Patent Publication Number: US-10760063-B2

Title: Modified template-independent enzymes for polydeoxynucleotide synthesis

Description:
RELATED APPLICATIONS 
     This Application is a continuation-in-part of U.S. Non-provisional application Ser. No. 14/918,212, filed Oct. 20, 2015, which claims priority to U.S. Provisional Application No. 62/065,976, filed Oct. 20, 2014, which is incorporated by reference in its entirety. 
    
    
     FIELD OF THE INVENTION 
     The invention relates to modified enzymes for de novo synthesis of polynucleotides with a desired sequence, and without the use of a template. As such, the invention provides the capability to make libraries of polynucleotides of varying sequence and varying length for research, genetic engineering, and gene therapy. 
     BACKGROUND 
     Most de novo nucleic acid sequences are synthesized using solid phase phosphoramidite-techniques developed more than 30 years ago. The technique involves the sequential de-protection and synthesis of sequences built from phosphoramidite reagents corresponding to natural (or non-natural) nucleic acid bases. Phosphoramidite nucleic acid synthesis is length-limited, however, in that nucleic acids greater than 200 base pairs (bp) in length experience high rates of breakage and side reactions. Additionally, phosphoramidite synthesis produces toxic by-products, and the disposal of this waste limits the availability of nucleic acid synthesizers, and increases the costs of contract oligo production. (It is estimated that the annual demand for oligonucleotide synthesis is responsible for greater than 300,000 gallons of hazardous chemical waste, including acetonitrile, trichloroacetic acid, toluene, tetrahydrofuran, and pyridine. See LeProust et al.,  Nucleic Acids Res ., vol. 38(8), p. 2522-2540, (2010), incorporated by reference herein in its entirety). Thus, there is a need for more efficient and cost-effective methods for oligonucleotide synthesis. 
     SUMMARY 
     The invention discloses modified terminal deoxynucleotidyl transferase (TdT) enzymes that can be used for de novo sequencing of oligonucleotides in the absence of a template. Methods for creating a template-independent polymerase through a combination of computational guidance and saturation mutagenesis, with a subsequent screen to identify functional mutants, are also disclosed. In some embodiments, the modified TdTs will include a mutation in the GGFRR or TGSR motifs, which interact with the nucleotide during synthesis. 
     Using the resulting enzymes, it will possible to synthesize de novo polynucleotides faster and cheaper. As such, the invention dramatically reduces the overall cost of synthesizing custom nucleic acids. In particular, the methods can be used to create template-independent transferases that can synthesize custom oligos in a stepwise fashion using modified 3′ hydroxyl-blocked nucleotides. Because of the terminating group, synthesis pauses with the addition of each new base, whereupon the terminating group is cleaved, leaving a polynucleotide that is essentially identical to a naturally occurring nucleotide (i.e., is recognized by the enzyme as a substrate for further nucleotide incorporation). 
     The methods and enzymes of the invention represent an important step forward in synthetic biology because the enzymes will allow for aqueous phase, template-independent oligonucleotide synthesis. Such methods represent an improvement over the prior art in that they will greatly reduce the chemical waste produced during oligonucleotide synthesis while allowing for the production of longer polynucleotides. Furthermore, because the methods replace a chemical process with a biological one, costs will be reduced, and the complexity of automated synthetic systems will also be reduced. In an embodiment, a simple five-reagent delivery system can be used to build oligonucleotides in a stepwise fashion and will enable recycling of unused reagents. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows an agarose gel of a solution phase polymerization reaction composed of terminal deoxynucleotidyl transferase (TdT), deoxyadenosine triphosphate (dATP) and fluorescent strand initiator 5′-Cy5-dA10 at different time points from Tjong et al. “Amplified on-chip fluorescence detection of DNA hybridization by surface-initiated enzymatic polymerization,”  Anal. Chem.,  2011; 83:5153-5159 (2011). 
         FIG. 2  illustrates an exemplary modified terminal deoxynucleotidyl transferase (TdT) mediated polynucleotide synthesis cycle using a support bound initiator and 3′-O-blocked nucleotide triphosphate including (A) incorporation of a nucleotide analog comprising a cleavable 3′-O-blocking group (indicated by R), and (B) removal of the 3′-O-blocking group thus enabling the next 3′-O-blocked nucleotide analog to be incorporated, wherein N=A, G, C, or T. 
         FIG. 3  shows the polyacrylamide gel analysis of a solution phase reaction time course of commercially-available TDT and a nucleic acid initiator with 3′-O-azidomethyl-dCTP or 3′-O-azidomethyl-dATP. Lane 1—100 bp ladder size standard, Lane 2—oligonucleotide standard, Lane 3—3′-O-azidomethyl-dCTP+TdT 15′ reaction time, Lane 4—1 hour, Lane 5—2 hours, Lane 6—4 hours, Lane 7—24 hours, Lane 8—3′-O-azidomethyl-dATP+TdT 15′ reaction time, Lane 9—1 hour, Lane 10—2 hours, Lane 10—4 hours, Lane 11—24 hours, Lane 12—dATP+TdT 15′ reaction time, Lane 13—1 hour, Lane 14—4 hours, Lane 15—24 hours. 
         FIG. 4  shows a computer-generated image of the active site of TdT using the PDB crystal structure 4129, showing the computationally docked catalytically productive position a 3′-O-dATP analog (blue, red, orange frame), each complexed to the two active-site metal ions (large greenspheres). Shown are the residues, that are in close proximity to the incoming dNTP and the targets of mutagenesis and screening. 
         FIG. 5  shows a table of TdT variants that were selected for increased incorporation of selected 3′-O-blocked dNTP analogs as described herein. 
         FIG. 6  shows exemplary 3′-O-azidomethyl deoxynucleotides that can be used to synthesize custom DNA oligomers using modified TdTs, as described herein. 
         FIG. 7  shows a synthetic scheme for producing 3′-O-azidomethyl deoxyadenosine triphosphate (3′-O-azidomethyl-dATP). 
         FIG. 8  shows a synthetic scheme for producing 3′-O-azidomethyl deoxythymidine triphosphate (3′-O-azidomethyl-dTTP). 
         FIG. 9  shows a synthetic scheme for producing 3′-O-azidomethyl deoxycytidine triphosphate (3′-O-azidomethyl-dCTP). 
         FIG. 10  shows a synthetic scheme for producing 3′-O-azidomethyl deoxyguanosine triphosphate (3′-O-azidomethyl-dGTP). 
         FIG. 11  shows a synthetic scheme for producing 3′-O-methoxymethyl deoxythymidine triphosphate (3′-O-MOM-dTTP). 
         FIG. 12  shows a synthetic scheme for producing 3′-O-thiomethyl deoxycytidine triphosphate (3′-O-MTM-dCTP). 
     
    
    
     DESCRIPTION OF THE INVENTION 
     The invention facilitates the synthesis of polynucleotides, such as DNA, by providing modified enzymes that can be used with nucleic acid analogs. Using the disclosed methods, a modified template-independent terminal deoxynucleotidyl transferase (TdT) is obtained that allows the enzymatically mediated synthesis of de novo oligodeoxynucleotides, thereby enabling their use in routine assembly for gene synthesis. The enzymes of the invention lend themselves to aqueous-based, enzyme-mediated methods of synthesizing polynucleotides of a predetermined sequence on a solid support. 
     The modified enzymes of the invention will allow 3′-O-blocked dNTP analogs to be used in a step-by-step method to extend an initiating nucleic acid into a user defined sequence (see  FIG. 2 ). Furthermore, after each nucleotide extension step, the reactants can be recovered and recycled from the solid support back to the original reagent reservoir. Once that step is complete, the 3′-O-blocking group will be removed, allowing the cycle to start anew. At the conclusion of n cycles of extension-recover-deblock-wash, the full length, single strand polydeoxynucleotide will be cleaved from the solid support and isolated for subsequent use. A variety of 3′-O-blocked deoxynucleotides, may be used, but the choice of specific 3′-O-blocking groups is dictated by: 1) the smallest possible bulk to maximize substrate utilization by TdT and 2) removal of the blocking group with the mildest and preferably aqueous conditions in the shortest period of time. 
     Cost savings by this approach will be achieved by exploiting the higher yield of final oligonucleotide product at a lower starting scale than currently being used as the existing industry standard (i.e., less than 1 nanomole). Future adaptation of this enzymatic approach to array based formats will allow even further and more dramatic reductions in the cost of synthesis of long oligonucleotides achievable by highly parallel synthesis. Furthermore, the enzymatic synthesis process that we propose uses only aqueous based chemistries like buffers and salts, thus greatly reducing the environmental burden of the organic waste generated by the existing phosphoramidite method. 
     The methods of the invention may be used to modify terminal deoxynucleotidyl transferases (TdT), however other enzymes could be modified with similar methods. TdT is likely to be a successful starting enzyme because it is capable of 3′-extension activity using single strand initiating primers in a template-independent polymerization. However, prior to the invention described herein, there have been no reports of 3′-O-blocked nucleotides being incorporated into single-stranded oligonucleotide by an enzyme in the absence of a template. In fact, as Chang and Bollum reported, substitution of the 3′-hydroxyl group results in complete inactivity of available transferase enzymes. See Chang and Bollum, “Molecular Biology of Terminal Transferase,  CRC Critical Reviews in Biochemistry , vol. 21 (1), p. 27-52 (1986), incorporated herein by reference in its entirety. Nonetheless, when TdT is used with natural dNTPs (i.e., not 3′-O-blocked), and without a template, oligonucleotide extension continues without stopping. Such uncontrolled incorporation is evidenced by the time-dependent gel electrophoresis images shown in  FIG. 1 .  FIG. 1  shows an agarose gel of a solution phase polymerization reaction composed of terminal deoxynucleotidyl transferase (TdT), deoxyadenosine triphosphate (dATP) and fluorescent strand initiator 5′-Cy5-dA10 at different time points. (Adapted with permission from Tjong et al. “Amplified on-chip fluorescence detection of DNA hybridization by surface-initiated enzymatic polymerization,”  Anal. Chem.,  2011; 83:5153-5159 (2011), incorporated by reference herein in its entirety.) Additionally, TdT can extend primers in a near quantitative manner resulting in the addition of thousands of nucleotides, while TdT is likely to accept a wide variety of modified and substituted dNTPs as efficient substrates. Furthermore, a substantial library of mechanistic and structural information regarding TdT already exists. See Delarue et al.,  EMBO J.  2002; 21(3):427-39; Gouge et al.,  J Mol Biol.  2013 Nov. 15; 425(22):4334-52 and Romain et al.,  Nucleic Acids Res.  2009; 37(14):4642-56, both of which are incorporated by reference in their entireties. 
     It is known that TdT can use substrates having modifications and/or substitutions at the deoxyribose sugar ring as well as the purine/pyrimidine nucleobases. For example, TdT accepts bulky modifications at the C5 of pyrimidines and the C7 of purines. See Sorensen et al., “Enzymatic Ligation of Large Biomolecules to DNA,”  ACS Nano  2013, 7(9):8098-104; Figeys et al.,  Anal. Chem.  1994, 66(23):4382-3; Li et al.,  Cytometry,  1995, 20(2):172-80, all of which are incorporated by reference in their entireties. In some instances, TdT can even accept non-nucleotide triphosphates. See Barone et al.,  Nucleotides and Nucleic Acids  2001, 20(4-7):1141-5, and Alexandrova et al.,  Bioconjug Chem.,  2007, 18(3):886-93, both of which are incorporated by reference in their entireties. However, there is little evidence in the prior art that TdT can accept 3′-O-blocked nucleotides. See, for example, Knapp et al.,  Chem. Eur. J.,  2011, 17:2903, incorporated herein by reference in its entirety. While the lack of activity of TdT was not a focus of Knapp et al., the authors reported that they tested their 3′-OH modified analog with TdT, and saw no incorporation of this relatively small 3′-OH modification into an oligonucleotide. 
     Native TdT is a very efficient enzyme. It has been demonstrated that TdT can polymerize extremely long homopolydeoxynucleotides of 1000 to 10,000 nucleotides in length (see Hoard et al.,  J of Biol Chem,  1969 244(19):5363-73; Bollum,  The Enzymes , Volume 10, New York: Academic Press; 1974. p. 141-71; Tjong et al.,  Anal Chem,  2011, 83:5153-59, all of which are incorporated by reference in their entireties). Random sequence oligomers consisting of all four nucleotides have also been polymerized by TdT, however there are no reports of ordered polynucleotides being synthesized in the absence of a template. See Damiani, et al.,  Nucleic Acids Res,  1982, 10(20):6401-10, incorporated by reference herein in its entirety. Support-bound synthesis of polynucleotides by TdT is additionally supported by reports of homopolymer synthesis of 150 bps initiators covalently attached to self-assembled monolayers on gold surfaces. See Chow et al.,  J Am Chem Soc  2005; 127:14122-3, and Chow and Chilikoti,  Langmuir  2007, 23:11712-7, both of which are incorporated by reference in their entireties. These authors also observed preference by TdT of dATP&gt;dTTP&gt;&gt;dGTP≈dCTP for incorporation of homopolymers. In a more recent report, Tjong et al. demonstrated the TdT mediated synthesis of long (&gt;1 Kb) homopolymer ssDNA from initiator primers immobilized on glass surfaces. 
     The distributive behavior of TdT is reinforced by  FIG. 3 , which shows a time course of a solution phase synthesis of 1-1.5 kb homopolymers. After each addition of an unmodified (natural) dNTP, the enzyme dissociates, thus allowing the random extension of any strand in the population. The distribution of product lengths in such a system should follow a Poisson distribution, as reported by Bollum and co-workers in 1974. If TdT were used with a terminating nucleotide species, i.e., one with the 3′-O-position blocked, the reaction should proceed to completion, resulting not in a distribution of product lengths, but essentially a pure product of a single nucleotide addition. 
     Nonetheless, as described above, nucleotide synthesis with 3′-O-blocked dNTPs does not proceed with commercially-available TdT proteins. This fact is reinforced by  FIG. 3 , which shows a gel shift assay used to monitor the solution phase incorporation kinetics of 3′-O-azidomethyl dATP and 3′-O-azidomethyl dCTP using a commercially-available, recombinant TdT. The data in  FIG. 3  clearly show that neither 3′-O-modified dNTP analog is a substrate for TdT, i.e., there is no polynucleotide extension when compared to reactions containing dATP as a positive control (lanes 12 thru 15).  FIG. 3 , thus, adds further evidence that commercially-available TdTs are not able to synthesize oligomers by incorporating dNTPs with modified 3′-OHs. 
     With suitable modifications, a variety of different 3′-O-blocked dNTP analogs will be suitable for the controlled addition of nucleotides by TdT. Modified 3′-O-blocked dNTP analogs include, but are not limited to, the 3′-O-allyl, 3′-O-azidomethyl, 3′-O—NH 2 , and 3′-OCH 2 CN blocking groups. Overall, the choice of the 3′-O-blocking group will be dictated by: 1) the smallest possible bulk to maximize substrate utilization by TdT, which is likely to affect kinetic uptake, and 2) the blocking group with the mildest removal conditions, preferably aqueous, and in the shortest period of time. 3′-O-blocking groups that are the suitable for use with this invention are described in WO 2003/048387; WO 2004/018497; WO 1996/023807; WO 2008/037568; Hutter D, et al.  Nucleosides Nucleotides Nucleic Acids,  2010, 29(11): 879-95; and Knapp et al.,  Chem. Eur. J.,  2011, 17:2903, all of which are incorporated by reference in their entireties. 
     A computational model of the active site of murine TdT was created to understand the structural basis for the lack of utilization of 3′-O-blocked dNTPs by TdT. Additionally, the computer model made it possible to “fit” various modified dNTPs into the active site.  FIG. 4  shows the docking of a -dATP (shown in blue, red, magenta, orange) with murine TdT (see SEQ ID NO. 9, below) using the PDB crystal structure 4129 and AutoDock 4.2 (Molecular Graphics Laboratory, Scripps Research Institute, La Jolla, Calif.). 
     The phosphate portions of the dATPs (orange) are in complex with the catalytic metal ions (green) while the alpha phosphate is positioned to be attacked by the 3′-OH of the bound oligonucleotide. The model shown in  FIG. 4  indicates the choice of amino acid residues likely to interfere with the formation of a catalytically productive complex when a 3′-O-blocked dNTP is present. Other residues that may interact with the closest residues, like Glu 180 or Met 192, are also targets of modification. 
     AutoDock&#39;s predicted binding mode suggests that modification to the 3′-OH will change the electrostatic interactions between two residues, Arg336 and Arg454. Although Arg336 is near the reaction center in the active site, Arg 336 is highly conserved, and early studies found that replacement of Arg336 with Gly or Ala reduced dNTP activity by 10-fold (Yang B et al. J. Mol. Biol. 1994; 269(16):11859-68). Accordingly, one motif for modification is the GGFRR motif including Arg 336 in the above structural model. 
     Additionally, it is thought that Gly452 and Ser453 exist in a cis-peptide bond conformation (see Delarue et al.,  EMBO J.,  2002; 21(3):427-39, incorporated herein by reference in its entirety) and that the guanidinium group of Arg336 assists in the stabilization of this conformation. The stability provided by Arg336 may help explain why substitutions at this position have a negative impact on the reactivity of modified TdT proteins. In some instances, the instability created by modifying position 336 may be overcome by using proline residues to stabilize cis-peptide bond conformation. However, if Arg336 is substituted, e.g., with alanine or glycine, the entire TGSR motif (positions 451, 452, 435, 454) may also have to be modified to compensate for this change. For example, the TGSR motif may be modified to TPSR or TGPR. Accordingly, the TGSR motif, including Gly452 in the above structural model was targeted for modification. 
     On the other hand, sequence analysis of the TdT family demonstrates a wide range of amino acids that can be accommodated at position 454. This analysis suggests structural flexibility at position 454, and surrounding residues. In another embodiment, substitutions at Arg454 to accommodate the steric bulk of a 3′-O-blocking group may require additional modifications to the α14 region to compensate for substitutions of glycine or alanine at Arg454. In other embodiments, substitutions to other residues in the all region may be required to compensate for substitution to Arg336 either instead of, or in addition to, modification of the TGSR motif. 
     While modification to Arg336 and Arg454 may change the binding interactions of 3′-O-modified dNTPs, it may also be necessary to explore substitutions that would result in improved steric interactions of 3′-O-modified dNTPs with TdT. In order to test computationally predicted enzyme variants that show increased substrate utilization of 3′-O-blocked dNTPs, synthetic genes specifying specific amino acid substitutions were generated in appropriate plasmid vectors and introduced into cells. After expression and isolation, protein variants were screened for activity by a polymerase incorporation assay with selected 3′-O-blocked dNTP analogs.  FIG. 5  shows the results of the screening of various synthetically generated murine TdT variants. In some embodiments, single amino acid changes are important while in other, combinations of one &amp; two amino acids also produce increased incorporation of 3′-O-blocked dNTPs. Interactions with residues such as Gly332, Gly333, Gly452, Thr451, Trp450, Ser453, and Q455 of murine TdT are important. Each of these residues is within 0.6 nm of the 3′-OH of a typical dNTP. These residues are also potential targets for substitution to allow the extra steric bulk of a 3′-blocking group like 3′-O-azidomethyl or 3′-O-aminoxy. Residues that are within 1.2 nm of the 3′-OH such as Glu457, Ala510, Asp509, Arg508, Lys199, Ser196, Met192, Glu180 or Leu161 may also potentially interfere with the substrate utilization of a 3′-O-blocked dNTP and are thus targets for substitution in addition to or in combination with Arg336 and Arg454. Additional residues of interest include Arg461 and Asn474. In addition to amino acid substitutions at positions 500-510 it may be necessary to delete residues to remove interference with a 3′-O-blocking group. Since these amino acids are located near the C-terminus of the protein, and exist in a relatively unstructured region, they may be deleted singly or altogether, either instead of or in combination with the modifications described above. 
     Modified TdT&#39;s of the invention include those described in  FIG. 5 . Modified TdT&#39;s may include one or more of a modification to Glu180 including E180L, E180R, E180D, or E180K. Contemplated modifications to Met192 include, for example, M192E, M192W, M192K, or M192R. Contemplated modifications to Gln455 include, for example, Q455I. Contemplated modifications to Trp450 include, for example, W450H. Contemplated modifications to ARG454 include, for example, R454I, R454K, R454A, or R454T. Contemplated modifications to Arg461 include, for example, R461V and modifications to Asn474 may include N474R. In various embodiments combinations of two or more modified residues may be used such as, for example, E180D+W450H, E180K+R454A, M192K+E180K, E180K+R454I, E180D+M192E, E180D+M192E+R454T, or E180K+W450H. 
     As shown below, most TdTs include the GGFRR and TGSR motifs. In the following sequences, the GGFRR and TGSR motifs have been bolded and underlined for easy reference. Native calf thymus TdT is a candidate for alteration of the primary structure to achieve a suitable template-independent polymerase. However, a variety of other proteins may be explored to identify a candidate suitable for the use with 3′-O-blocked dNTP analogs, including human and murine TdT. The amino acid sequence corresponding to native calf TdT is listed in Table 1 as SEQ ID NO. 1, while the nucleic acid sequence is listed in Table 2 as SEQ ID NO. 2. In some embodiments, the resulting protein, adapted for sequence-specific de novo polynucleotide synthesis with 3′-O-modified dNTPs and NTPs, will be at least 85% identical, i.e., at least 90% identical, i.e., at least 93% identical, i.e., at least 95% identical, i.e., at least 97% identical, i.e., at least 98% identical, i.e., at least 99% identical, with SEQ ID NO. 1. Furthermore, it may be possible to truncate portions of the amino acid sequence of bovine TdT and still maintain catalytic activity. 
     
       
         
           
               
             
               
                 TABLE 1 
               
               
                   
               
               
                 Amino Acid Sequence of Bovine TdT 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 SEQ ID NO. 1: (520 aa) 
               
               
                   
                 MAQQRQHQRL PMDPLCTASS GPRKKRPRQV GASMASPPHD 
               
               
                   
               
               
                   
                 IKFQNLVLFI LEKKMGTTRR NFLMELARRK GFRVENELSD 
               
               
                   
               
               
                   
                 SVTHIVAENN SGSEVLEWLQ VQNIRASSQL ELLDVSWLIE 
               
               
                   
               
               
                   
                 SMGAGKPVEI TGKHQLVVRT DYSATPNPGF QKTPPLAVKK 
               
               
                   
               
               
                   
                 ISQYACQRKT TLNNYNHIFT DAFEILAENS EFKENEVSYV 
               
               
                   
               
               
                   
                 TFMRAASVLK SLPFTIISMK DTEGIPCLGD KVKCIIEEII 
               
               
                   
               
               
                   
                 EDGESSEVKA VLNDERYQSF KLFTSVFGVG LKTSEKWFRM 
               
               
                   
               
               
                   
                 GFRSLSKIMS DKTLKFTKMQ KAGFLYYEDL VSCVTRAEAE 
               
               
                   
               
               
                   
                 AVGVLVKEAV WAFLPDAFVT MT   GGFRR   GKK IGHDVDFLIT 
               
               
                   
               
               
                   
                 SPGSAEDEEQ LLPKVINLWE KKGLLLYYDL VESTFEKFKL 
               
               
                   
               
               
                   
                 PSRQVDTLDH FQKCFLILKL HHQRVDSSKS NQQEGKTWKA 
               
               
                   
               
               
                   
                 IRVDLVMCPY ENRAFALLGW    TGSR   QFERDI RRYATHERKM 
               
               
                   
               
               
                   
                 MLDNHALYDK TKRVFLKAES EEEIFAHLGL DYIEPWERNA 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 2 
               
               
                   
               
               
                 Nucleic Acid Sequence of Bovine TdT 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 SEQ ID NO. 2: (1923 nt) 
               
               
                   
                 ctcttctgga gataccactt gatggcacag cagaggcagc 
               
               
                   
               
               
                   
                 atcagcgtct tcccatggat ccgctgtgca cagcctcctc 
               
               
                   
               
               
                   
                 aggccctcgg aagaagagac ccaggcaggt gggtgcctca 
               
               
                   
               
               
                   
                 atggcctccc ctcctcatga catcaagttt caaaatttgg 
               
               
                   
               
               
                   
                 tcctcttcat tttggagaag aaaatgggaa ccacccgcag 
               
               
                   
               
               
                   
                 aaacttcctc atggagctgg ctcgaaggaa aggtttcagg 
               
               
                   
               
               
                   
                 gttgaaaatg agctcagtga ttctgtcacc cacattgtag 
               
               
                   
               
               
                   
                 cagaaaacaa ctctggttca gaggttctcg agtggcttca 
               
               
                   
               
               
                   
                 ggtacagaac ataagagcca gctcgcagct agaactcctt 
               
               
                   
               
               
                   
                 gatgtctcct ggctgatcga aagtatggga gcaggaaaac 
               
               
                   
               
               
                   
                 cagtggagat tacaggaaaa caccagcttg ttgtgagaac 
               
               
                   
               
               
                   
                 agactattca gctaccccaa acccaggctt ccagaagact 
               
               
                   
               
               
                   
                 ccaccacttg ctgtaaaaaa gatctcccag tacgcgtgtc 
               
               
                   
               
               
                   
                 aaagaaaaac cactttgaac aactataacc acatattcac 
               
               
                   
               
               
                   
                 ggatgccttt gagatactgg ctgaaaattc tgagtttaaa 
               
               
                   
               
               
                   
                 gaaaatgaag tctcttatgt gacatttatg agagcagctt 
               
               
                   
               
               
                   
                 ctgtacttaa atctctgcca ttcacaatca tcagtatgaa 
               
               
                   
               
               
                   
                 ggatacagaa ggaattccct gcctggggga caaggtgaag 
               
               
                   
               
               
                   
                 tgtatcatag aggaaattat tgaagatgga gaaagttctg 
               
               
                   
               
               
                   
                 aagttaaagc tgtgttaaat gatgaacgat atcagtcctt 
               
               
                   
               
               
                   
                 caaactcttt acttctgttt ttggagtggg actgaagaca 
               
               
                   
               
               
                   
                 tctgagaaat ggttcaggat ggggttcaga tctctgagta 
               
               
                   
               
               
                   
                 aaataatgtc agacaaaacc ctgaaattca caaaaatgca 
               
               
                   
               
               
                   
                 gaaagcagga tttctctatt atgaagacct tgtcagctgc 
               
               
                   
               
               
                   
                 gtgaccaggg ccgaagcaga ggcggttggc gtgctggtta 
               
               
                   
               
               
                   
                 aagaggctgt gtgggcattt ctgccggatg cctttgtcac 
               
               
                   
               
               
                   
                 catgacagga ggattccgca ggggtaagaa gattgggcat 
               
               
                   
               
               
                   
                 gatgtagatt ttttaattac cagcccagga tcagcagagg 
               
               
                   
               
               
                   
                 atgaagagca acttttgcct aaagtgataa acttatggga 
               
               
                   
               
               
                   
                 aaaaaaggga ttacttttat attatgacct tgtggagtca 
               
               
                   
               
               
                   
                 acatttgaaa agttcaagtt gccaagcagg caggtggata 
               
               
                   
               
               
                   
                 ctttagatca ttttcaaaaa tgctttctga ttttaaaatt 
               
               
                   
               
               
                   
                 gcaccatcag agagtagaca gtagcaagtc caaccagcag 
               
               
                   
               
               
                   
                 gaaggaaaga cctggaaggc catccgtgtg gacctggtta 
               
               
                   
               
               
                   
                 tgtgccccta cgagaaccgt gcctttgccc tgctaggctg 
               
               
                   
               
               
                   
                 gactggctcc cggcagtttg agagagacat ccggcgctat 
               
               
                   
               
               
                   
                 gccacacacg agcggaagat gatgctggat aaccacgctt 
               
               
                   
               
               
                   
                 tatatgacaa gaccaagagg gtatttctca aagcggaaag 
               
               
                   
               
               
                   
                 tgaagaagaa atctttgcac atctgggatt ggactacatt 
               
               
                   
               
               
                   
                 gaaccatggg aaagaaatgc ttaggagaaa gctgtcaact 
               
               
                   
               
               
                   
                 tttttctttt ctgttctttt tttcaggtta gacaaattat 
               
               
                   
               
               
                   
                 gcttcatatt ataatgaaag atgccttagt caagtttggg 
               
               
                   
               
               
                   
                 attctttaca ttttaccaag atgtagattg cttctagaaa 
               
               
                   
               
               
                   
                 taagtagttt tggaaacgtg atcaggcacc ccctgggtta 
               
               
                   
               
               
                   
                 tgctctggca agccatttgc aggactgatg tgtagaactc 
               
               
                   
               
               
                   
                 gcaatgcatt ttccatagaa acagtgttgg aattggtggc 
               
               
                   
               
               
                   
                 tcatttccag ggaagttcat caaagcccac tttgcccaca 
               
               
                   
               
               
                   
                 gtgtagctga aatactgtat acttgccaat aaaaatagga 
               
               
                   
               
               
                   
                 aac 
               
               
                   
               
            
           
         
       
     
     Additionally, to make isolation of recombinant proteins easier, it is common to append an N-terminal His tag sequence to the recombinant protein (see Boule J-B et al.,  Molecular Biotechnology,  1998; 10:199-208, incorporated by reference herein in its entirety), which is used in combination with an affinity column (Hitrap, Amersham Pharmacia Biotech, Uppsala, Sweden). Alternatively, N-terminal truncated forms of the enzyme with appended His-tag sequence will work with the current invention (see, e.g., U.S. Pat. No. 7,494,797, incorporated by reference herein in its entirety). His-tagged Bovine TdT amino acid sequences are shown below in Tables 3, 5, and 7, while His-tagged Bovine TdT nucleic acid sequences are shown below in Tables 4, 6, and 8. His tags may be engineered at other positions as required. In some embodiments, the resulting protein, adapted for sequence-specific de novo polynucleotide synthesis with 3′-O-modified dNTPs and NTPs, will be at least 85% identical, i.e., at least 90% identical, i.e., at least 93% identical, i.e., at least 95% identical, i.e., at least 97% identical, i.e., at least 98% identical, i.e., at least 99% identical, with SEQ ID NOS. 3, 5, or 7. 
     
       
         
           
               
             
               
                 TABLE 3 
               
               
                   
               
               
                 Amino Acid Sequence of a Δ138 and 
               
               
                 His-tagged Bovine TdT. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 SEQ ID No. 3: (392 aa) 
               
               
                 Met Arg Gly Ser His His His His His His Arg Thr 
               
               
                   
               
               
                 Asp Tyr Ser Ala Thr Pro Asn Pro Gly Phe Gln Lys 
               
               
                   
               
               
                 Thr Pro Pro Leu Ala Val Lys Lys Ile Ser Gln Tyr 
               
               
                   
               
               
                 Ala Cys Gln Arg Lys Thr Thr Leu Asn Asn Tyr Asn 
               
               
                   
               
               
                 His Ile Asp Ala Phe Glu Ile Leu Ala Glu Asn Ser 
               
               
                   
               
               
                 Glu Phe Lys Glu Asn Glu Val Ser Tyr Val Thr Phe 
               
               
                   
               
               
                 Met Arg Ala Ala Ser Val Leu Lys Ser Leu Pro Phe 
               
               
                   
               
               
                 Thr Ile Ile Ser Met Lys Asp Thr Phe Thr Glu Gly 
               
               
                   
               
               
                 Ile Pro Cys Leu Gly Asp Lys Val Lys Cys Ile Ile 
               
               
                   
               
               
                 Glu Glu Ile Ile Glu Asp Gly Glu Ser Ser Glu Val 
               
               
                   
               
               
                 Lys Ala Val Leu Asn Asp Glu Arg Tyr Gln Ser Phe 
               
               
                   
               
               
                 Lys Leu Ser Val Phe Gly Val Gly Leu Lys Thr Ser 
               
               
                   
               
               
                 Glu Lys Trp Phe Arg Met Gly Phe Thr Phe Arg Ser 
               
               
                   
               
               
                 Leu Ser Lys Ile Met Ser Asp Lys Thr Leu Lys Lys 
               
               
                   
               
               
                 Met Gln Lys Ala Gly Phe Leu Tyr Tyr Glu Asp Leu 
               
               
                   
               
               
                 Val Ser Cys Val Thr Arg Ala Glu Ala Glu Ala Val 
               
               
                   
               
               
                 Gly Val Leu Val Lys Glu Ala Val Trp Ala Phe Leu 
               
               
                   
               
               
                 Pro Asp Ala Phe Val Thr Met Thr    Gly Gly Phe Arg     
               
               
                   
               
               
                     Arg    Gly Lys Lys Ile Gly His Asp Val Asp Phe Leu 
               
               
                   
               
               
                 Ile Thr Ser Pro Gly Ser Ala Glu Asp Glu Glu Gln 
               
               
                   
               
               
                 Leu Leu Pro Lys Val Ile Asn Leu Trp Glu Lys Lys 
               
               
                   
               
               
                 Gly Leu Leu Leu Tyr Tyr Asp Leu Val Glu Ser Thr 
               
               
                   
               
               
                 Phe Glu Lys Phe Lys Phe Thr Leu Pro Ser Arg Gln 
               
               
                   
               
               
                 Val Asp Thr Leu Asp His Phe Gln Lys Cys Phe Leu 
               
               
                   
               
               
                 Ile Leu Lys Leu His His Gln Arg Val Asp Ser Ser 
               
               
                   
               
               
                 Lys Ser Asn Gln Gln Glu Gly Lys Thr Trp Lys Ala 
               
               
                   
               
               
                 Ile Arg Val Asp Leu Val Met Cys Pro Tyr Glu Asn 
               
               
                   
               
               
                 Arg Ala Phe Ala Leu Leu Gly Trp    Thr Gly Ser Arg     
               
               
                   
               
               
                 Gln Phe Glu Arg Asp Ile Arg Arg Tyr Ala Thr His 
               
               
                   
               
               
                 Glu Arg Lys Met Met Leu Asp Asn His Ala Leu Tyr 
               
               
                   
               
               
                 Asp Lys Thr Lys Arg Val Phe Leu Lys Ala Glu Ser 
               
               
                   
               
               
                 Glu Glu Glu Ile Phe Ala His Leu Gly Leu Asp Tyr 
               
               
                   
               
               
                 Ile Glu Pro Trp Glu Arg Asn Ala 
               
               
                   
               
            
           
         
       
     
     Table 4. Nucleotide Sequence of a 4138 and His-Tagged Bovine TdT. 
     SEQ ID No. 4: (1187 nt) 
     atgagaggat cgcatcacca tcaccatcac agaacagact attcagctac cccaaaccca ggcttccaga agactccacc acttgctgta aaaaagatct cccagtacgc gtgtcaaaga aaaaccactt tgaacaacta taaccacata ttcacggatg cctttgagat actggctgaa aattctgagt ttaaagaaaa tgaagtctct tatgtgacat ttatgagagc agcttctgta cttaaatctc tgccattcac aatcatcagt atgaaggata cagaaggaat tccctgcctg ggggacaagg tgaagtgtat catagaggaa attattgaag atggagaaag ttctgaagtt aaagctgtgt taaatgatga acgatatcag tccttcaaac tctttacttc tgtttttgga gtgggactga agacatctga gaaatggttc aggatggggt tcagatctct gagtaaaata atgtcagaca aaaccctgaa attcacaaaa atgcagaaag caggatttct ctattatgaa gaccttgtca gctgcgtgac cagggccgaa gcagaggcgg ttggcgtgct ggttaaagag gctgtgtggg catttctgcc ggatgccttt gtcaccatga caggaggatt ccgcaggggt aagaagattg ggcatgatgt agatttttta attaccagcc caggatcagc agaggatgaa gagcaacttt tgcctaaagt gataaactta tgggaaaaaa agggattact tttatattat gaccttgtgg agtcaacatt tgaaaagttc aagttgccaa gcaggcaggt ggatacttta gatcattttc aaaaatgctt tctgatttta aaattgcacc atcagagagt agacagtagc aagtccaacc agcaggaagg aaagacctgg aaggccatcc gtgtggacct ggttatgtgc ccctacgaga accgtgcctt tgccctgcta ggctggactg gctcccggca gtttgagaga gacatccggc gctatgccac acacgagcgg aagatgatgc tggataacca cgctttatat gacaagacca agagggtatt tctcaaagcg gaaagtgaag aagaaatctt tgcacatctg ggattggact acattgaacc atgggaaaga aatgcttaag cttgcgc 
     
       
         
           
               
             
               
                 TABLE 5 
               
               
                   
               
               
                 Amino Acid Sequence of a Δ151 and 
               
               
                 His-tagged Bovine TdT. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 SEQ ID No. 5: (379 aa) 
               
               
                 Met Arg Gly Ser His His His His His His Lys Thr 
               
               
                   
               
               
                 Pro Pro Leu Ala Val Lys Lys Ile Ser Gln Tyr Ala 
               
               
                   
               
               
                 Cys Gln Arg Lys Thr Thr Leu Asn Asn Tyr Asn His 
               
               
                   
               
               
                 Ile Asp Ala Phe Glu Ile Leu Ala Glu Asn Ser Glu 
               
               
                   
               
               
                 Phe Lys Glu Asn Glu Val Ser Tyr Val Thr Phe Met 
               
               
                   
               
               
                 Arg Ala Ala Ser Val Leu Lys Ser Leu Pro Phe Thr 
               
               
                   
               
               
                 Ile Ile Ser Met Lys Asp Thr Phe Thr Glu Gly Ile 
               
               
                   
               
               
                 Pro Cys Leu Gly Asp Lys Val Lys Cys Ile Ile Glu 
               
               
                   
               
               
                 Glu Ile Ile Glu Asp Gly Glu Ser Ser Glu Val Lys 
               
               
                   
               
               
                 Ala Val Leu Asn Asp Glu Arg Tyr Gln Ser Phe Lys 
               
               
                   
               
               
                 Leu Ser Val Phe Gly Val Gly Leu Lys Thr Ser Glu 
               
               
                   
               
               
                 Lys Trp Phe Arg Met Gly Phe Thr Phe Arg Ser Leu 
               
               
                   
               
               
                 Ser Lys Ile Met Ser Asp Lys Thr Leu Lys Lys Met 
               
               
                   
               
               
                 Gln Lys Ala Gly Phe Leu Tyr Tyr Glu Asp Leu Val 
               
               
                   
               
               
                 Ser Cys Val Thr Arg Ala Glu Ala Glu Ala Val Gly 
               
               
                   
               
               
                 Val Leu Val Lys Glu Ala Val Trp Ala Phe Leu Pro 
               
               
                   
               
               
                 Asp Ala Phe Val Thr Met Thr    Gly Gly Phe Arg Arg     
               
               
                   
               
               
                 Gly Lys Lys Ile Gly His Asp Val Asp Phe Leu Ile 
               
               
                   
               
               
                 Thr Ser Pro Gly Ser Ala Glu Asp Glu Glu Gln Leu 
               
               
                   
               
               
                 Leu Pro Lys Val Ile Asn Leu Trp Glu Lys Lys Gly 
               
               
                   
               
               
                 Leu Leu Leu Tyr Tyr Asp Leu Val Glu Ser Thr Phe 
               
               
                   
               
               
                 Glu Lys Phe Lys Phe Thr Leu Pro Ser Arg Gln Val 
               
               
                   
               
               
                 Asp Thr Leu Asp His Phe Gln Lys Cys Phe Leu Ile 
               
               
                   
               
               
                 Leu Lys Leu His His Gln Arg Val Asp Ser Ser Lys 
               
               
                   
               
               
                 Ser Asn Gln Gln Glu Gly Lys Thr Trp Lys Ala Ile 
               
               
                   
               
               
                 Arg Val Asp Leu Val Met Cys Pro Tyr Glu Asn Arg 
               
               
                   
               
               
                 Ala Phe Ala Leu Leu Gly Trp    Thr Gly Ser Arg    Gln 
               
               
                   
               
               
                 Phe Glu Arg Asp Ile Arg Arg Tyr Ala Thr His Glu 
               
               
                   
               
               
                 Arg Lys Met Met Leu Asp Asn His Ala Leu Tyr Asp 
               
               
                   
               
               
                 Lys Thr Lys Arg Val Phe Leu Lys Ala Glu Ser Glu 
               
               
                   
               
               
                 Glu Glu Ile Phe Ala His Leu Gly Leu Asp Tyr Ile 
               
               
                   
               
               
                 Glu Pro Trp Glu Arg Asn Ala 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 6 
               
               
                   
               
               
                 Nucleotide Sequence of a Δ151 and 
               
               
                 His-tagged Bovine TdT. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 SEQ ID No. 6: (1148 nt) 
               
               
                   
                 atgagaggat cgcatcacca tcaccatcac aagactccac 
               
               
                   
               
               
                   
                 cacttgctgt aaaaaagatc tcccagtacg cgtgtcaaag 
               
               
                   
               
               
                   
                 aaaaaccact ttgaacaact ataaccacat attcacggat 
               
               
                   
               
               
                   
                 gcctttgaga tactggctga aaattctgag tttaaagaaa 
               
               
                   
               
               
                   
                 atgaagtctc ttatgtgaca tttatgagag cagcttctgt 
               
               
                   
               
               
                   
                 acttaaatct ctgccattca caatcatcag tatgaaggat 
               
               
                   
               
               
                   
                 acagaaggaa ttccctgcct gggggacaag gtgaagtgta 
               
               
                   
               
               
                   
                 tcatagagga aattattgaa gatggagaaa gttctgaagt 
               
               
                   
               
               
                   
                 taaagctgtg ttaaatgatg aacgatatca gtccttcaaa 
               
               
                   
               
               
                   
                 ctctttactt ctgtttttgg agtgggactg aagacatctg 
               
               
                   
               
               
                   
                 agaaatggtt caggatgggg ttcagatctc tgagtaaaat 
               
               
                   
               
               
                   
                 aatgtcagac aaaaccctga aattcacaaa aatgcagaaa 
               
               
                   
               
               
                   
                 gcaggatttc tctattatga agaccttgtc agctgcgtga 
               
               
                   
               
               
                   
                 ccagggccga agcagaggcg gttggcgtgc tggttaaaga 
               
               
                   
               
               
                   
                 ggctgtgtgg gcatttctgc cggatgcctt tgtcaccatg 
               
               
                   
               
               
                   
                 acaggaggat tccgcagggg taagaagatt gggcatgatg 
               
               
                   
               
               
                   
                 tagatttttt aattaccagc ccaggatcag cagaggatga 
               
               
                   
               
               
                   
                 agagcaactt ttgcctaaag tgataaactt atgggaaaaa 
               
               
                   
               
               
                   
                 aagggattac ttttatatta tgaccttgtg gagtcaacat 
               
               
                   
               
               
                   
                 ttgaaaagtt caagttgcca agcaggcagg tggatacttt 
               
               
                   
               
               
                   
                 agatcatttt caaaaatgct ttctgatttt aaaattgcac 
               
               
                   
               
               
                   
                 catcagagag tagacagtag caagtccaac cagcaggaag 
               
               
                   
               
               
                   
                 gaaagacctg gaaggccatc cgtgtggacc tggttatgtg 
               
               
                   
               
               
                   
                 cccctacgag aaccgtgcct ttgccctgct aggctggact 
               
               
                   
               
               
                   
                 ggctcccggc agtttgagag agacatccgg cgctatgcca 
               
               
                   
               
               
                   
                 cacacgagcg gaagatgatg ctggataacc acgctttata 
               
               
                   
               
               
                   
                 tgacaagacc aagagggtat ttctcaaagc ggaaagtgaa 
               
               
                   
               
               
                   
                 gaagaaatct ttgcacatct gggattggac tacattgaac 
               
               
                   
               
               
                   
                 catgggaaag aaatgcttaa gcttgcgc 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 7 
               
               
                   
               
               
                 Amino Acid Sequence of a Δ160 and 
               
               
                 His-tagged Bovine TdT. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
            
               
                 SEQ ID No. 7: (370 aa) 
               
               
                 Met Arg Gly Ser His His His His His His Ile Ser 
               
               
                   
               
               
                 Gln Tyr Ala Cys Gln Arg Lys Thr Thr Leu Asn Asn 
               
               
                   
               
               
                 Tyr Asn His Ile Asp Ala Phe Glu Ile Leu Ala Glu 
               
               
                   
               
               
                 Asn Ser Glu Phe Lys Glu Asn Glu Val Ser Tyr Val 
               
               
                   
               
               
                 Thr Phe Met Arg Ala Ala Ser Val Leu Lys Ser Leu 
               
               
                   
               
               
                 Pro Phe Thr Ile Ile Ser Met Lys Asp Thr Phe Thr 
               
               
                   
               
               
                 Glu Gly Ile Pro Cys Leu Gly Asp Lys Val Lys Cys 
               
               
                   
               
               
                 Ile Ile Glu Glu Ile Ile Glu Asp Gly Glu Ser Ser 
               
               
                   
               
               
                 Glu Val Lys Ala Val Leu Asn Asp Glu Arg Tyr Gln 
               
               
                   
               
               
                 Ser Phe Lys Leu Ser Val Phe Gly Val Gly Leu Lys 
               
               
                   
               
               
                 Thr Ser Glu Lys Trp Phe Arg Met Gly Phe Thr Phe 
               
               
                   
               
               
                 Arg Ser Leu Ser Lys Ile Met Ser Asp Lys Thr Leu 
               
               
                   
               
               
                 Lys Lys Met Gln Lys Ala Gly Phe Leu Tyr Tyr Glu 
               
               
                   
               
               
                 Asp Leu Val Ser Cys Val Thr Arg Ala Glu Ala Glu 
               
               
                   
               
               
                 Ala Val Gly Val Leu Val Lys Glu Ala Val Trp Ala 
               
               
                   
               
               
                 Phe Leu Pro Asp Ala Phe Val Thr Met Thr    Gly Gly     
               
               
                   
               
               
                     Phe Arg Arg    Gly Lys Lys Ile Gly His Asp Val Asp 
               
               
                   
               
               
                 Phe Leu Ile Thr Ser Pro Gly Ser Ala Glu Asp Glu 
               
               
                   
               
               
                 Glu Gln Leu Leu Pro Lys Val Ile Asn Leu Trp Glu 
               
               
                   
               
               
                 Lys Lys Gly Leu Leu Leu Tyr Tyr Asp Leu Val Glu 
               
               
                   
               
               
                 Ser Thr Phe Glu Lys Phe Lys Phe Thr Leu Pro Ser 
               
               
                   
               
               
                 Arg Gln Val Asp Thr Leu Asp His Phe Gln Lys Cys 
               
               
                   
               
               
                 Phe Leu Ile Leu Lys Leu His His Gln Arg Val Asp 
               
               
                   
               
               
                 Ser Ser Lys Ser Asn Gln Gln Glu Gly Lys Thr Trp 
               
               
                   
               
               
                 Lys Ala Ile Arg Val Asp Leu Val Met Cys Pro Tyr 
               
               
                   
               
               
                 Glu Asn Arg Ala Phe Ala Leu Leu Gly Trp    Thr Gly     
               
               
                   
               
               
                     Ser Arg    Gln Phe Glu Arg Asp Ile Arg Arg Tyr Ala 
               
               
                   
               
               
                 Thr His Glu Arg Lys Met Met Leu Asp Asn His Ala 
               
               
                   
               
               
                 Leu Tyr Asp Lys Thr Lys Arg Val Phe Leu Lys Ala 
               
               
                   
               
               
                 Glu Ser Glu Glu Glu Ile Phe Ala His Leu Gly Leu 
               
               
                   
               
               
                 Asp Tyr Ile Glu Pro Trp Glu Arg Asn Ala 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 8 
               
               
                   
               
               
                 Nucleotide Sequence of a Δ160 and 
               
               
                 His-tagged Bovine TdT. 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 SEQ ID No. 8: (1121 nt) 
               
               
                   
                 atgagaggat cgcatcacca tcaccatcac atctcccagt 
               
               
                   
               
               
                   
                 acgcgtgtca aagaaaaacc actttgaaca actataacca 
               
               
                   
               
               
                   
                 catattcacg gatgcctttg agatactggc tgaaaattct 
               
               
                   
               
               
                   
                 gagtttaaag aaaatgaagt ctcttatgtg acatttatga 
               
               
                   
               
               
                   
                 gagcagcttc tgtacttaaa tctctgccat tcacaatcat 
               
               
                   
               
               
                   
                 cagtatgaag gatacagaag gaattccctg cctgggggac 
               
               
                   
               
               
                   
                 aaggtgaagt gtatcataga ggaaattatt gaagatggag 
               
               
                   
               
               
                   
                 aaagttctga agttaaagct gtgttaaatg atgaacgata 
               
               
                   
               
               
                   
                 tcagtccttc aaactcttta cttctgtttt tggagtggga 
               
               
                   
               
               
                   
                 ctgaagacat ctgagaaatg gttcaggatg gggttcagat 
               
               
                   
               
               
                   
                 ctctgagtaa aataatgtca gacaaaaccc tgaaattcac 
               
               
                   
               
               
                   
                 aaaaatgcag aaagcaggat ttctctatta tgaagacctt 
               
               
                   
               
               
                   
                 gtcagctgcg tgaccagggc cgaagcagag gcggttggcg 
               
               
                   
               
               
                   
                 tgctggttaa agaggctgtg tgggcatttc tgccggatgc 
               
               
                   
               
               
                   
                 ctttgtcacc atgacaggag gattccgcag gggtaagaag 
               
               
                   
               
               
                   
                 attgggcatg atgtagattt tttaattacc agcccaggat 
               
               
                   
               
               
                   
                 cagcagagga tgaagagcaa cttttgccta aagtgataaa 
               
               
                   
               
               
                   
                 cttatgggaa aaaaagggat tacttttata ttatgacctt 
               
               
                   
               
               
                   
                 gtggagtcaa catttgaaaa gttcaagttg ccaagcaggc 
               
               
                   
               
               
                   
                 aggtggatac tttagatcat tttcaaaaat gctttctgat 
               
               
                   
               
               
                   
                 tttaaaattg caccatcaga gagtagacag tagcaagtcc 
               
               
                   
               
               
                   
                 aaccagcagg aaggaaagac ctggaaggcc atccgtgtgg 
               
               
                   
               
               
                   
                 acctggttat gtgcccctac gagaaccgtg cctttgccct 
               
               
                   
               
               
                   
                 gctaggctgg actggctccc ggcagtttga gagagacatc 
               
               
                   
               
               
                   
                 cggcgctatg ccacacacga gcggaagatg atgctggata 
               
               
                   
               
               
                   
                 accacgcttt atatgacaag accaagaggg tatttctcaa 
               
               
                   
               
               
                   
                 agcggaaagt gaagaagaaa tctttgcaca tctgggattg 
               
               
                   
               
               
                   
                 gactacattg aaccatggga aagaaatgct taagcttgcg 
               
               
                   
               
               
                   
                 c 
               
               
                   
               
            
           
         
       
     
     
       
         
           
               
             
               
                 TABLE 9 
               
               
                   
               
               
                 Amino Acid Sequence of murine TdT 
               
               
                   
               
             
            
               
                   
               
            
           
           
               
               
            
               
                   
                 SEQ ID NO. 9: (510 aa) 
               
               
                   
                 MDPLQAVHLG PRKKRPRQLG TPVASTPYDI RFRDLVLFIL 
               
               
                   
               
               
                   
                 EKKMGTTRRA FLMELARRKG FRVENELSDS VTHIVAENNS 
               
               
                   
               
               
                   
                 GSDVLEWLQL QNIKASSELE LLDISWLIEC MGAGKPVEMM 
               
               
                   
               
               
                   
                 GRHQLVVNRN SSPSPVPGSQ NVPAPAVKKI SQYACQRRTT 
               
               
                   
               
               
                   
                 LNNYNQLFTD ALDILAENDE LRENEGSCLA FMRASSVLKS 
               
               
                   
               
               
                   
                 LPFPITSMKD TEGIPCLGDK VKSIIEGIIE DGESSEAKAV 
               
               
                   
               
               
                   
                 LNDERYKSFK LFTSVFGVGL KTAEKWFRMG FRTLSKIQSD 
               
               
                   
               
               
                   
                 KSLRFTQMQK AGFLYYEDLV SCVNRPEAEA VSMLVKEAVV 
               
               
                   
               
               
                   
                 TFLPDALVTM T   GGFRR   GKMT GHDVDFLITS PEATEDEEQQ 
               
               
                   
               
               
                   
                 LLHKVTDFWK QQGLLLYCDI LESTFEKFKQ PSRKVDALDH 
               
               
                   
               
               
                   
                 FQKCFLILKL DHGRVHSEKS GQQEGKGWKA IRVDLVMCPY 
               
               
                   
               
               
                   
                 DRRAFALLGW    TGSR   QFERDL RRYATHERKM MLDNHALYDR 
               
               
                   
               
               
                   
                 TKRVFLEAES EEEIFAHLGL DYIEPWERNA 
               
               
                   
               
            
           
         
       
     
     A variety of 3′-O-modified dNTPs and NTPs may be used with the disclosed proteins for de novo synthesis. In some embodiments, the preferred removable 3′-O-blocking group is a 3′-O-amino, a 3′-O-allyl or a 3′-O-azidomethyl. In other embodiments, the removable 3′-O-blocking moiety is selected from the group consisting of O-phenoxyacetyl; O-methoxyacetyl; O-acetyl; O-(p-toluene)-sulfonate; O-phosphate; O-nitrate; O-[4-methoxy]-tetrahydrothiopyranyl; O-tetrahydrothiopyranyl; O-[5-methyl]-tetrahydrofuranyl; O-[2-methyl,4-methoxy]-tetrahydropyranyl; O-[5-methyl]-tetrahydropyranyl; and O-tetrahydrothiofuranyl (see U.S. Pat. No. 8,133,669). In other embodiments the removable blocking moiety is selected from the group consisting of esters, ethers, carbonitriles, phosphates, carbonates, carbamates, hydroxylamine, borates, nitrates, sugars, phosphoramide, phosphoramidates, phenylsulfenates, sulfates, sulfones and amino acids (see Metzker M L et al. Nuc Acids Res. 1994; 22(20):4259-67, U.S. Pat. Nos. 5,763,594, 6,232,465, 7,414,116; and 7,279,563, all of which are incorporated by reference in their entireties). 
     Synthesis of Exemplary 3′-O-Blocked dNTP Analogs 
       FIG. 6  shows four exemplary 3′-O-blocked dNTP analogs, namely 3′-O-azidomethyl-dATP, 3′-O-azidomethyl-dCTP, 3′-O-azidomethyl-dGTP, and 3′-O-azidomethyl-dTTP. The synthesis of each 3′-O-azidomethyl analog is described below and detailed in  FIGS. 7-12 . The 3′-O-blocked dNTP analogs can also be purchased from specialty suppliers, such as Azco Biotech, Oceanside, Calif. It is to be understood that corresponding 3′-O-blocked ribonucleotides can be formed with similar synthetic methods to enable the creation of custom RNA oligos. 
     3′-O-azidomethyl-dATP: 
     With reference to  FIG. 7 , a solution of N 6 -benzoyl-5′-O-(tert-butyldimethylsilyl)-2′-deoxyadenosine (3.0 g; 6.38 mmol) [CNH Technologies, Woburn, Mass.] in DMSO (12 ml), acetic acid (5.5 ml) and acetic anhydride (17.6 ml) was prepared. The mixture was stirred at room temperature for 48 h. Approximately 100 ml of a saturated NaHCO 3  solution was added and the aqueous layer was extracted with CH 2 Cl 2 . The combined organic extract was washed with saturated NaHCO 3  solution and dried over Na 2 SO 4 . The residue was purified by flash column chromatography (hexane/ethyl acetate, 1:1 to 1:4) to recover N 6 -Benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyadenosine (shown as compound 1 in  FIG. 7 ) as a white powder (2.4 g; 71% yield). 400 mg of N 6 -Benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyadenosine was dissolved in dry CH 2 Cl 2  (7 ml) under nitrogen to create a solution (0.76 mmol). Cyclohexene (400 μl), and SO 2 Cl 2  (155 μl; 1.91 mmol, redistilled) were then added. The reaction mixture was stirred at 0° C. for 2 h. The solvent was then removed under reduced pressure and then under a high-vacuum pump for 10 min. The resulting residue was dissolved in dry DMF (5 ml) and reacted with NaN 3  (400 mg; 6.6 mmol) at room temperature for 3 h. The reaction mixture was dispersed in distilled water (50 ml) and extracted with CH 2 Cl 2 . The combined organic layer was dried over Na 2 SO 4  and concentrated under reduced pressure. The residue was dissolved in MeOH (5 ml) and stirred with NH 4 F (300 mg; 8.1 mmol) at room temperature for 24 h. The solvent was then removed under reduced pressure. The reaction mixture was concentrated under reduced pressure and partitioned between water and CH 2 Cl 2 . The organic layer was separated and dried over Na 2 SO 4 . After concentration, the crude product was purified by flash column chromatography (ethyl acetate/methanol) to produce N 6 -Benzoyl-3′-O-(azidomethyl)-2′-deoxyadenosine (compound 2;  FIG. 7 ) as a white powder (150 mg; 48% yield). N 6 —Benzoyl-3′-O-(azidomethyl)-2′-deoxyadenosine (123 mg; 0.3 mmol) and a proton sponge (75.8 mg; 0.35 mmol) were then dried in a vacuum desiccator over P 2 O 5  overnight before dissolving in trimethyl phosphate (600 μl). Next freshly distilled POCl 3  (40 μl; 0.35 mmol) was added dropwise at 0° C. and the mixture was stirred at 0° C. for 2 h. Subsequently, a mixture of tributylammonium pyrophosphate (552 mg) and tributylamine (0.55 ml; 2.31 mmol) in anhydrous DMF (2.33 ml) was added at room temperature and stirred for 30 min. Triethyl ammonium bicarbonate solution (TEAB) (0.1 M; pH 8.0; 15 ml) was then added, and the mixture was stirred for 1 hour at room temperature. Subsequently, concentrated NH 4 OH (15 ml) was added and stirred overnight at room temperature. The resulting mixture was concentrated under vacuum and the residue was diluted with 5 ml of water. The crude mixture was then purified with anion exchange chromatography on DEAE-Sephadex A-25 at 4° C. using a gradient of TEAB (pH 8.0; 0.1-1.0 M). The crude product was purified with reverse-phase HPLC to produce 3′-O-azidomethyl-dATP ( FIG. 7 , compound 3), a nucleotide analog to be used for later synthesis. 
     3′-O-azidomethyl-dTTP: 
     Acetic acid (4.8 ml) and acetic anhydride (15.4 ml) were added to a stirred solution of 5′-O-(tertbutyldimethylsilyl)thymidine (2.0 g; 5.6 mmol) [CNH Technologies, Woburn, Mass.] in DMSO. The reaction mixture was stirred at room temperature for 48 h. A saturated NaHCO 3  solution (100 ml) was added, and the aqueous layer was extracted with ethyl acetate (3×100 ml). The combined organic extract was washed with a saturated solution of NaHCO 3  and dried over Na 2 SO 4 . After concentration, the crude product was purified by flash column chromatography (hexane/ethyl acetate) to produce 3′-O-(Methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)thymidine ( FIG. 8 ; Compound 4) as a white powder (1.75 g; 75% yield). Approximately 1 gram of 3′-O-(Methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)thymidine was then dissolved in dry CH 2 Cl 2  (10 ml) under nitrogen. To this mixture cyclohexene (1.33 ml) and SO 2 Cl 2  (2840; 3.5 mmol, redistilled) were added. The resulting mixture was then stirred at 0° C. for 1.5 h. The solvent was then removed under reduced pressure and then under high vacuum for 10 min. The residue was dissolved in dry DMF (5 ml) and reacted with NaN 3  (926 mg; 15.4 mmol) at room temperature for 3 h. That reaction mixture was next dispersed in distilled water (50 ml) and extracted with CH 2 Cl 2  (3×50 ml). The combined organic extract was dried over Na 2 SO 4  and concentrated under reduced pressure. The residue was dissolved in MeOH (5 ml) and reacted with NH 4 F (600 mg; 16.2 mmol) at room temperature for 24 h. The reaction mixture was concentrated under reduced pressure and partitioned between water and CH 2 Cl 2 . The organic layer was then separated and dried over Na 2 SO 4 . After concentration, the residue was purified by flash column chromatography (hexane/ethyl acetate) to produce 3′-O-(azidomethyl)thymidine ( FIG. 8 , Compound 5) as a white powder (550 mg; 71% yield). Next, the 3′-O-(azidomethyl)thymidine and a proton sponge (0.35 mmol) were dried in a vacuum desiccator over P 2 O 5  overnight before dissolving in trimethyl phosphate (600 μl). Next, freshly distilled POCl 3  (40 μl; 0.35 mmol) was added dropwise at 0° C. and the mixture was stirred at 0° C. for 2 h. Subsequently, a mixture of tributylammonium pyrophosphate (552 mg) and tributylamine (0.55 ml; 2.31 mmol) in anhydrous DMF (2.33 ml) was added at room temperature and stirred for 30 min. Triethyl ammonium bicarbonate solution (TEAB) (0.1 M; pH 8.0; 15 ml) was then added, and the mixture was stirred for 1 hour at room temperature. Subsequently, concentrated NH 4 OH (15 ml) was added and stirred overnight at room temperature. The resulting mixture was concentrated under vacuum and the residue was diluted with 5 ml of water. The crude mixture was then purified with anion exchange chromatography on DEAE-Sephadex A-25 at 4° C. using a gradient of TEAB (pH 8.0; 0.1-1.0 M). The crude product was purified with reverse-phase HPLC to produce 3′-O-azidomethyl-dTTP ( FIG. 8 , compound 6), a nucleotide analog to be used for later synthesis. 
     3′-O-azidomethyl-dCTP: 
     Three and a half grams of N 4 -benzoyl-5′-O-(tert-butyldimethylsilyl)-2′-deoxycytidine [CNH Technologies, Woburn, Mass.] was added to 14.7 ml of DMSO to produce a 7.65 mmol solution. To this solution, acetic acid (6.7 ml) and acetic anhydride (21.6 ml) were added, and the reaction mixture was stirred at room temperature for 48 h. A saturated NaHCO 3  solution (100 ml) was then added and the aqueous layer was extracted with CH 2 Cl 2  (3×100 ml). The combined organic extract was washed with a saturated solution of NaHCO 3  and then dried over Na 2 SO 4 . After concentration, the crude product was purified by flash column chromatography (ethyl acetate/hexane) to produce N 4 -Benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxycytidine ( FIG. 9 ; compound 7) as a white powder (2.9 g; 73% yield). In 8 ml of CH 2 Cl 2  N 4 -Benzoyl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxycytidine (558 mg; 1.04 mmol) was dissolved and then cyclohexene (560 μl) and SO 2 Cl 2  (220 μl; 2.7 mmol) were added. The reaction mixture was stirred at 0° C. for 1 h. The volatiles were then removed with reduced pressure. The remaining residue was dissolved in dry DMF (5 ml) and reacted with NaN 3  (400 mg; 6.6 mmol) at room temperature for 2 h. The reaction mixture was dispersed in distilled water (50 ml) and extracted with CH 2 Cl 2  (3×50 ml). The combined organic extract was dried over Na 2 SO 4  and concentrated under reduced pressure. The residue was dissolved in MeOH (5 ml) and reacted with NH 4 F (600 mg; 16.2 mmol) at room temperature for 24 h. The solvent was removed under reduced pressure. The resulting residue was suspended in water (50 ml) and extracted with CH 2 Cl 2  (3×50 ml). The combined organic extract was dried over Na 2 SO 4  and concentrated under reduced pressure. The crude product was purified by flash column chromatography (hexane/ethyl acetate) to produce N 4 -Benzoyl-3′-O-(azidomethyl)-2′-deoxycytidine ( FIG. 9 , compound 8) as a white powder (200 mg; 50% yield). Next, the N 4 -Benzoyl-3′-O-(azidomethyl)-2′-deoxycytidine and a proton sponge (0.35 mmol) were dried in a vacuum desiccator over P 2 O 5  overnight before dissolving in trimethyl phosphate (600 μl). Then freshly distilled POCl 3  (40 μl; 0.35 mmol) was added dropwise at 0° C. and the mixture was stirred at 0° C. for 2 h. Subsequently, a mixture of tributylammonium pyrophosphate (552 mg) and tributylamine (0.55 ml; 2.31 mmol) in anhydrous DMF (2.33 ml) was added at room temperature and stirred for 30 min. Triethyl ammonium bicarbonate solution (TEAB) (0.1 M; pH 8.0; 15 ml) was then added, and the mixture was stirred for 1 hour at room temperature. Subsequently, concentrated NH 4 OH (15 ml) was added and stirred overnight at room temperature. The resulting mixture was concentrated under vacuum and the residue was diluted with 5 ml of water. The crude mixture was then purified with anion exchange chromatography on DEAE-Sephadex A-25 at 4° C. using a gradient of TEAB (pH 8.0; 0.1-1.0 M). The crude product was purified with reverse-phase HPLC to produce 3′-O-azidomethyl-dCTP ( FIG. 9 , compound 9), a nucleotide analog to be used for later synthesis. 
     3′-O-azidomethyl-dGTP: 
     To a stirred solution of N 2 -isobutyryl-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine (5 g; 11.0 mmol) [CNH Technologies, Woburn, Mass.] in dry DMSO (21 ml), acetic acid (10 ml) and acetic anhydride (32 ml) were added. The reaction mixture was stirred at room temperature for 48 h. A saturated NaHCO 3  solution (100 ml) was added and the aqueous layer was extracted with ethyl acetate (3×100 ml). The combined organic extract was washed with a saturated NaHCO 3  solution and dried over Na 2 SO 4 . After concentration, the crude product was purified by flash column chromatography (CH 2 Cl 2 /MeOH) to produce N 2 -Isobutyryl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine ( FIG. 10 , compound 10) as a white powder (3.9 g; 69% yield). One gram of N 2 -Isobutyryl-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine was subsequently added to dry pyridine (22 ml; 2.0 mmol) along with diphenylcarbamoyl chloride (677 mg; 2.92 mmol) and DIEA (N,N-diisopropylethylamine; SIGMA) (1.02 ml; 5.9 mmol). The reaction mixture was stirred under nitrogen atmosphere at room temperature for 3 h. The solvent was removed under high vacuum. The crude product was purified by flash column chromatography (ethyl acetate/hexane) to produce N 2 -Isobutyryl-O 6 -(diphenylcarbamoyl)-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine ( FIG. 10 , compound 11), which appeared as a yellowish powder (1.09 g; 80% yield). N 2 —Isobutyryl-O 6 -(diphenylcarbamoyl)-3′-O-(methylthiomethyl)-5′-O-(tert-butyldimethylsilyl)-2′-deoxyguanosine was then dissolved in dry CH 2 Cl 2  (1.1 mmol) and stirred under nitrogen atmosphere at 0° C. for 1.5 h. The solvent was removed under reduced pressure and then under high vacuum for 10 min. The resulting residue was dissolved in dry DMF (5 ml) and reacted with NaN 3  (600 mg; 10 mmol) at room temperature for 3 h. The reaction mixture was then dispersed in distilled water (50 ml) and extracted with CH 2 Cl 2  (3×50 ml). The combined organic extract was dried over Na 2 SO 4  and concentrated under reduced pressure. The resultant residue was dissolved in MeOH (5 ml) and reacted with NH 4 F (500 mg; 13.5 mmol) at room temperature for 24 h. The solvent was removed under reduced pressure. The residue was suspended in water (50 ml) and extracted with CH 2 Cl 2  (3×50 ml). The combined organic extract was dried over Na 2 SO 4  and concentrated under reduced pressure. The crude product was purified by flash column chromatography (hexane/ethyl acetate) to produce N 2 -Isobutyryl-O 6 -(diphenylcarbamoyl)-3′-O-azidomethyl-2′-deoxyguanosine ( FIG. 10 , compound 12) as a white powder (230 mg; 36% yield). Finally, the N 2 -Isobutyryl-O 6 -(diphenylcarbamoyl)-3′-O-azidomethyl-2′-deoxyguanosine and a proton sponge (0.35 mmol) were dried in a vacuum desiccator over P 2 O 5  overnight before dissolving in trimethyl phosphate (600 μl). Then freshly distilled POCl 3  (40 μl; 0.35 mmol) was added dropwise at 0° C. and the mixture was stirred at 0° C. for 2 h. Subsequently, a mixture of tributylammonium pyrophosphate (552 mg) and tributylamine (0.55 ml; 2.31 mmol) in anhydrous DMF (2.33 ml) was added at room temperature and stirred for 30 min. Triethyl ammonium bicarbonate solution (TEAB) (0.1 M; pH 8.0; 15 ml) was then added, and the mixture was stirred for 1 hour at room temperature. Subsequently, concentrated NH 4 OH (15 ml) was added and stirred overnight at room temperature. The resulting mixture was concentrated under vacuum and the residue was diluted with 5 ml of water. The crude mixture was then purified with anion exchange chromatography on DEAE-Sephadex A-25 at 4° C. using a gradient of TEAB (pH 8.0; 0.1-1.0 M). The crude product was purified with reverse-phase HPLC to produce 3′-O-azidomethyl-dGTP ( FIG. 10 , compound 13), a nucleotide analog to be used for later synthesis. 
     As described with respect to  FIG. 2 , once a 3′-O-blocked dNTP or 3′-O-blocked rNTP is added, it will be necessary to remove the blocking group so that additional dNTPs or rNTPs can be added. In some embodiments, the 3′-O-blocking group can be removed with a palladium catalyst in neutral aqueous solution at elevated temperature hydrochloric acid to pH 2, a reducing agent such as mercaptoethanol, or by the addition of tris-(2-carboxyethyl) phosphine. See, e.g., U.S. Pat. No. 6,664,079; Meng, et al.  J. Org. Chem.,  2006, 71(81):3248-52; Bi et al.,  J. Amer. Chem. Soc.  2006; 2542-2543, U.S. Pat. Nos. 7,279,563, and 7,414,116, all of which are incorporated herein by reference in their entireties. In other embodiments, the 3′-substitution group may be removed by UV irradiation (see, e.g., WO 92/10587, incorporated by reference herein in its entirety). In some embodiments, the removal of the 3′-O-blocking group does not include chemical cleavage but uses a cleaving enzyme such as alkaline phosphatase. 
     3′-O-Methoxymethyl-dTTP: 
     5′-O-Benzoylthymidine (173 mg, 0.5 mmol, 1 equiv) was dissolved in 10 mL of dichloromethane under argon at ambient T. Di-isopropylethylamine (128 mg, 1 mmol, 2 equiv) was added followed by methoxymethyl bromide (124 mg, 1 mmol, 2 equiv). The mixture was stirred at ambient T for 18 h. The mixture was diluted with 10 mL dichloromethane and this was washed successively with 20 mL of 5% aq HCl, and brine. The organic layer was dried with sodium sulfate and evaporated. 5′-O-Benzoyl-3′-O-methoxymethylthymidine (50 mg, 0.13 mmol) was dissolved in 5 mL of concentrated ammonium hydroxide at ambient temperature. The mixture was stirred at ambient T overnight. The mixture was diluted extracted 3 times with 10 mL portions of dichloromethane. The combined extracts were washed with brine. The organic layer was dried with sodium sulfate and evaporated. 3′-O-Methoxymethylthymidine (23 mg, 0.08 mmol) was co-evaporated with pyridine (1.5 mL×3) and dried overnight under high vacuum. The nucleoside was dissolved in a mixture of 1.5 mL of trimethylphosphate and 0.6 mL dry pyridine under Ar. The mixture was cooled in an ice bath. a first aliquot of 10 uL of POCl 3  was added dropwise. Five minutes later, a second aliquot of 10 uL was added. The mixture was stirred an additional 30 min. A solution of the TBA phosphate salt in dry DMF (1.25 mL) was cooled in an ice bath in a vial under Ar. This was added to the rxn mixture dropwise over 10 sec. Immediately the pre-weighed solid proton sponge (21 mg, 1.25 equiv) was added as a solid in one portion. The mixture was stirred for 25 min after this addition and was quenched with 5 mL of cold TEAB buffer. The mixture was stirred in the ice bath for 10 min and then transferred to a small RB flask for FPLC separation. Final separation was accomplished by reverse phase HPLC using a water/acetonitrile gradient containing 0.1 mM formic acid. 
     3′-O-Methylthiomethyl-dCTP: 
     To a suspension of deoxycytidine (1 g, 4.4 mmol) in 25 mL of methanol was added N,N-dimethylformamide dimethyl acetal (1.75 mL, 13.2 mmol). The mixture was stirred overnight at ambient temperature. The reaction mixture was evaporated, and the residue was purified by flash chromatography using a DCM/methanol gradient as eluant. N6-Formamidino-5′-O-benzoyldeoxy-3′-O-methylthiomethyldeoxycytidine (250 mg, 0.41 mmol) was dissolved in 10 mL of methanol and 10 mL conc aqueous ammonium hydroxide. The mixture was stirred at ambient temperature for 18 h and then evaporated under reduced pressure. The residue was purified by column chromatography (DCM/Methanol 98:2 to 90:10) to afford 170 mg (93%) of the desired nucleoside as a slightly yellow solid. 3′-O-Methylthiomethyl dexoxycytidine (25.0 mg, 0.09 mmol) in a 25 mL vial was co-evaporated with anhydrous pyridine (3×1 mL) and dried over the weekend. Trimethyl phosphate (0.7 mL) was added to dissolve the nucleoside and cooled in an ice bath to 0° C. Phosphoryl chloride (28 μL, 0.3 mmol) was added slowly (12 μL, 5 min later 8 μL, 30 min later 8 μL) and the reaction was stirred for 2 h at 0° C. The di(tetrabutylammonium) hydrogen pyrophosphate was dissolved in anhydrous DMF (1 mL), this mixture was cooled to 0° C. and added to the reaction mixture. Proton sponge (9.2 mg, 0.04 mmol) was added and the reaction was stirred at 0° C. for 2 h. To the reaction mixture was added 1 M triethylammonium bicarbonate buffer (TEAB) (2 mL) and the mixture was stirred for 1 h. The mixture was then transferred to round-bottom flask, 50 mL×3 of miliQ water was added and mixture was concentrated to dryness. The residue was dissolved in miliQ water (11 mL) and loaded onto an AKTA FPLC at room temperature. The fractions containing the triphosphate (F48-F52) were evaporated under reduced pressure at 40° C., and the residue was then lyophilized. The triphosphate was dried to afford the desired triphosphate (12 mg, 16.5%). 
     Examples 
     Protein Modifications. 
     Murine (mur) TdT variants originated from 380 aa synthetic gene. This backbone is a truncated version of WT murine TdT and represents a catalytic core starting with amino acid XX and ending amino acid XXX of the ET sequence. Chemically synthesized TdT constructs were cloned into a pRSET A bacterial expression vector, featuring an N-terminal 6×-histidine tag and enterokinase cleavage site (ThermoFisher Scientific GeneArt Gene Synthesis). Synthetic TdT plasmids were maintained in DH5 alpha cells (Biopioneer) plated on LB agar plates containing 100 ug/ml carbenicillin. For expression, the pRSETA-murine TdT plasmids were transformed into BL21 (DE3) pLysS cells (Thermo-Fisher) by incubating plasmids and cells on ice for 20 min., followed by a 30 sec. heat shock at 42° C., followed by addition of SOC media and incubation with shaking at 37° C. for 30-60 min. After addition of SOC media to cells, the entire volume (typically 60 ul) were plated on LB agar plates containing 100 ug/mL carbenicillin plus 34 ug/mL chloramphenicol. 
     Cells from 10 mL cultures (24-well plates, Corning) were harvested by centrifugation (3000×g, 15 min), then lysed in B-PER lysis buffer (Thermo-Fisher) containing lysozyme, protease inhibitors, and 100 mM NaCl. Pellets were soaked 1×60 min. in TBS buffer and supernatants collected for purification. The supernatant was bound onto 50 uL Ni-NTA bead (GE Life Sciences) slurry in 24-well plates for 30 min. The bead slurry was then washed 3×50 mM Tris-HCl, pH 8, 500 mM NaCl (500 uL), followed by washing 4×50 mM Tris-HCl, pH 8, 500 mM NaCl, 50 mM Imidazole (200 uL). The protein was then recovered by treating with 50 mM Tris-HCl, pH 8, 500 mM NaCl, 300 mM Imidazole (50 uL), then 50 mM Tris-HCl, pH 8, 500 mM NaCl, 300 mM Imidazole (130 uL), and finally 50 mM Tris-HCl, pH 8, 500 mM NaCl, 1M Imidazole (50 uL). 
     Recovered fractions were analyzed by taking 2.5 ul sample and running on 8% NuPage gel (Thermo-Fisher), 200 V for 50 min, denaturing conditions. Gel stained with Coomassie Blue. The eluted protein was buffer exchanged using a 7.5 MWCO desalting column (Thermo-Fisher) and scored at −80° C. (Storage Buffer=20 mM Tris-HCl, pH 6.8, 50 mM NaOAc; 0.01% Triton X-100 and 10% Glycerol). 
     Activity Screens: 
     TdT activity screening was performed via a dNTP polymerase extension reaction using different 3′-O-blocked dNTP analogs and a biotinylated oligonucleotide: 
     
       
         
           
               
            
               
                 5BiosG/TAATAATAATAATAATAATAATAATAATAATAATAATTTTTT 
               
               
                 (ChemGenes Corporation) 
               
            
           
         
       
     
     Reactions were typically set up in a 96 well plate. Reactions were performed by making a master mix with final concentrations of the following components: 0.2U PPase (Thermo-Fisher), 10 pmol of oligonucleotide, 75 uM dNTP (see below), 1× TdT reaction buffer (5× from Thermo-Fisher) to a final volume of 10 ul. Reactions were initiated by adding a defined volume (typically 2 ul) of TdT variants in different wells and incubating the reaction mix at 37° C. for 5 min and 60 min time points. Reactions were terminated by removal of a 10 ul aliquot and adding to 5 ul of 250 mM EDTA. 
     dNTPs tested: 
     
         
         3′-O-azidomethyl-dTTP see description above 
         3′-O-azidomethyl-dATP see description above 
         3′-O-azidomethyl-dGTP see description above 
         3′-O-MOM-dTTP see description above 
         3′-O-MTM-dCTP see description above 
         3′-aminoxy-dTTP Firebird BioMolecular Sciences LLC 
         3′-aminoxy-dATP Firebird BioMolecular Sciences LLC 
         3′-aminoxy-dGTP Firebird BioMolecular Sciences LLC 
         3′-O-methyl-dATP TriLink BioTechnologies LLC 
         3′-O-methyl-dGTP TriLink BioTechnologies LLC 
         3′-O-methyl-dCTP TriLink BioTechnologies LLC 
       
    
     Biotinylated oligos in the quenched reaction mix were bound to Streptavidin beads (0.77 um, Spherotech). The beads were then transferred to filter plates (Pall Corporation) and washed several times with water. The oligonucleotides were cleaved from the solid support by incubating the plate with cleavage buffer (10% Diisopropyl-amine in methanol) at 50° C. for 30 min followed by elution in water. The eluted samples were dried and dissolved in 30 ul water containing oligonucleotide sizing standards (two oligonucleotides (ChemGenes Corporation) that are approximately 15-20 bases smaller or larger than the starting 42-mer oligonucleotide). Oligonucleotides were then analyzed for extension efficiency by Capillary Gel Electrophoresis (Oligo Pro II, Advanced Analytical Technologies Inc.). 
     INCORPORATION BY REFERENCE 
     References and citations to other documents, such as patents, patent applications, patent publications, journals, books, papers, web contents, have been made throughout this disclosure. All such documents are hereby incorporated herein by reference in their entirety for all purposes. 
     EQUIVALENTS 
     Various modifications of the invention and many further embodiments thereof, in addition to those shown and described herein, will become apparent to those skilled in the art from the full contents of this document, including references to the scientific and patent literature cited herein. The subject matter herein contains important information, exemplification and guidance that can be adapted to the practice of this invention in its various embodiments and equivalents thereof.