 2 to 3·1010 cells per ml.
Plasmid Construction
Three types of fusion were prepared:

    
    
        fusion of an antibiotic resistance gene to the Ostreococcus tauri histone H4 promoter (G418 resistance gene: PotLuc vector and clonat resistance gene: pH4Nat1 vector);
        fusion of the HAPT Promoter to the firefly luciferase gene (PotLuc-HAPT vector);
        fusion of the PPR1 (Pseudo Response Regulator 1) gene of Ostreococcus tauri to the firefly luciferase gene (PotLuc-PPR1 vector).
    
    


All DNA manipulations were carried out essentially as described by Sambrook et al. (Molecular cloning. A laboratory Manual Cold Spring Harbor Laboratory Press, 1989). The Ostreococcus tauri histone H4 promoter corresponds to a DNA fragment of Ostreococcus tauri amplified by PCR by using the two primers ot-H4 For and ot-H4 Rev. The sequence for ot-H4 For is 5′-GCG GAT CCC ACG GAG CGC AAC GGT ACC-3′ (SEQ ID No: 13); the sequence for ot-H4 Rev is 5′-CC AGC GCC AGC CAT GGT TTT CGA ACG-3′ (SEQ ID No: 14).
The TEF promoter of the PAG25 vector (SEQ ID No 11) encoding the nourseothricin acetyltransferase (Clonat) resistance gene (Nat1) was replaced by the Ostreococcus tauri Histone H4 promoter using the BamHI and NcoI sites.
The Ostreococcus tauri histone H4 promoter was fused to the G418 antibiotic resistance gene of KanMx module (Genbank accession number S78175) between the sites BamHI and NcoI from PUG6.5 (to replace the TEF promoter). The obtained plasmid was named PotH4KanMx. The luciferase+gene from pSP-luc+NF of Promega was inserted into PotH4KanMx using the sites XbaI and NheI. The resulting plasmid, named Potluc makes it possible to fuse genes to the luciferase+gene. The Potluc plasmid has a replication origin for E. coli and an ampicillin resistance gene already present in pUC 19. The Potluc plasmid has also a gene coding for the firefly luciferase gene (SEQ ID No 10), as well as a G418 resistance gene (SEQ ID No 6) used as a selection marker in Ostreococcus tauri under control of the strong promoter of the Ostreococcus tauri histone H4 (SEQ ID No 1), and followed by the TEF terminator (SEQ ID No 7) (FIG. 1).
The whole Ostreococcus tauri gene PRR1 (SEQ ID No 2) comprising 200 nucleotides of the promoter region was amplified by using the specific primers TOC1fullfuRNco1 and TOC1fullFNhe1 and was cloned between the sites NheI and NcoI of PotLuc, to generate plasmid PotLuc-PRR1 (FIG. 2A). The sequence of TOC1fullfuRNco1 is TTTCCATGGACTTGGAGCCGTCGCGAGA (SEQ ID No: 15) and the sequence of TOC1fullFNhe1 is TTTGCTAGCACCTCGAGCCGGGACCAAAAA (SEQ ID No: 16).
The HAPT promoter (SEQ ID No 3) consisting of 100 by upstream of the ATG, was cloned into Potluc between the BgIII and NcoI sites, to generate plasmid PotLuc-HAPT (FIG. 2B).
DNA Preparation for Ostreococcus tauri Transformation
The four DNA constructs were linearized using an accurate restriction enzyme that cuts upstream of promoters driving the expression of the different genes of interest: the Ostreococcus tauri histone H4 promoter fused to the Clonat resistance; the Ostreococcus tauri histone H4 promoter fused to the G418 resistance; the Ostreococcus tauri histone H4 promoter fused to the G418 resistance gene followed by either the Ostreococcus tauri PRR1 gene or HAPT promoter fused to the luciferase gene. Linearized plasmids were purified by ethanol precipitation after tRNA addition (1 mg/ml final concentration) (Sigma-Aldrich, RS636) and resuspended in water to obtain a final DNA concentration of 1 μg/μl.
Ostreococcus tauri transformation by electroporation was optimized as shown in FIG. 6. The linearized DNA construct issued from the PotLuc-HAPT plasmid (by Xmnl) containing the HAPT promoter (strong promoter) fused to the luciferase (ca 6 kb) was used to optimize electroporation conditions by monitoring the luciferase activity. The experimental conditions described below are those with the highest transformation efficiency and the highest viability (Sorbitol buffer, Field strength 600V/cm, resistor 400 For one transformation, 50 ml of competent cells (2·1010 cells) prepared as described above were incubated with 5 μl of DNA on ice for at least 10 min and transferred to in a 1 mm electroporation cuvette and left to warm up at RT for ca 2 min (Bridge Biosciences). Electroporation was performed in a Gene pulser apparatus (Biorad) with the following electrical parameters: Field strength 600V/cm, resistor 400Ω Capacitor 25 μF. Time constant was between 8 and 9 ms. The electroporated cells were resuspended in 40 ml of culture medium as described in paragraph [0053] during 2 days (expression phase). After 48 hours, transformed cells were selected in a solid medium (Keller Medium (SIGMA Aldrich reference number K1630) containing 0.2% of agarose and the appropriate selection antibiotic). The solid medium was prepared as follows: the autoclaved low melting agarose (Invitrogen) at 2.1% w/v was maintained at 90° C. and was mixed at a rate of 1 Vol. for 9 Vol. of culture medium. 500 μl to 1 ml of transformed cells were mixed with ten milliliters of this medium and the mixture was poured on a Petri dish. Petri dishes were placed in a wet chamber under the same conditions of illumination as previously described.
Optimisation of Ostreococcus Cells Transformation by Electroporation
The linearized DNA construct (with Xmnl) containing the high affinity phosphate transporter (HAPT) promoter fused to luciferase+(ca 6 kb) was electroporated in different conditions. Transient expression of the luciferase gene under the strong homologous promoter of HAPT was checked in the electroporated cells after one day. Luminescence signal per 106 viable cells is normalized throughout different experiment as the percentage of the maximum signal (in Black). Mortality was defined by the percentage of cells with a low red fluorescence as assessed by flow cytometry after one day (In White). Results are shown in FIG. 6 (A to D).
FIG. 6A shows the effect of the osmoticum. Cells were resuspended either in 1M sucrose or 1M sorbitol and electroporated at 800V/cm 200Ω and 400Ω (25 μF) to increase the time constant. The viability was equivalent in both osmoticum but transformation efficiency was higher in sorbitol, notably for higher time constant and was therefore used for further optimization.
FIG. 6B shows the effect of the pulse duration. Cells were electroporated at 800V/cm and the resistance was adjusted to increase the pulse duration (time constant). Transformation efficiency was greatly improved for constant time above 5 ms while viability was about 80% in this experiment.
FIG. 6C shows the effect of field strength. Electroporation was almost inefficient at 500V and was maximal at 600V still efficient but reduced by ca 50% until 800 V. Voltage above greatly affected cell viability and probably thereby decreased efficiency.
FIG. 6D shows the Effect of DNA quantity (concentration of 1 mg/ml). The Optimal quantity of DNA was 5 μg.
Results:
Stable Ostreococcus tauri Transformants which are Resistant to Antibiotics
Ostreococcus tauri cells were transformed by the linearized PotLuc plasmid (by Xmnl) carrying resistance to G418 under control of the Ostreococcus tauri histone H4 promoter. Four independent transformations were carried out. The positive clones were selected on G418 at 1 mg/ml. 50 to 1000 transformants were obtained per microgram of DNA. No transformants were obtained in the negative controls electropored under the same conditions without DNA. I. No transformants were obtained in the negative controls electropored under the same conditions without DNA.
Ostreococcus tauri cells were transformed by the linearized pH4Nat1 plasmid (by Xmnl) carrying resistance to Clonat under control of the Ostreococcus tauri histone H4 promoter. Two independent transformations were carried out. The positive clones were selected on Clonat at 700 mg/ml. Up to 500 transformants were obtained per microgram of DNA.
This example shows that it is possible to use exogenous selection genes with no bias in codon usage in Ostreococcus tauri. 
Expression of a Luciferase Recombinant Protein in Ostreococcus tauri 
Ostreococcus tauri cells were tranformed by the linearized PotLuc-PRR1 plasmid (with NheI) carrying resistance to G418 and the luciferase+gene fused to the whole gene PRR1 of Ostreococcus tauri (FIG. 2A). 1000 transformants per microgram of DNA were obtained on the basis of selection into G4181 mg/ml, which is the lethal dose for wild type Ostreococcus tauri. Luminescence was measured using a luminometer Berthold LB 360, in the presence of luciferin, either directly on the cultures into microplaques or on protein extract (FIG. 3).
The results show that more than 90% of the tested clones had luciferase levels higher than transformed negative control which were transformed without DNA (2 to more 100 000 times). The luciferase activity of different transformants were tested in vivo on 200 μl of culture cells at 3·107 cells/ml incubated in the presence of luciferine 200 μM, cultivated in alternation day/night (FIG. 4A). Similar profiles of cyclic expression of luciferase were observed in these various clones. These profiles reflect the expression of PRR1 mRNA measured with quantitative RT-PCR (FIG. 4B), which suggests that the PRR1 gene is entirely inserted. DNA of 10 transformants were extracted (Kit Dneasy Plant mini Kit from Qiagen), digested by the enzyme of restriction NcoI and used for Southern Blot. A probe corresponding to 600 pb of the 3′ coding region of PRR1 was used to detect insertions of the construction in the genome of Ostreococcus tauri (FIG. 5).
The results show that the presence of endogenous gene PRR1 in wild type strain 0TTH0595 (C) was detected by an hybridization signal at 4 kb. This band was found in all transformants. Supernumerary bands correspond to multiple insertions (1 to 3). On average, 2 events of integration by transformants were observed.
Different transformants accumulated the luciferase at various levels of expression. The analysis of transformants showed multiple events of integration in the genome and suggested that the introduced fragments were not truncated since more than 90% of the cells resistant to G418 presented an expression of luciferase reflecting the expression of gene PRR1.
This example shows the efficiently of the expression of the recombinant protein luciferase+in Ostreococcus tauri. 
Example 2
Construction of the Potox Vector
The TEF promoter upstream of the Nat1 coding sequence in the PAG25 vector was replaced by the histone H4 promoter from Ostreococcus between BamHI and NcoI cloning sites. A synthetic multiple cloning site (MCS) followed by a sequence encoding a 6× histidine tag and is complementary sequence (SEQ ID No 17 and SEQ ID No 18) were subsequently introduced together with the Ostreococcus tauri HAPT promoter, generating the Potox vector. This vector allows to clone and express the coding sequence of interest under control of the strong HAPT promoter. The transformants can be selected on the basis of CLONAT resistance at 750 μg/ml. The protein of interest can be purified using a 6× histidine TAG. This TAG can be removed, if required, using the Xho I restriction nuclease.
All documents referenced in this application, including those listed above, are herein incorporated by reference in their entirety. A variety of modifications to the embodiments described above will be apparent to those skilled in the art from the disclosure provided herein. Thus, the technology in this disclosure may be embodied in other specific forms without departing from the spirit or essential attributes thereof.

## Claims

1. A method of producing at least one polypeptide from the nuclear genome of Ostreococcus sp., the method comprising: (i) introducing at least one recombinant nucleic acid molecule into the nuclear genome of Ostreococcus sp., wherein said recombinant nucleic acid molecule comprises a first polynucleotide operatively linked to a second polynucleotide, wherein said second polynucleotide encodes at least one polypeptide and wherein said first polynucleotide comprises a promoter sequence allowing expression of said at least one polypeptide in Ostreococcus sp.

2. The method according to claim 1, wherein the Ostreococcus sp. is at least one chosen from the group consisting of Ostreococcus sp. available at the Roscoff Culture Collection of Marine Phytoplankton (RCC) at Roscoff in France under the references RCC141, RCC143, RCC343, RCC344, RCC356, RCC371, RCC371, RCC393, RCC410, RCC420 and RCC501 and Ostreococcus tauri available at the Culture Collection of Algae and Protozoa (CCAP) in the United Kingdom (SAMS Research Services Ltd. OBAN, Argyll PA37 IQA, Scotland) with the accession number given by the International Depositary Authority CCAP 157/1.

3. The method according to claim 1, wherein the Ostreococcus sp. is Ostreococcus tauri available at the Culture Collection of Algae and Protozoa (CCAP) in the United Kingdom (SAMS Research Services Ltd. OBAN, Argyll PA37 IOA, Scotland) with the accession number given by the International Depositary Authority CCAP 157/1.

4. The method according to claim 1, wherein the promoter sequence is at least one chosen from the group consisting of the Ostreococcus tauri histone H4 promoter sequence (identified by SEQ ID No 1), the Ostreococcus tauri cpx promoter sequence (identified by SEQ ID No 5), the Ostreococcus tauri crd1 promoter sequence (identified by SEQ ID No 4), the Ostreo-coccus tauri high affinity phosphate transporter (HAPT) promoter (identified by SEQ ID No 3).

5. The method according to claim 4, wherein the promoter sequence comprises SEQ ID No 3.

6. The method according to claim 1, wherein the second polynucleotide comprises at least one exogenous nucleotide sequence coding at least one polypeptide.

7. The method according to claim 6, wherein said at least one exogenous nucleotide sequence is a marker gene which induces resistance to antibiotics.

8. The method according to claim 6, wherein said at least one exogenous nucleotide sequence is a sequence of therapeutic interest.

9. The method according to claim 1, wherein the second polynucleotide encodes a first polypeptide and at least a second polypeptide.

10. The method according to claim 9, wherein the first polypeptide and the at least second polypeptide correspond to a fusion protein.

11. The method according to claim 10, wherein the first polypeptide comprises an immunoglobulin heavy chain or a variable region thereof, and the second polypeptide comprises an immunoglobulin light chain or a variable region thereof.

12. The method according to claim 1, wherein the second polynucleotide consists of 0.5 to 10 kb.

13. The method according to claim 1, wherein the second polynucleotide comprises a secretion signal allowing secretion of the at least one polypeptide in Ostreococcus sp.

14. The method according to claim 13, wherein the secretion signal is Ostreococcus tauri predicted aqualysin/subtilisin secreted protease sequence peptide (SEQ ID No 17).

15. The method according to claim 1, wherein the step (i) of introducing is carried out by electroporation.

16. The method according to claim 1, further comprising: (ii) harvesting the at least one polypeptide expressed in Ostreococcus sp.

17. The method according to claim 1, wherein Ostreococcus sp. is grown in a bioreactor.

18. The method according to claim 1, wherein Ostreococcus sp. is grown in a culture medium comprising at least one compound stimulating the growth of Ostreococcus sp., wherein the at least one compound is at least one chosen from the group consisting of nitrate, ammonium, phosphate and carbon dioxyde.

19. An expression cassette for expression of at least one polypeptide in Ostreococcus sp., wherein said cassette comprises: (a) a promoter sequence operatively linked to and positioned upstream of a cloning site for insertion of a nucleotide sequence coding for said at least one polypeptide, wherein the promoter sequence allows expression of the at least one polypeptide in Ostreococcus sp.

20. The expression cassette according to claim 19, wherein the promoter sequence is at least one chosen from the group consisting of the Ostreococcus tauri histone H4 promoter sequence (identified by SEQ ID No 1), the Ostreococcus tauri cpx promoter sequence (identified by SEQ ID No 5), the Ostreococcus tauri crdl promoter sequence (identified by SEQ ID No 4), the Ostreococcus tauri high affinity phosphate transporter (HAPT) promoter (identified by SEQ ID No 3).

21. The expression cassette according to claim 19, wherein the promoter sequence comprises SEQ ID No 3.

22. The expression cassette according to claim 19, wherein the cloning site is chosen from the group consisting of at least one restriction endonuclease recognition site and at least one recombinase recognition site.

23. A vector comprising at least one expression cassette according to claim 19.

24. The vector according to claim 23, further comprising a prokaryotic origin replication.

25. The vector according to claim 23, wherein the origin of replication is an E. Coli origin replication.

26. A cell comprising: (i) at least one recombinant nucleic acid molecule comprising a first polynucleotide operatively linked to a second polynucleotide, wherein said second polynucleotide encodes at least one polypeptide, and wherein said first polynucleotide comprises a promoter sequence allowing expression of said at least one polypeptide in Ostreococcus sp., wherein the cell is an Ostreococcus sp. cell chosen from the group consisting of Ostreococcus tauri available at the Culture Collection of Algae and Protozoa (CCAP) in the United Kingdom (SAMS Research Services Ltd. OBAN, Argyll PA37 IQA, Scotland) with the accession number given by the International Depositary Authority CCAP 157/1, Ostreococcus oceanica, Ostreococcus sp. available at the Roscoff Culture Collection of Marine Phytoplankton (RCC) at Roscoff in France under the references RCC141, RCC143, RCC343, RCC344, RCC356, RCC371, RCC371, RCC393, RCC410, RCC420 and RCC501, preferably Ostreococcus tauri.

27. The cell according to claim 26, which is the Ostreococcus tauri cell available at the Culture Collection of Algae and Protozoa (CCAP) in the United Kingdom (SAMS Research Services Ltd. OBAN, Argyll PA37 IQA, Scotland) with the accession number given by the International Deposit Authority CCAP 157/1.

28. The method of claim 2, wherein the Ostreococcus sp. is Ostreococcus tauri.

29. The method of claim 7, wherein the antibiotic resistance gene provides resistance to antibiotics chosen from the group consisting of G418 and nourseothricin acetyltransferase.

30. The method of claim 7, wherein the reporter gene is luciferase.

31. The method of claim 12, wherein the second polynucleotide consists of 0.5 to 5 kb.

32. The method of claim 12, wherein the second polynucleotide consists of 0.5 to 3 kb.

33. The method of claim 20, wherein the promoter sequence comprises SEQ ID No 3.

34. The method of claim 26, wherein the Ostreococcus sp. is Ostreococcus tauri.