Abstract:
Fusion proteins having thermostable blunt-end ligase activity are provided. Blunt-end ligases are useful for DNA amplification, sequencing, production of recombinant DNA and recombinant fusion proteins, and other purposes. These thermostable blunt-end DNA ligases are useful in ligation schemes which include, e.g., an incubation at about 60-65° C. or higher, or as high as about 94° C., or at other temperatures. The ligases disclosed herein may enable high temperature blunt-end ligation without need for molecular crowding agents, and so may be useful for many nucleic acid ligation-amplification schemes, e.g., ones which operate at a uniform temperature (e.g., at about 60° C. or higher), including ones which require temperature cycling, e.g., from about 94° C. to about 60° C. (or higher) for one, two, three, or more cycles. The thermostable blunt-end DNA ligases disclosed herein enable higher specificity target amplification, for example, by permitting temperature denaturation of double-stranded DNA templates as well as specific primer binding.

Description:
SEQUENCE LISTING 
       [0001]    The instant application contains a Sequence Listing which has been submitted electronically in ASCII format and is hereby incorporated by reference in its entirety. Said ASCII copy, created on May 22, 2014, is named 2028.201_SL.txt and is 35,411 bytes in size. 
       BACKGROUND 
       [0002]    A variety of methods for the amplification of nucleic acids are known. For example, polymerase chain reaction (“PCR”) (see, e.g. U.S. Pat. No. 4,683,202) is a popular method for the amplification of nucleic acids. To successfully perform a PCR reaction, the reaction must be performed at multiple different temperatures. This requires hardware or other mechanisms for repeatedly changing the temperature of the PCR reaction. Another method for amplification of nucleic acids is referred to as loop-mediated isothermal amplification (“LAMP”) (see, e.g. U.S. Pat. No. 6,410,278). LAMP reactions may be performed isothermally, but typically involve the use of four different primers which recognize a total of six distinct sequences on the target nucleic acid. 
         [0003]    Some amplification methods utilize DNA ligation. DNA ligation is a common molecular biology method for joining multiple DNA fragments. Ligases can seal single-strand nicks in duplex DNA, join two pieces with complementary “sticky” ends, and in some cases join two pieces with blunt, non-sticky ends. These methods find use in molecular cloning, nucleic acid diagnostics/detection, nucleic acid amplification, and related activities. Ligases and ligation methods are described, for example, in U.S. Patent Application Publication No. 20120214208; U.S. Pat. No. 7,927,853; de Lumley et al., J. Mol. Biol. (2004) 339, 1059-1075; and Rolland et al., FEMS Microbiol. Lett. (2004) 236, 267-273. 
         [0004]    Ligation of two strands of “sticky-ended” DNA is possible, where the substrates to be joined are already pre-positioned in the case of single-strand nick sealing, and the affinity of sticky ends for each other helps drive sticky-end ligation. However, since DNA ligation depends upon the juxtaposition of the two substrates to be joined, DNA ligation is more difficult in situations where the juxtaposition of the two substrates is more difficult, or the strands are likely to become mis-aligned or to separate readily. For this reason, blunt-end and high temperature ligations are two types of ligation that are particularly difficult. Blunt-end ligation, however, depends upon random interactions of the substrates. Two adjustments in blunt-end ligation are commonly made to drive the substrate interactions: low temperature to slow down molecular motion so that random interactions last longer and thus give the ligase a better chance to join the fragments; and molecular crowding reagents such as polyethylene glycol to increase the local concentrations of substrates. Likewise, high temperature ligations are challenging because the interactions of DNA substrates are briefer. The least efficient case is thus a high temperature blunt-end ligation in which molecular crowding reagents cannot be used. 
         [0005]    A thermostable ligase, T4 DNA ligase, has been described that can perform blunt-end ligations; however, it is inactivated at temperature above approximately 45° C., so that the range of temperature at which T4 DNA ligase is stable is relatively small. A few other examples of ligases that can be induced to join blunt-ended fragments are known, but these ligases appear to do so only in the presence of high concentrations of molecular crowding agents such as 50% polyethylene glycol (PEG). However, the utility of ligases that require such molecular crowding agents is unclear, since 50% PEG which DNA polymerization (which is required for DNA amplification). 
         [0006]    Accordingly, in order to facilitate the generation of amplified nucleic acids for the many and growing number of applications which use amplified nucleic acids, new methods and reagents for the amplification of nucleic acids are desired. 
       INCORPORATION BY REFERENCE 
       [0007]    All publications, patents, and patent applications mentioned in this specification are herein incorporated by reference to the same extent as if each individual publication, patent, or patent application was specifically and individually indicated to be incorporated by reference. 
       SUMMARY 
       [0008]    Fusion proteins having thermostable blunt-end ligase activity are provided. Such thermostable blunt-end ligases are useful for DNA amplification, sequencing, production of recombinant DNA (e.g., as a result of such blunt-end fusions) and recombinant fusion proteins (e.g., proteins encoded by recombinant DNA produced as a result of such blunt-end fusions), and for other purposes. Many molecular biology schemes involve ligases; such schemes would benefit from the ability to perform such schemes at elevated temperatures. Thermostable blunt-end DNA ligases disclosed herein are suitable for use in ligation schemes which would benefit from at least one elevated temperature incubation, such as an incubation at about 60-65° C. or higher, including incubation schemes at temperatures as high as about 94° C. A thermostable blunt-end ligase as disclosed herein would be useful for such schemes. 
         [0009]    In addition, thermostable blunt-end ligases as disclosed herein may enable high temperature blunt-end ligation without the need for molecular crowding agents. Accordingly, thermostable blunt-end ligase as disclosed herein may be useful for many nucleic acid ligation-amplification schemes, including nucleic acid ligation-amplification schemes which operate at a uniform temperature (e.g., at about 60° C. or higher), and including nucleic acid ligation-amplification schemes which require temperature cycling such as cycling from, e.g., about 94° C. to about 60° C. (or higher) for one, two, three, or more cycles. Nucleic acid ligation-amplification schemes which may benefit from the use of thermostable blunt-end DNA ligases disclosed herein may operate at other temperatures as well, e.g., depending on the temperature resistance characteristics of the novel fusion proteins disclosed herein and the requirements of the difference nucleic acid ligation-amplification schemes. Thermostable blunt-end DNA ligases disclosed herein provide the ability to use high temperatures in nucleic acid ligation-amplification schemes and thereby enable higher specificity target amplification, for example, by permitting temperature denaturation of double-stranded DNA templates as well as specific primer binding. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWING 
         [0010]      FIG. 1  illustrates the results of a blunt-ended ligation reaction performed at 75° C., and demonstrates that fusion proteins as disclosed herein provide improved blunt-end ligation activity at high temperature. 
       
    
    
     DETAILED DESCRIPTION 
       [0011]    Thermostable, blunt-end ligases are provided herein. As disclosed herein, a thermostable, blunt-end ligase may be prepared by the fusion of a DNA-binding protein to a thermostable ligase. Such ligases produced by these fusions have features and capabilities not provided by their parent compounds alone; the different portions of these fusion proteins provide activities which, when combined, provide new capabilities and unexpectedly improved activity. The DNA-binding protein portion of such fusion proteins is effective to increase the affinity of the ligase to DNA substrates, resulting in enhanced ligation in the challenging conditions of high temperature, blunt-end ligation. The DNA ligase portion surprisingly retains its ability to ligate DNA when combined with a foreign protein (the DNA-binding protein portion). Together, the combined portions unite in novel fusion proteins that are able to ligate blunt-ended DNA substrates even at high temperatures, providing increased ligation activity at high temperatures unavailable by the use of the original, unmodified ligases. 
         [0012]    Methods, reagents, devices, systems, and articles of manufacture useful for the practice of the methods, and for the use of reagents, devices, systems, and articles of manufacture disclosed herein, are described, for example, in U.S. Patent Application Ser. No. 61/800,606 and U.S. Patent Application Ser. No. 61/800,925, the entire disclosures of which are hereby incorporated by reference in their entireties. 
       Definitions 
       [0013]    Before the present novel ligases and ligation methods are disclosed and described, it is to be understood that the terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting. It is also to be understood that the present disclosure provides explanatory and exemplary descriptions and examples, so that, unless otherwise indicated, the molecules, compositions, assays, methods, and kits disclosed herein are not limited to the specific embodiments described herein. 
         [0014]    It must be noted that, as used in the specification and the appended claims, the singular forms “a,” “an” and “the” include plural referents unless the context clearly dictates otherwise. Thus, for example, reference to “a salt” refers to a single salt or mixtures of different salts, and the like. 
         [0015]    As used in the description herein and throughout the claims that follow, the meaning of “in” includes “in” and “on” unless the context clearly dictates otherwise. 
         [0016]    As used in the description herein and throughout the claims that follow, the meaning of “or” includes both the conjunctive and disjunctive unless the context expressly dictates otherwise. Thus, the term “or” includes “and/or” unless the context expressly dictates otherwise. 
         [0017]    In this specification and in the claims that follow, reference will be made to a number of terms, which shall be defined to have the following meanings: 
         [0018]    The term “moiety” as used herein refers to any particular composition of matter, e.g., a molecular fragment, an intact molecule, or a mixture of materials. 
         [0019]    The term “nucleic acid” refers to nucleotides and nucleosides which make up, for example, deoxyribonucleic acid (DNA) macromolecules and ribonucleic acid (RNA) macromolecules. Nucleic acids may be identified by the base attached to the sugar (e.g., deoxyribose or ribose); as used herein, the following abbreviations for these bases are used to represent nucleic acids in sequence listings identifying and describing their structures (either upper-case or lower-case may be used). 
         [0000]    
       
         
               
               
               
             
           
               
                   
                 TABLE 1A 
               
               
                   
                   
               
               
                   
                 Base (in Nucleic Acid) 
                 Letter Code 
               
               
                   
                   
               
             
             
               
                   
                 Adenine 
                 A 
               
               
                   
                 Thymine 
                 T 
               
               
                   
                 Guanine 
                 G 
               
               
                   
                 Cytosine 
                 C 
               
               
                   
                 Uracil 
                 U 
               
               
                   
                   
               
             
          
         
       
     
         [0020]    As used herein, a “polynucleotide” refers to a polymeric chain containing two or more nucleotides. “Polynucleotides” includes primers, oligonucleotides, nucleic acid strands, etc. A polynucleotide may contain standard or non-standard nucleotides. Typically, a polynucleotide contains a 5′ phosphate at one terminus (“5′ terminus”) and a 3′ hydroxyl group at the other terminus (“3′ terminus) of the chain. The most 5′ nucleotide of a polynucleotide may be referred to herein as the “5′ terminal nucleotide” of the polynucleotide. The most 3′ nucleotide of a polynucleotide may be referred to herein as the “3′ terminal nucleotide” of the polynucleotide. 
         [0021]    The term “downstream” as used herein in the context of a polynucleotide containing a 5′ terminal nucleotide and a 3′ terminal nucleotide refers to a position in the polynucleotide which is closer to the 3′ terminal nucleotide than a reference position in the polynucleotide. For example, in a primer having the sequence: 5′ ATAAGC 3′, the “G” is downstream from the “T” and all of the “A”s. 
         [0022]    The term “upstream” as used herein in the context of a polynucleotide containing a 5′ terminal nucleotide and a 3′ terminal nucleotide, refers to a position in the polynucleotide which is closer to the 5′ terminal nucleotide than a reference position in the polynucleotide. For example, in a primer having the sequence: 5′ ATAAGC 3′, the “T” is upstream from the “G”, the “C”, and the two “A”s closest to the “G”. 
         [0023]    As used herein, “nucleic acid” includes both DNA and RNA, including DNA and RNA containing non-standard nucleotides. A “nucleic acid” contains at least one polynucleotide (a “nucleic acid strand”). A “nucleic acid” may be single-stranded or double-stranded. 
         [0024]    As used herein, a nucleic acid molecule which is described as containing the “sequence” of a template or other nucleic acid may also be considered to contain the template or other nucleic acid itself (e.g. a molecule which is described as containing the sequence of a template may also be described as containing the template), unless the context clearly dictates otherwise. 
         [0025]    As used herein, a “target” nucleic acid or molecule refers to a nucleic acid of interest. A target nucleic acid/molecule may be of any type, including single-stranded or double stranded DNA or RNA (e.g. mRNA). 
         [0026]    As used herein, “complementary” sequences refer to two nucleotide sequences which, when aligned anti-parallel to each other, contain multiple individual nucleotide bases which pair with each other. It is not necessary for every nucleotide base in two sequences to pair with each other for sequences to be considered “complementary”. Sequences may be considered complementary, for example, if at least 30%, 40%, 50%, 55%, 60%, 65%, 70%, 75%, 80%, 85%, 90%, 95%, 98%, 99%, or 100% of the nucleotide bases in two sequences pair with each other. In addition, sequences may still be considered “complementary” when the total lengths of the two sequences are significantly different from each other. For example, a primer of 15 nucleotides may be considered “complementary” to a longer polynucleotide containing hundreds of nucleotides if multiple individual nucleotide bases of the primer pair with nucleotide bases in the longer polynucleotide when the primer is aligned anti-parallel to a particular region of the longer polynucleotide. 
         [0027]    As used herein, the term “isolated” as applied to proteins, nucleic acids, or other biomolecules refers to a molecule that has been purified or separated from a component of its naturally-occurring environment (e.g. a protein purified from a cell in which it was naturally produced). An “isolated” molecule may be in contact with other molecules (for example, as part of a reaction mixture). As used herein, “isolated” molecules also include recombinantly-produced proteins or nucleic acids which have an amino acid or nucleotide sequence which occurs naturally. “Isolated” nucleic acids include polypeptide-encoding nucleic acid molecules contained in cells that ordinarily express the polypeptide where, for example, the nucleic acid molecule is at a chromosomal location different from that of natural cells. In some embodiments, “isolated” polypeptides are purified to at least 50%, 60%, 70%, 80%, 90%, 95%, 98%, or 100% homogeneity as evidenced by SDS-PAGE of the polypeptides followed by Coomassie blue, silver, or other protein staining method. 
         [0028]    The terms “polypeptide” and “protein” may be used interchangeably to refer to molecules comprised of amino acids linked by peptide bonds. Individual amino acids may be termed “residues” of a polypeptide or protein. The amino acid sequences of polypeptides disclosed herein may be identified by SEQ ID NO: presented as a string of letters, where the letters have the following meanings: 
         [0000]    
       
         
               
               
               
               
             
           
               
                   
                 TABLE 1B 
               
               
                   
                   
               
               
                   
                 Amino Acid 
                 3-Letter Code 
                 1-Letter Code 
               
               
                   
                   
               
             
             
               
                   
                 Alanine 
                 Ala 
                 A 
               
               
                   
                 Arginine 
                 Arg 
                 R 
               
               
                   
                 Asparagine 
                 Asn 
                 N 
               
               
                   
                 Aspartic acid 
                 Asp 
                 D 
               
               
                   
                 Cysteine 
                 Cys 
                 C 
               
               
                   
                 Glutamic acid 
                 Glu 
                 E 
               
               
                   
                 Glutamine 
                 Gln 
                 Q 
               
               
                   
                 Glycine 
                 Gly 
                 G 
               
               
                   
                 Histidine 
                 His 
                 H 
               
               
                   
                 Isoleucine 
                 Ile 
                 I 
               
               
                   
                 Leucine 
                 Leu 
                 L 
               
               
                   
                 Lysine 
                 Lys 
                 K 
               
               
                   
                 Methionine 
                 Met 
                 M 
               
               
                   
                 Phenylalanine 
                 Phe 
                 F 
               
               
                   
                 Proline 
                 Pro 
                 P 
               
               
                   
                 Serine 
                 Ser 
                 S 
               
               
                   
                 Threonine 
                 Thr 
                 T 
               
               
                   
                 Tryptophan 
                 Trp 
                 W 
               
               
                   
                 Tyrosine 
                 Tyr 
                 Y 
               
               
                   
                 Valine 
                 Val 
                 V 
               
               
                   
                   
               
             
          
         
       
     
         [0029]    A composition may include a buffer. Buffers include, without limitation, phosphate, citrate, ammonium, acetate, carbonate, tris(hydroxymethyl)aminomethane (TRIS), 3-(N-morpholino) propanesulfonic acid (MOPS), 3-morpholino-2-hydroxypropanesulfonic acid (MOPSO), 2-(N-morpholino)ethanesulfonic acid (MES), N-(2-Acetamido)-iminodiacetic acid (ADA), piperazine-N,N′-bis(2-ethanesulfonic acid) (PIPES), N-(2-Acetamido)-2-aminoethanesulfonic acid (ACES), cholamine chloride, N,N-Bis(2-hydroxyethyl)-2-aminoethanesulfonic acid (BES), 2-[[1,3-dihydroxy-2-(hydroxymethyl)propan-2-yl]amino]ethanesulfonic acid (TES), 4-(2-hydroxycthyl)-1-piperazine ethanesulfonic acid (HEPES), acetamidoglycine, tricine (N-(2-Hydroxy-1,1-bis(hydroxymethyl)ethyl)glycine), glycinamide, and bicine (2-(Bis(2-hydroxyethyl)amino)acetic acid) buffers. Buffers include other organic acid buffers in addition to the phosphate, citrate, ammonium, acetate, and carbonate buffers explicitly mentioned herein. 
         [0030]    An article of manufacture may comprise a container; and a composition contained within the container, wherein the composition comprises a thermostable ligase. An article of manufacture may comprise a container; and a composition contained within the container, wherein the composition comprises a blunt-end ligase. An article of manufacture may comprise a container; and a composition contained within the container, wherein the composition comprises a thermostable blunt-end ligase. The containers may be formed from a variety of materials such as glass or plastic, and may have a sterile access port (for example the container may be an intravenous solution bag or a vial having a stopper pierceable by a hypodermic injection needle). The article of manufacture may further comprise a label or package insert on or associated with the container indicating that the composition may be used to ligate blunt-ended DNA. 
         [0031]    Embodiments of methods and compositions provided herein may be described with reference to  FIG. 1 . 
         [0032]    The novel thermostable blunt-end DNA ligases disclosed herein are comprised of the fusion of a DNA-binding protein with a thermostable ligase. Such fusion proteins as disclosed herein may further comprise other components, such as, for example, peptide linkers, N-terminal or C-terminal additions, tag peptides, and other amino acid sequences, including D-amino acids and peptide mimetics. In addition, fusion protein as disclosed herein may further comprise other components, such as, for example, sugars (e.g., the fusion proteins may be glycosylated), polymers such as polyethylene glycol (e.g., the fusion proteins may be PEGylated), organic moieties other than amino acids, and other additions linked to the fusion proteins. The DNA-binding protein portion increases the affinity of the ligase to DNA substrates, resulting in enhanced ligation in the challenging conditions of high temperature, blunt-end ligation. Many combinations of binding proteins and ligases are possible. For example, the binding protein may be N-terminal or C-terminal relative to the ligase. 
         [0033]    It will be understood that multiple combinations of DNA ligases and DBA-binding proteins may be joined to produce the fusion proteins disclosed herein. For example, the following 16 combinations have been constructed: 
         [0000]    
       
         
               
             
           
               
                 TABLE 2 
               
               
                   
               
             
             
               
                 Fusion Proteins Constructed 
               
               
                 
                   
                             
                     
                         
                         
                     
                   
                 
               
               
                   
               
               
                 * Light gray refers to the DNA-binding protein. Descriptions of CTF and p50 are 
               
               
                 given below. 
               
               
                 * Dark gray refers to the ligase. 
               
               
                 T4 = T4 phage, 
               
               
                 Tth = Thermus thermophilus, 
               
               
                 Pfu = Pyrococcus furiosus, 
               
               
                 Pfu C23 = Pfu with a C-terminal 23-amino acid deletion. 
               
             
          
         
       
     
         [0034]    Additional variations are possible, including: (1) different DNA-binding proteins, including thermostable proteins; (2) other DNA ligases, including T4 DNA ligase variants engineered to be thermostable; (3) multiple DNA-binding proteins per ligase, or multiple ligases per DNA-binding protein; (4) transient, non-covalent linkages between the ligase and DNA-binding protein, rather than a protein fusion, to better enable multiple ligation events per ligase molecule; (5) dimers or higher-order multimers of fusion proteins to increase the local concentration of ligase once DNA substrates are bound; (6) different degrees of affinity between the DNA substrate and the DNA-binding protein—for example, using the natural target sequence of p50, GGGAATTCCC (SEQ ID NO: 1), in the DNA target, to enable low picomolar affinity; (7) other nucleic acid modifying enzymes instead of DNA ligase to perform other molecular reactions. 
         [0035]    An exemplary method for producing the fusion proteins disclosed herein may be as follows. Fusion proteins may be made by inducing  E. coli  to express DNA constructs made by standard recombinant DNA methods, followed by standard chromatographic protein purification methods. An affinity tag such as polyhistidine may be employed to assist in the protein purification. In the case of thermostable proteins, purification may be assisted by employing heat to denature most  E. coli  host proteins. The purified fusion protein may be used in the same way that a standard, non-heat-stable ligase would be used in the application/reaction of choice, for example in a scheme that depends upon ligation of DNA substrates to make a template for amplification. 
         [0036]    The methods disclosed herein can be readily incorporated into and used in device for processing a sample, or a system for processing a sample, which may be an automated assay device, or may be an automated assay system. Such assay devices and assay systems may comprise devices and systems disclosed, for example, in U.S. Pat. No. 8,088,593; U.S. Pat. No. 8,380,541; U.S. patent application Ser. No. 13/769,798, filed Feb. 18, 2013; U.S. patent application Ser. No. 13/769,779, filed Feb. 18, 2013; U.S. patent application Ser. No. 13/244,947 filed Sep. 26, 2011; PCT/US2012/57155, filed Sep. 25, 2012; U.S. application Ser. No. 13/244,946, filed Sep. 26, 2011; U.S. patent application Ser. No. 13/244,949, filed Sep. 26, 2011; and U.S. Application Ser. No. 61/673,245, filed Sep. 26, 2011, the disclosures of which patents and patent applications are all hereby incorporated by reference in their entireties. 
       EXAMPLES 
       [0037]    Examples of individual protein components suitable for use in providing fusion proteins as disclosed herein include (the amino acid sequences are provided using the one-letter code for amino acids): 
         [0000]    
       
         
               
               
               
             
           
               
                 TABLE 3  
               
               
                   
               
               
                   
                   
                 SEQ ID 
               
               
                 Protein Name 
                 Protein Sequence 
                 NO: 
               
               
                   
               
             
             
               
                 p50 DNA-binding 
                 adgpylqileqpkqrgfrfryvcegpshgglpgasseknk 
                 2 
               
               
                 protein (fragment from 
                 ksypqvkicnyvgpakvivqlvtngknihlhahslvgkhc 
                   
               
               
                 the human NF-kappa-B 
                 edgictvtagpkdmvvgfanlgilhvtkkkvfetlearmt 
                   
               
               
                 protein, accession 
                 eacirgynpgllvhpdlaylqaegggdrqlgdrekelirq 
                   
               
               
                 number NP_003989, 
                 aalqqtkemdlsvvrlmftaflpdstgsftrrlepvvsda 
                   
               
               
                 amino acids 40-366) 
                 iydskapnasnlkivrmdrtagcvtggeeiyllcdkvqkd 
                   
               
               
                   
                 diqirfyeeeenggvwegfgdfsptdvhrgfaivfktpky 
                   
               
               
                   
                 kdinitkpasvfvqlrrksdletsepkpflyypeikdkee 
                   
               
               
                   
                 vqrkrqk 
                   
               
               
                   
               
               
                 CTF DNA-binding 
                 sptsymspslpaldwqlpshsgpyelrievqpkshhrahy 
                 3 
               
               
                 protein (a hybrid from 
                 etegsrgavkasagghpivqlhgylenepltlqlfigtad 
                   
               
               
                 the murine NFATc1 
                 drllrphafyqvhritgktvsttsheiilsntkvleipll 
                   
               
               
                 protein, accession 
                 pennmraiidcagilklrnsdielrkgetdigrkntrvrl 
                   
               
               
                 number NP_058071, 
                 vfrvhipqpngrtlslqasnlkivrmdrtagcvtggeeiy 
                   
               
               
                 amino acids 390-506; 
                 llcdkvqkddiqirfyeeeenggvwegfgdfsptdvhrqf 
                   
               
               
                 followed by an alanine 
                 aivfktpkykdinitkpasvfvqlrrksdletsepkpfly 
                   
               
               
                 spacer; followed by a 
                 ypeikdkeevqrkrqk 
                   
               
               
                 fragment from the 
                   
                   
               
               
                 human NF-kappa-B 
                   
                   
               
               
                 protein, accession 
                   
                   
               
               
                 number NP_003989, 
                   
                   
               
               
                 amino acids 249-366) 
                   
                   
               
               
                   
               
               
                 T4 DNA ligase 
                 Milkilneiasigstkqkqaileknkdnellkrvyrltys 
                 4 
               
               
                 (accession number 
                 rglqyyikkwpkpgiatqsfgmltltdmldfieftlatrk 
                   
               
               
                 NP_049813) 
                 ltgnaaieeltgyitdgkkddvevlrrvmmrdlecgasvs 
                   
               
               
                   
                 iankvwpglipeqpqmlassydekginknikfpafaqlka 
                   
               
               
                   
                 dgarcfaevrgdelddvrllsragneylgldllkeelikm 
                   
               
               
                   
                 taearqihpegvlidgelvyheqvkkepegldflfdaype 
                   
               
               
                   
                 nskakefaevaesrtasngiankslkgtisekeaqcmkfq 
                   
               
               
                   
                 vwdyvplveiyslpafrlkydvrfskleqmtsgydkvili 
                   
               
               
                   
                 enqvvnnldeakviykkyidqglegiilknidglwenars 
                   
               
               
                   
                 knlykfkevidvdlkivgiyphrkdptkaggfilesecgk 
                   
               
               
                   
                 ikvnagsglkdkagvksheldrtrimenqnyyigkilece 
                   
               
               
                   
                 cngwlksdgrtdyvklflpiairlredktkantfedvfgd 
                   
               
               
                   
                 fhevtgl 
                   
               
               
                   
               
               
                 Tth DNA ligase 
                 mtleearkrvnelrdliryhnyryyvladpeisdaeydrl 
                 5 
               
               
                 ( Thermus Thermophilus   
                 lrelkeleerfpelkspdsptlqvgarpleatfrpvrhpt 
                   
               
               
                 strain HB8, accession 
                 rmysldnafnldelkafeerieralgrkgpfaytvehkvd 
                   
               
               
                 YP_144363.1) 
                 glsvnlyyeegvlvygatrgdgevgeevtqnlltiptipr 
                   
               
               
                   
                 rlkgvperlevrgevympieaflrlneeleergerifknp 
                   
               
               
                   
                 rnaaagslrqkdpritakrglratfyalglgleeveregv 
                   
               
               
                   
                 atqfallhwlkekgfpvehgyaravgaegveavyqdwlkk 
                   
               
               
                   
                 rralpfeadgvvvkldelalwrelgytaraprfaiaykfp 
                   
               
               
                   
                 aeeketrlldvvfqvgrtgrvtpvgilepvflegsevsrv 
                   
               
               
                   
                 tlhnesyieeldirigdwvlvhkaggvipevlrvlkerrt 
                   
               
               
                   
                 geerpirwpetcpecghrllkegkvhrcpnplcpakrfea 
                   
               
               
                   
                 irhfasrkamdiqglgeklierllekglvkdvadlyrlrk 
                   
               
               
                   
                 edlvglermgeksaqnllrqieeskkrglerllyalglpg 
                   
               
               
                   
                 vgevlarnlaarfgnmdrlleasleelleveevgeltara 
                   
               
               
                   
                 iletlkdpafrdlvrrlkeagvemeakekggealkgltfv 
                   
               
               
                   
                 itgelsrpreevkallrrlgakvtdsysrktsylvvgenp 
                   
               
               
                   
                 gsklekaralgvptlteeelyrlleartgkkaeelv 
                   
               
               
                   
               
               
                 Pfu DNA ligase 
                 mrylelaqlyqklekttmkliktrlvadflkkvpddhlef 
                 6 
               
               
                 ( Pyrococcus furiosus   
                 ipylilgdvfpewderelgvgekllikavamatgidanei 
                   
               
               
                 strain ST04, accession 
                 ensvkdtgdlgesialavkkrkqksffsqpltikrvyqtl 
                   
               
               
                 YP_006355162) 
                 vkvaettgegsqekkmkylanlfmdaepieakyiartvlg 
                   
               
               
                   
                 tmrtgvaegllrdaialafhvkvelveraymltsdfgfva 
                   
               
               
                   
                 kvaklegneglakvqvgigkpikpmlaqqaanikeallem 
                   
               
               
                   
                 ggeaefeikydgarvqvhkdgdkiivysrrlenvtraipe 
                   
               
               
                   
                 ivealkqsvkpnkaivegelvaigedgrplpfqyvlrrfr 
                   
               
               
                   
                 rkhniqemmkkiplelnlfdvlyvdgesmidvkfidrrkk 
                   
               
               
                   
                 leeiiepngkikvaenlitkkveeaeafykkalemghegl 
                   
               
               
                   
                 makrldatyepgnrgkkwlkikptmenldlviigaewgeg 
                   
               
               
                   
                 rrahllgsfilgaydpetgeflevgkvgsgftdedlveft 
                   
               
               
                   
                 kmlkpliikeegkrvwiepkivievtyqeiqkspkyksgf 
                   
               
               
                   
                 alrfpryvalrddkgpedadtieriaglyelgermkgkv 
                   
               
               
                   
               
               
                 Pfu C23 DNA ligase 
                 mrylelaglyqklekttmkliktrlvadflkkvpddhlef 
                 7 
               
               
                 (C-terminal 23-amino 
                 ipylilgdvfpewderelgvgekllikavamatgidanei 
                   
               
               
                 acid deletion of Pfu) 
                 ensvkdtgdlgesialavkkrkqksffsgpltikrvyqtl 
                   
               
               
                   
                 vkvaettgegsqekkmkylanlfmdaepieakyiartvlg 
                   
               
               
                   
                 tmrtgvaegllrdaialafhvkvelveraymltsdfgfva 
                   
               
               
                   
                 kvaklegneglakvqvqigkpikpmlaqqaanikeallem 
                   
               
               
                   
                 ggeaefeikydgarvqvhkdgdkiivysrrlenvtraipe 
                   
               
               
                   
                 ivealkqsvkpnkaivegelvaigedgrplpfqyvlrrfr 
                   
               
               
                   
                 rkhniqemmkkiplelnlfdvlyvdgesmidvkfidrrkk 
                   
               
               
                   
                 leeiiepngkikvaenlitkkveeaeafykkalemghegl 
                   
               
               
                   
                 makrldatyepgnrgkkwlkikptmenldlviigaewgeg 
                   
               
               
                   
                 rrahllgsfilgaydpetgeflevgkvgsgftdedlveft 
                   
               
               
                   
                 kmlkpliikeegkrvwiepkivievtyqeiqkspkyksgf 
                   
               
               
                   
                 alrfpryvalrddkgp 
               
               
                   
               
             
          
         
       
     
         [0038]    The following provides an example of a fusion protein providing thermostable blunt-end DNA ligase activity:
       p50-Tth ligase fusion, Theranos construct EE0217, e.g. protein prep RDP0078, consists of a His10-containing leader (“His10” disclosed as SEQ ID NO: 11), p50 (shown in italics), a flexible glycine-rich sequence, and the Tth DNA ligase (shown underlined).       
 
         [0000]    
       
         
               
               
             
           
               
                 mghhhhhhhhhhssghiegras adgpylqileqpkqrgfrfryvcegpshgglpgasseknkksypqvkicnyvgpakvivqlvtng   
                   
               
               
                   
               
               
                 
                   knihlhahslvgkhcedgictvtagpkdmvvgfanlgilhvtkkkvfetlearmteacirgynpgllvhpdlaylqaegggdrqlgdrek 
                 
               
               
                   
               
               
                 
                   elirqaalqqtkemdlsvvrlmftaflpdstgsftrrlepvvsdaiydskapnasnlkivrmdrtagcvtggeeiyllcdkvqkddiqirfye 
                 
               
               
                   
               
               
                   eeenggvwegfgdfsptdvhrqfaivfktpkykdinitkpasvfvqlrrksdletsepkpflyypeikdkeevqrkrqk gssgtsgggsgg 
               
               
                   
               
               
                 g mtleearkrvnelrdliryhnyryyvladpeisdaeydrllrelkeleerfpelkspdsptlqvgarpleatfrpvrhptrmysldnafnld   
               
               
                   
               
               
                 
                   elkafeerieralgrkgpfaytvehkydglsvnlyyeegvlvvgatrgdgevgeevtqnlltiptiprrlkgvperlevrgevympieaflrl 
                 
               
               
                   
               
               
                 
                   neeleergerifknprnaaagslrqkdpritakrglratfyalglgleeveregvatqfallhwlIkekgfpvehgyaravgaegveavyqd 
                 
               
               
                   
               
               
                 
                   wlkkrralpfeadgvvvkldelalwrelgytaraprfaiaykfpaeeketrlldvvfqvgrtgrvtpvgilepvflegsevsrvtlhnesyiee 
                 
               
               
                   
               
               
                 
                   ldirigdwvlvhkaggvipevlrvlkerrtgeerpirwpetcpecghrllkegkvhrcpnplcpakrfeairhfasrkamdiqglgeklierl 
                 
               
               
                   
               
               
                 
                   lekglvkdvadlyrlrkedlyglermgeksaqnllrqieeskkrglerllyalglpgvgevlarnlaarfgnmdrlleasleelleveevgelta 
                 
               
               
                   
               
               
                 
                   railetlkdpafrdlyrrlkeagvemeakekggealkgltfvitgelsrpreevkallrrlgakvtdsvsrktsylvvgenpgsklekaralgvp 
                 
               
               
                   
               
               
                 
                   tlteeelyrlleartgkkaeelv 
                 
               
             
          
         
       
     
         [0040]    An example of a blunt-ended ligation reaction performed at 75° C. is provided below. The DNA substrate was a 49-bp duplex DNA made by annealing oligonucleotides EE0139 and EE0140 (sequences below). This blunt-ended duplex was capable of making concatamers upon multiple ligation events. The results of the reaction products were separated by size on an agarose gel, as shown below, demonstrate that: (1) Tth ligase alone (lanes 2-3), although it is known to be capable of sealing DNA nicks at 75° C., performed very little or no blunt ligation in these conditions; (2) T4 DNA ligase with an N-terminal p50 fusion (lanes 4-5) also performed very little blunt ligation in these conditions, although there was some evidence of more ligation than for Tth alone. This observation was believed to be surprising given the temperature sensitivity of T4 ligase alone; (3) Tth DNA ligase with an N-terminal p50 fusion (lanes 6-7) demonstrated a much higher level of blunt-end ligation at 75° C. Thus, the results shown in  FIG. 1  below demonstrate that fusion proteins as disclosed herein provide improved blunt-end ligation activity at high temperature.