Abstract:
A method of direct construction and isolation of full-length target cDNA molecules obtains a large number of full-length target cDNA clones with minimum screening. This new method saves time and resources that are needed in traditional methods requiring large screenings of clones that do not contain the desired sequence. A characteristic of this method includes using a total RNA population to purify mRNA for synthesizing a mixed population of first strand cDNA. Another characteristic is the use of a short sequence of the target sequence as a forward primer for the generation of the second strand cDNAs and then separating out all non-hybridizing single stranded cDNAs. Only the double stranded cDNAs are then cloned and sequenced. In this manner, only target cDNA clones are obtained and a large amount of screening is avoided. Another characteristic is the use of directed expression of the cDNA during cloning by modifying each of the two ends of the cDNA with a different restriction enzyme, each end of the cDNA then being distinct and incompatible with the other end. The directional target cDNA is then ligated into a vector compatible with the ends of the directional cDNA. The vector-ligated cDNA is transformed into a bacterial host cell. The sequence of the isolated full-length target cDNA molecule is confirmed by sequencing.

Description:
This is a continuation-in-part of U.S. patent application Ser. No. 08/937,365, filed Sep. 25, 1997, now abandoned. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to the isolation of target cDNA molecules. More particularly, the invention relates to methods of direct construction of full-length target cDNA molecules with a primer from the target molecule, such that all isolated clones contain a full-length target cDNA. 
     BACKGROUND OF THE INVENTION 
     Several cDNA cloning strategies have been developed, however cDNA library cloning techniques rely on the selection and screening for expression of the gene of interest. Selection and screening can be time consuming and expensive. Also, traditional methods isolate partial target sequences, and the complete sequence must be pieced together from several clones or synthesized with techniques such as the Polymerase Chain Reaction. 
     Molecular cloning of cDNAs was first reported by Rougeon and Mach, PNAS USA 73, 3418-3422 (1976) and by Efstradiadis et al., Cell 7, 279-288 (1976). Since then, the technology has continued to be refined. Library cloning techniques are now commonplace and described in textbooks, such as, MOLECULAR CELL BIOLOGY, Darnell, Lodish and Baltimore (second edition, Scientific American Books, Inc., 1990). These techniques generally start with total RNA from cells. Generally, it is preferable to use mRNA from cells believed to synthesize the target protein. To make the complimentary DNA (cDNA), a short primer strand is hybridized to the mRNA near the 3&#39; end. Most eukaryotic mRNA have a 3&#39; poly(A) +  tail, therefore, the primer is often poly(dT). Reverse transcriptase is then employed to add nucleotides to the primer to generate a cDNA, a DNA copy of the RNA molecule. The newly generated cDNA is called the first strand cDNA. The mRNA is then removed from the first strand cDNA, leaving the single stranded cDNA. 
     Next, a second strand cDNA is synthesized using the first strand cDNA as a template strand, to form a double stranded cDNA molecule. Double stranded cDNA can be made via several methods, for example, terminal transferase will add bases to a free 3&#39; end of the single strand cDNA. If a poly(dC) tail is added to the 3&#39; end of the first strand cDNA, then a poly(dG) primer and DNA polymerase can be employed to add nucleotides to the primer, thus creating the second strand cDNA. The second strand cDNA is annealed to the first strand cDNA and forms a complete double stranded DNA copy of the original mRNA. 
     The double stranded cDNA is then inserted into a plasmid vector having a marker. One method for inserting the cDNA into a plasmid vector involves cutting the plasmid vector with a restriction enzyme, and adding homopolymeric tails onto both ends of the cDNA and to both of the cut plasmid ends. Once the double stranded cDNA ends are annealed to the plasmid ends, the strands are joined by DNA ligase. 
     The plasmids are then transformed into an organism, such as E. coli, which lacks the marker of the plasmid vector. Cell colonies are grown and colonies having the marker are selected. For example, if the marker was ampicilin resistance and the cells were grown in the presence of ampicilin, only those cells having a plasmid conferring ampicilin resistance will grow. Then, radiolabeled DNA probe will be used to screen the cDNA library to look for the hybridized clones. Whether an inserted cDNA is present in the plasmid, and if so, if it is a partial or full-length target cDNA, is tested by sequencing and functional assays. It is common that many of the isolated and copied cDNAs, or clones, must be sequenced and analyzed before a partial or full-length target cDNA is isolated. This process of screening plasmids for a cDNA insert is expensive and time-consuming. 
     Okayama and Berg, Mol. Cell. Biol. 2, 161-170 (1982), transcribe mRNA into cDNA using a T-tailed cloning vector as primer. This method also requires a large amount of screening of plasmid vectors in search of vectors containing a partial or full-length target cDNA. 
     Gubler and Hoffman, Gene 25, 263-269 (1983), which describes synthesis of DNA in two steps by combining classical oligo(dT)-primed first-strand synthesis with RNase H-DNA polymerase I-mediated second strand synthesis. This method also requires a large amount of screening of plasmid vectors in search of vectors containing a partial or full-length target cDNA. 
     Heidecker and Messing, in Meth. in Enz. 154, 28-41 (1987), describe extending the 3&#39; ends of a linearized pUC plasmid with thymidine residues. The oligo(dT) tails are annealed to the poly(A) tails of mRNA and are used to prime cDNA synthesis, thus covalently linking the cDNA to the vector DNA. The plasmid-cDNA molecules are extended with oligo(dG) tails, alkali denatured and sized on alkaline sucrose gradients (thereby also removing the RNA and denaturing the vector). An excess of oligo(dC)-tailed pUC plasmid is added. The vector strands are reannealed, the oligo(dC) tail of the second strand vector serves as primer for synthesis of the second strand of the cDNA insert after which the recombination plasmids are transfected into E. coli. This method also requires a large amount of screening of plasmid vectors in search of vectors containing a partial or full-length target cDNA. 
     In Coleclough, Meth. In Enz. 154, 64-83 (1987), single stranded cDNA is directly inserted into double stranded cloning vehicles with synthetic oligonucleotides called primer-restriction end adapters, which serve both as primers for polymerase reactions and as ligation substrates. This method also requires a large amount of screening of plasmid vectors in search of vectors containing a partial or full-length target cDNA. 
     Another method of obtaining full length cDNA clones is the &#34;RACE&#34; protocol, described in Frohman et al., Proc. Natl. Acad. Sci. USA 85, 8998-9002 (1988). The RACE method obtains cDNA clones from rare transcripts, that is, those having a low-abundance of mRNAs. An mRNA selection step is required. Then, cDNA clones are generated by using a modified version of the DNA polymerase chain reaction technique to amplify copies of the region between a single point in the transcript and the unknown 3&#39; or 5&#39; end. The method requires a single primer which must be a short stretch of sequence within the mRNA to be cloned. 
     In Joyce et al., Proc. Natl. Acad. Sci. USA 80, 1830-1834 (1983), mass production of the Klenow fragment by directed expression during cloning is described. However, this method also requires a large amount of screening for expression of the lacZ gene, screening by an alkaline lysis procedure, assays for β-galactosidase activity and characterization of fusion junctions by DNA sequence analysis. 
     SUMMARY OF THE INVENTION 
     The present invention comprises methods and compounds of direct construction and isolation of full-length target cDNA molecules. These novel methods and compounds can be used to obtain a large number of full-length target cDNA clones with minimum screening. These novel methods and compounds save time and resources that are needed by traditional methods requiring large screenings of isolated cloning vectors for those that contain the desired cDNA sequence. 
     A preferred embodiment of the methods of the present invention is summarized in FIG. 1. First, a mixed population of mRNA is obtained, using known techniques, from any biological source. First strand cDNA synthesis from the entire mRNA population is primed with a poly(dT) 20  -Not I adapter. With this particular primer, a Not I site is strategically added to the 5&#39; end of the first strand cDNA. The mRNA strands are degenerated. A poly(dC) 20-30  tail is polymerized on the 3&#39; end of the first strand cDNA. 
     The primer for the synthesis of the second strand cDNA ensures isolation of only cDNAs of interest. A sequence fragment from the target DNA is used as the primer for the second strand synthesis. Only first strand cDNA sharing this sequence will hybridize with the primer and be duplicated in the 5&#39; to 3&#39; direction. These double stranded cDNAs are then separated from the single stranded cDNA. Because the primer may not have hybridized on the 3&#39; end of the first strand cDNA and thereby they may not provide a full-length cDNA, the 3&#39; end of the first strand cDNA must also be duplicated. This is accomplished by employing as a primer an Sse 8387 I adapter-poly(dG) 16  -primer. The poly(dG) portion of this primer will hybridize to the poly(dC) 20-30  tail that was polymerized on the 3&#39; end of the first strand cDNA and strategically adds an Sse 8387 I restriction site to the 5&#39; end of the second strand cDNA. Then the space between the poly(dG) 16  -primer and the previously duplicated section of the target cDNA is filled in. 
     Next, cohesive termini on both ends of the double stranded target cDNA and in a suitable vector are produced by digestion with restriction enzymes Not I and Sse 8387 I. The double stranded cDNA and the vector are then ligated together and the vector-ligated cDNA is transformed into a bacterial host cell. The host cells are grown and the target cDNAs isolated through standard techniques. The isolated full-length target cDNA molecule is confirmed by sequencing the double stranded cDNA fragment with T7 and SP6 primers. 
     A characteristic of the methods of the present invention comprises the use of an entire RNA genome to synthesize a mixed population of mRNA to synthesize a mixed population of cDNA for use in the production of the target cDNA. 
     Another characteristic of the methods of the present invention comprises using a short stretch of the DNA target sequence as a forward primer (as opposed to a poly(dG) primer) for the generation of the second strand cDNA and then separating out all non-hybridizing single stranded cDNA. Only the double stranded cDNAs are then synthesized to full-length, cloned and sequenced. In this manner, only target cDNA clones are obtained and a large amount of screening is avoided. 
     Another unique step in the preparation of full-length target cDNA of the methods of the present invention includes inserting the cDNA into the vector in a specific orientation for directed expression during cloning. Imparting a specific orientation is important for understanding that the outer strand cDNA in the vector represents the mRNA in the same direction and same sequence. Also, T7 may now be used as a promoter in functional studies, but not SP6. Finally, one can synthesize the antisense RNA with SP6 RNA polymerase. 
     Insertion of the cDNA in a specific orientation is accomplished by cutting the ends of the cDNA with two different restriction enzymes. By cutting one end with one enzyme and the other end with a different enzyme, each end of the cDNA is then distinct and incompatible with the other end. The directional target cDNA is then ligated into a vector compatible with the ends of the directional cDNA. 
     It is an object of the present invention to provide methods of obtaining full-length target cDNA clones without having to construct a cDNA library and to screen an excessive number of clones. 
     It is also an object of the present invention to provide methods to obtain a large number of copies of a full-length clone on the first screen. 
     It is an object of the present invention that only target cDNA is isolated, amplified and sequenced thereby saving time and expense in screening and sequencing many clones when isolating the full-length target. 
     It is an object of the present invention that no fragments of target molecules from cDNA libraries are competing with the full-length cDNAs for ligation into the vector. 
     It is an object of the present invention that the methods can be performed without the use of radioactive reagents. 
     It is an object of the present invention that double stranded target cDNA clones are separated from single stranded (non-target) cDNA clones with oligo (dA) cellulose. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a flow chart showing an embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The methods, intermediate compounds and compounds used in the disclosed invention for isolating full-length cDNA clones of a target molecule from an mRNA population are described below. FIG. 1 and the following examples provide a detailed disclosure of one embodiment of the invention. It will be understood by those in the art that alternate techniques in many of the following steps may be employed. For example, alternate techniques may be used in the preparation of mRNA, in the separation of single stranded and double stranded cDNA, and in the introduction of vectors into the host cells. Other variables include substitution of the particular vectors and bacterial host cells employed in this disclosure as well as the restriction enzymes used for directed expression of the cDNA. 
     1. mRNA Isolation 
     The construction and isolation of full-length cDNA of a target molecule relies on the preparation of a high quality mixed population of template mRNA. Extraction of total RNA may be from different biological sources. A preferred extraction method is with Tri-zol Reagent [TRIzol®reagent, Cat.No.: 15596, Life Technologies]. Preferably the mixed population of template mRNA is purified over an affinity column [oligo (dT) cellulose, Bethesda Research Laboratories, Cat. No.: 15940-018] twice to select polyadenylated [poly(A) +  ] RNA from total RNA (Aviv et al., Proc. Nat. Acad. Sci. USA 69:1408-1412 (1972)). 
     Generally, 5 μg mRNA will be sufficent for one reaction to construct the full-length cDNA of target molecule in an mRNA population. 
     2. First Strand Synthesis 
     A reverse transcriptase that no longer contains RNAse H activity is recommended for the mixed population of first strand cDNA synthesis from the mixed population of template mRNA. A preferred reverse transcriptase is Modified Moloney Murine Leukemia Virus (M-MLV RT). 
     A preferred primer is a poly(dT)-adapter having a recognition sequence for a rare enzyme, such as Not I or Sse 8387 I which occur approximately once in 10 6  base pairs. The number of (dT)s in the primer is variable. In the disclosed embodiment the preferred hybridized primer is a poly(dT) 20  -Not I adapter, 5&#39;-pATGTTCTGAGCGGCCGCCC(T) 20  -3&#39; (SEQ ID NO:1) wherein the underlined portion is the Not I recognition site (Kotewicz et al., Gene 35:249-258 (1985); Coleclough, Meth. Enz., 154, 64-83 (1987)). (See FIG. 1, Steps 1 and 2.) The poly(dT) portion of the primer anneals to the poly(A) tails of the template mRNA. 
     Preferred first strand synthesis reaction conditions are: 
     Reaction Temperature 37° C. 
     Reaction volume 20 μl 
     M-MLV RT 200 u/per μg mRNA 
     After completion of the mixed population of first strand cDNA sequences synthesis reaction (FIG. 1, step 2), the primary sequence of the hybridized template mRNA is degenerated by denaturation. This separates the mRNA from the first strand cDNA, and protects the cDNA from digestion of RNase A and RNase T 1  (Kunitz, J Biol. Chem. 164:563-568 (1946); Arima et al., Biochem. J 106:609-613 (1968)) (FIG. 1, step 3.) 
     3. Adding Deoxyribonucleotides(dC) 20-30  to the 3&#39; Ends of the First Single Stranded cDNAs 
     Terminal Deoxynucleotideyl Transferase generates poly(dC) 20-30  tails on the 3&#39; ends of the first strand cDNAs (Roychoudhury, Method in Enzymology 65:43-62 (1980)), which are the most preferred substrate for the enzyme. (FIG. 1, step 4). The optimal reaction conditions of generating 20 to 30 residues include a temperature of 37° C., and a reaction time of approximately 10 min. The reaction is terminated by adding 20 μl of 0.5 M EDTA and subsequent phenol extraction. 
     4. Second Strand Synthesis of Target Molecule 
     To generate target cDNA a short stretch of the target DNA sequence is added to the first strand cDNA mixed population as a forward primer under conditions allowing for hybridization. Only target first strand cDNAs have the complementary target sequence, so only target first strand cDNAs will hybridize with the forward primer. Non-target first strand cDNAs remain single stranded. 
     The length of the forward primer is preferably between 10 to 50 base pairs, and more preferably between 22 and 35 base pairs. In FIG. 1, step 5, the primer is shown as &#34;5&#39;→3&#39;&#34;. Preferably the primer hybridizes in the middle or close to the 3&#39; end of the first strand cDNA sequences. Upon hybridization of the primer, only this hybridized first strand cDNA can be used as a template in second strand cDNA synthesis. 
     The synthesis of the second strand between the forward primer and the 3&#39; end of the second strand cDNA sequences is catalyzed with a DNA dependent DNA polymerase, such as Klenow (Klenow et al., Proc. Nat. Acad. Sci. USA 65:168-175 (1970)). Double stranded target cDNA is now formed. 
     The optimal reaction conditions for the second strand cDNA synthesis are: 
     Reaction Temperature 37° C. 
     Reaction Volume 50 μl 
     Klenow Fragment 5-10 u 
     5. Purification of Double Strand cDNA 
     A distinguishing difference between the double stranded cDNA and the leftover single stranded cDNAs is that the single stranded cDNAs have exposed poly(dT) 20  tails on the 5&#39; ends. FIG. 1, step 5 shows the double stranded target molecule, wherein the poly(dT) 20  tail has been covered by the newly synthesized second strand cDNA. Therefore an affinity column of oligo (dA) cellulose is a preferred method for separating and isolating the double stranded target cDNA from the non-target single stranded cDNAs (Batra et al., GATA 8:129-133 (1990)). Preferably the double stranded cDNA is run through the affinity column twice. 
     To ensure that no copies of the single stranded cDNA exist in the last elution of double stranded cDNA, residual single-stranded cDNA can be further degraded by the addition of a 5&#39; exonuclease, such as mung bean nuclease [5&#39;→3&#39; exonuclease, United States Biochemical, E.C.3.1.30.1.] (Henikoff, Gene 28:351-359 (1984)). 
     6. Directional Cloning Construction--Completion of the Second Strand cDNA Synthesis and Introducing Asymmetry into cDNA 
     Directional expression during cloning is obtained by introducing two different restriction endonuclease sites at the ends of the cDNA. The Not I site was previously added to the 5&#39; end of the second strand cDNA in part 2 of the Detailed Description and shown in Steps 1 and 2 of FIG. 1. Now an Sse 8387 I site will be hybridized to the 3&#39; end of the first strand cDNA, to be incorporated into the 5&#39; end of the second strand cDNA. 
     First a poly(dG)-primer-Sse 8387 I adapter, 5&#39;-pAGTATGCGAACCTGCAGGGAA(G) 16  -3&#39; (SEQ ID NO:2) wherein the underlined portion is the Sse 8387 I recognition site, is hybridized to the poly(dC) 20-30  tail on the 3&#39; end of the first strand cDNA sequences. The number of (dG)s in the primer is variable. In the disclosed embodiment, the poly(dG) 16  primer-Sse 8387 I adapter will prime the completion of the second strand cDNA synthesis between the 5&#39;-Sse 8387 I adapter-poly(dG)-primer-3&#39; and the forward primer after the unpaired deoxynucleotide on 3&#39; ends of both Sse 8387 I adapter-poly(dG) 16  -primer and first single strand cDNA with Exonuclease I (United States Biochemical, Cat. No.: 70073) are removed (Lehman et al., The J Biol Chem. 239:2628-2636 (1964)). This creates a Sse 8387 I restriction enzyme site beyond the cap sequence of the double stranded cDNA shown in step 8 of FIG. 1. T 4  bacteriophage DNA polymerase is recommended for completing the second strand synthesis (Nossal, J Biol. Chem. 249:5668 (1974)), since it does not contain 5&#39;→3&#39; exonuclease activity during the elongation of the second strand and the first strand cDNA. Thus the double stranded target cDNA is modified into directional target cDNA, each double stranded target cDNA having two ends, one end of each of the double stranded target cDNA being distinct and incompatible with the other end, and a full-length double stranded target cDNA is formed. 
     The optimal reaction conditions are: 
     Reaction temperature 37° C. 
     Reaction volume 50 μl 
     Enzyme 20 u 
     A nick is left unfilled when the second strand cDNA sequences between the 5&#39;-Sse 8387 I adapter-poly(dG)-primer-3&#39; and the forward primer are synthesized. T 4  DNA ligase is preferred for sealing the nick in the double stranded cDNA. The optimal reaction conditions are: 
     Reaction temperature 22° C. 
     Reaction volume 50 μl 
     Enzyme 6 u 
     Before ligating the full-length double stranded target cDNA into a vector, restriction enzymes Not I and Sse 8387 I (United States Biochemical) are used to digest the nucleic acids on both the double stranded cDNA and the vector to create two different coherence terminus on the ends of cDNA and vector. A preferred vector is pSPORT 1 (Life Technologies, Cat. No.: 15382-013). The eight base recognition sequence of Not I and Sse 8387 I are chosen for their extreme rarity in most cDNAs, occurring approximately once in 10 6  base pairs. This creates a directional target cDNA and a vector having compatible ends to the directional target cDNA. 
     7. Ligation of Double Stranded Target cDNA to the Plasmid Vector and Introduction into E. coli 
     The directional target cDNa is ligated into a vector compatible with the ends of the directional target cDNA. The ligation reaction (in Example 8) in the methods of the present invention will suffice for all applications. It is preferred that 10 ng of cDNA to 50 ng of cDNA vector is used in the ligation reaction (as little as 1 ng of cDNA can be used in the ligation reaction). 
     pSPORT 1 is chosen for its multifunctional capabilities. It acts as an expression vector for cDNA cloning, in vitro transcription, dideoxy DNA sequencing and is useful in subtraction library procedures. The plasmid contains a unique multiple cloning site with sites for 19 restriction endonucleases, including Not I and Sse 8387 I, flanked by SP6 and T7 RNA polymerase promoters. These promoters may be used to generate RNA for probes, in vitro translations, or subtracted cDNA libraries. DNA inserts can be sequenced from double strand DNA using SP6/T7 Forward or Reverse Sequencing Primers. The ampicilin-resistance gene on the plasmid provides for convenient selection. Any plasmid sharing these characteristics may be used in the embodiment shown in FIG. 1. 
     The ligated cDNA vector can be introduced into bacterial host cells, such as E. coli. A preferred means of introduction is electroporation (Calvin, J. Bacteriol. 170:2796-2801 (1988); Dower, Nucl. Acids Research 16:6127-6145 (1988)), which generally will yield a greater number of transformants (up to 1×10 9  transformants/μg of cDNA). However, any means of introduction of the ligated cDNA vector into the host cells will suffice. 
     8. Sequencing Confirmation of Isolated Full-length Target cDNA Insert 
     A. Titer Determination of the Transformants 
     Portions of the cells are plated on LB plates containing 100 μg/ml ampicilin. The equivalent of 1.0, 0.1, and 0.01 μl, are plated in serial dilution in LB medium. The plates are incubated overnight at 37° C. and the remaining transformed cells are stored at 4° C. overnight. The colonies on each plate are counted, the correct titer is determined, and are then plated at a density suitable for single colony isolation. 
     B. Sequence Confirmation of Target cDNA Molecule 
     Molecular biology techniques for confirming sequence are applicable for this invention. A preferred method for confirming the correct construct of target cDNA molecule is by plasmid purification with Wizard Lambda Preps DNA Purification System [Promega, Cat.No.: A7290] and double stranded DNA sequencing [Perkin-Elmer Applied Biosystems 377 Prism DNA Sequencer] with primers T7 and SP6 on both ends of the insert. 
     The invention also contemplates a kit for direct construction and isolation of full-length target cDNA molecules from a mixed population of mRNAs comprising adapter-primers, enzymes, buffering agents and oligonucleotides well known in the art for synthesizing cDNA and other reagents necessary for carrying out the other reactions described in this application. Also possible to be included is a plasmid vector, DEPC water and oligo(dA) cellulose. 
     EXAMPLES 
     Example 1 
     Total RNA Isolation 
     Combine 50-100 mg tissue or cells with 1 ml Tri-zol Reagent [Life Technologies]. Homogenize 40×1000 rpm, 1 min., twice. Keep for 5 min. at room temperature. 
     Phase Separation: 
     Add chloroform 0.2 ml/1 ml Tri-zol Reagent. Shake for 15 sec., 2-3 min at room temperature. Centrifuge sample at 12,000×g for 30 min in 4° C. Collect supernantant. Add isopropyl alcohol 0.5 ml/1 ml Tri-zol Reagent. Keep 10 min. at room temperature. Centrifuge 12000×g for 10 min in 4° C. Wash the RNA pellet with 75% ethanol 20 ml (-20° C.). Centrifuge at 7500×g for 5 min at 4° C., twice. Dry the sample 2-5 min. Redissolve the sample in DEPC treated water (total RNA). 
     Example 2 
     mRNA Purification 
     The construction and isolation of full-length cDNA of a target molecule relies on the preparation of high quality mRNA. 
     Total RNA preparation 
     4 mg Total RNA in DEPC Treated water 65° C., 10 min, ice, 10 min. Add concentrated NaCl to a final concentration of 0.5 M. (Total volume is 1 ml). 
     Affinity Column Preparation of Oligo (dT) Cellulose 
     Dissolve 0.25-0.5 g dry weight oligo (dT) cellulose in 1 ml elution buffer. Put on column and wash column with 30 ml elution buffer. Equilibrate the column with 30 ml of binding buffer. 
     Apply sample on the column and collect the elution, re-apply the sample at least two more times. Wash the column with 10 ml binding buffer. Elute the mRNA with 1 ml elution buffer. The collected elution is the mRNA. 
     Repeat the elution three more times with either a new column or a reactivated column. The eluted mRNA is immediately precipitated by 1/10(v/v) 3 M CH 3  COOK and two volumes of ethanol (-20° C.). Dry the pellet. Dissolve the mRNA with DEPC treated water to a final concentration of 2-3 μg/μl, store the sample in -80° C. for the next step. 
     
         ______________________________________Binding buffer     Elution buffer______________________________________0.5M NaCl          10 mM Tris-HCl (pH 7.5)  10 mM Tris-HCI (pH 7.5) 1.0 mM EDTA  1.0 mM EDTA 0.2% SDS  0.2% SDS______________________________________ 
    
     Example 3 
     First Strand cDNA Synthesis 
     Place poly(A) +  mRNA (5.0 μg) in DEPC-treated water with 50 mg/ml Not I adapter-poly(dT) 20  -primer. 
     Incubate at 70° C., 10 min. Keep on ice, 10 min. 
     
         ______________________________________10X Reaction buffer I______________________________________50         mM Tris-HCl (pH 8.3)  75 mM KCl  6 mM MgCl.sub.2  10 mM DTT  500 μM each dATP, dCTP, dGTP, dTTP______________________________________ 
    
     Incubate at 37° C., 2 min. Add SuperScript II RT 30,000 u/ml and incubate at 37° C., 1 h. This completes the first strand synthesis (final volume 20 μl). 
     
         ______________________________________Example of a 20 μl reaction:______________________________________    2   μl mRNA  13 μl water  2 μl 10X Reaction Buffer I  2 μl dNTP  1 μl enzyme______________________________________ 
    
     Precipitate 
     Add 130 μl DEPC-treated water, 70 μl of 7.5 M NH 4  OAc (-20° C.), and 0.5 ml of absolute ethanol (-20° C.). Centrifuge for 10 min at 14,000×g, and remove the supernatant. Wash the pellet with 800 μl of 70% ethanol (-20° C.) and centrifuge. Dry the sample. The pellet will contain mRNA and the first strand cDNA. 
     mRNA Degradation 
     Add DEPC-treated water to the dried pellet and incubate at 100° C., 5 min. Keep on ice for 5 min. Add 10× RNase A, RNaseT1 reaction buffer and incubate at 37° C., 1 h (final volume 150 μl). 
     
         ______________________________________RNase A, RNase T.sub.1 reaction buffer______________________________________    10   μM Tris-HCl (pH 7.5)  0.4 μM EDTA  100 μg/ml RNase A  100 μg/ml RNase T.sub.1______________________________________ 
    
     Extract, Precipitate 
     Add 150 μl of phenol:chloroform:isoamyl alcohol (25:24:1) and vortex thoroughly. Centrifuge at room temperature for 5 min at 14,000×g to separate the phase. Remove 145 μl of the upper aqueous layer to a fresh 1.5 ml microcentrifuge tube. Add 70 μl of 7.5 M NH 4  OAc and 0.5 ml of absolute ethanol (-20° C.). Centrifuge for 20 min at 14,000×g. Remove the supernatant. Wash the pellet with 800 μl of 70% ethanol (-20° C.) and centrifuge. Dry the sample which now contains only single strand cDNA. 
     Example 4 
     Poly (dC) 20-30  Tail Generation on the 3&#39; End of First Strand cDNA 
     Combine 1-10 μg of first strand cDNA (1-10 μg) with reaction buffer II to a final volume of 150 μl. Incubate at 37° C., 10 min. 
     
         ______________________________________Reaction buffer II______________________________________100  μM          dCTP  1 mM MgCl.sub.2  100 mM Potassium cacodylate pH 7.0  200 μM Mercaptoethanol  60 u/pmole of 3&#39;-OH end Terminal Deoxynucleotidyl Transferase______________________________________ 
    
     To terminate the reaction add 20 μl 0.5 M EDTA and 100 μl neutralized phenol:chloroform:isoamyl alcohol (25:24:1). Vortex thoroughly. Keep in 0° C., 10 min. Centrifuge at 14,000×g for 5 min. Collect the supernatant. Add 70 μl of 7.5 M NH 4  OAc and 0.5 ml of absolute ethanol (-20° C.). Centrifuge for 20 min at 14,000×g, and remove the supernatant. Wash the pellet with 800 μl of 70% ethanol (-20° C.) and centrifuge. Dry the sample, which now contains only single strand cDNA with poly(dC) 20-30  tail. 
     Example 5 
     Priming and Beginning Second cDNA Strand Synthesis 
     The primer is a DNA fragment obtainable from many sources, such as GenBank, off the internet or a cDNA fragment. Preferably, the primer is a maximum of 50 base pairs. More preferably, the primer is between 22 to 35 base pairs. 
     Place the first strand cDNA in DEPC-treated water with 22 to 35 base pair primer (2 pmol/per reaction). Incubate at 100° C., 5 min. Place on ice for 5 min. Incubate at 65° C., 1 h. Place on ice, 10 min. Add reaction buffer III to a final volume of 50 μl and incubate at 37° C. for 1 hour. 
     
         ______________________________________Reaction buffer III______________________________________67       mM KPO.sub.4, pH 7.4  6 mM MgCl.sub.2  1 mM 2-Mercaptoethanol  35 μM each dATP, dCTP, dGTP, dTTP   Klenow Fragment of Polymerase I 5-10 u______________________________________ 
    
     Extract Precipitate 
     Add 50 μl of phenol:chloroform:isoamyl ethanol (25:24:1). Vortex thoroughly. Centrifuge at 14,000×g for 5 min. Transfer the upper layer to a fresh 1.5 ml microcentrifuge tube. Add 12.5 μl of 7.5 M NH 4  OAc and 150 μl of absolute ethanol (-20° C.). Centrifuge at 14,000×g for 20 min. Remove the supernatant. Wash the pellet with 800 μl of 70% ethanol (-20° C.) and centrifuge. Dry the sample, which now contains the partially complete double stranded cDNA and the non-hybridized single stranded cDNAs. 
     Example 6 
     Double Strand cDNA Purification from the First Strand cDNA Pool 
     Column Purification 
     Sample Preparation 
     Combine cDNA sample and DEPC-treated water. Add concentrated NaCl to a final concentration of 1.0 M. Adjust the final volume with DEPC-treated water to 100 μl. 
     Column Preparation 
     Mix 0.25 g dry weight oligo(dA) cellulose and 1 ml column preparation buffer. Put on column and wash with 30 ml column preparation buffer. Equilibrate the column with 30 ml binding buffer. 
     
         ______________________________________Binding buffer    Column preparation buffer______________________________________1.0M NaCl         10 mM Tris-HCl pH 7.5  10 mM Tris-HCl pH 7.5 1.0 mM EDTA  1.0 mM EDTA______________________________________ 
    
     Apply the sample on the column and collect the elution, apply the sample to the column at least two more times. Collect the last elution. Add 100 μl more binding buffer on the column and collect it. Repeat this procedure [oligo (dA) cellulose chromatography] before proceeding to the next step. The eluted double stranded cDNAs are immediately precipitated by adding 1/10 volume of 3 M CH 3  COOK and two volumes of ethanol (-20° C.). Centrifuge at 14,000×g, 20 min, washing twice with 800 μl of 70% ethanol (-20° C.). Dry the pellet. Dissolve the double stranded cDNA in 5 μl DEPC-treated water. 
     Mung Bean Nuclease Digestion 
     Add 45 μl Reaction Buffer IV to the 5 μl of resuspended double-stranded cDNA. Incubate at 37° C. for 30 min (final volume 50 μl). 
     
         ______________________________________Reaction buffer IV______________________________________    30  mM NaOAc (pH 4.6)  50 mM NaCl  1 mM ZnCl.sub.2  5% Glycerol______________________________________ 
    
     Extract, Precipitate 
     Add 50 μl of phenol:chloroform:isoamyl ethanol (25:24:1) and vortex thoroughly. Centrifuge at 14,000×g for 5 min. Transfer the upper layer to a fresh 1.5 ml microcentrifuge tube. Add 12.5 μl of 7.5 M NH 4  OAc, 5 μl yeast tRNA (GIBCO BRL, Life Technologies), and 150 μl of absolute ethanol (-20° C.). Centrifuge at 14,000×g for 20 min. Remove the supernatant. Wash the pellet with 800 μl of 70% ethanol (-20° C.) and centrifuge. Dry the sample, which is now partially complete double stranded target cDNA. 
     Example 7 
     Completion of the Second Strand of the Target cDNA 
     Resuspend the double stranded cDNA in 8 μl DEPC-treated water. Add 10 pM/2 μl Sse 8387 I adapter-oligo (dG) 16  -primer. Incubate at 65° C., 10 min. Place on ice, 10 min. 
     3&#39; OH End Preparation 
     Add reaction buffer V to a final volume of 20 μl and incubate at 37° C. for 30 min. 
     
         ______________________________________Reaction buffer V______________________________________   67  mM Glycine buffer (pH 9.5)  10 mM 2-Mercaptoethanol  6.7 mM MgCl.sub.2  2 u Exonuclease I______________________________________ 
    
     Add reaction buffer VI and T4 Bacteriophage DNA Polymerase 20 u to a final volume of 100 μl. Incubate at 37° C., 2 h. 
     
         ______________________________________Reaction buffer VI______________________________________67         mM Tris-HCl (pH 8.8)  16.6 mM (NH.sub.4).sub.2 SO.sub.4  6.7 μM EDTA  167 μg/ml BSA  10 mM β-Mercaptoethanol  33 μM dATP, dCTP, dGTP, dTTP  T4 Bacteriophage DNA Polymerase 20 u______________________________________ 
    
     Extract, Precipitate 
     Add 100 μl of phenol:chloroform:isoamyl ethanol(25:24:1) and vortex thoroughly. Centrifuge at 14,000×g for 5 min. Transfer the upper layer to a fresh 1.5 ml microcentrifuge tube. Add 50 μl of 7.5 M NH 4  OAc, 5 μl yeast tRNA (GIBCO BRL, Life Technologies) and 300 μl of absolute ethanol (-20° C.). Centrifuge at 14,000×g for 20 min. Remove the supernatant. Wash the pellet with 800 μl of 70% ethanol (-20° C.) and centrifuge. Dry the sample, which is now full length double stranded target cDNA with a nick. 
     Example 8 
     Second Strand cDNA Ligation 
     Resuspend the double stranded target cDNA pellet in 10 μl DEPC-treated water. Add reaction buffer VII to a final volume 50 μl and incubate at 22° C., 1 h. 
     
         ______________________________________Reaction buffer VII______________________________________20        mM Tris-HCl (pH 7.6)  5 mM MgCl.sub.2  5 mM Dithiothizetol  500 μg/ml Bovine Serum Albumin (Fraction 5;   Sigma)  0.5 mM ATP   Bacteriophage T.sub.4 DNA Ligase 6 u______________________________________ 
    
     Extract, Precipitate 
     Add 50 μl of phenol:chloroform:isoamyl ethanol (25:24:1) and vortex thoroughly. Centrifuge at 14,000×g for 5 min. Transfer the upper layer to a fresh 1.5 ml microcentrifuge tube. Add 12.5 μl of 7.5 M NH 4  OAc, 5 μl yeast tRNA (Gibco BRL, Life Technologies), and 150 μl of absolute ethanol (-20° C.). Centrifuge at 14,000×g for 20 min. Remove the supernatant. Wash the pellet with 800 μl of 70% ethanol (-20° C.) and centrifuge. Dry the sample, which is now full-length double stranded cDNA. 
     Example 9 
     Coherence Termini Preparation (Not I/Sse 8387 I Digestion) 
     The ends of all of the double stranded cDNA and 5 μg plasmid DNA vector (pSPORT 1) are then prepared for ligation. Not I digestion is accomplished first. The cDNA and the plasmid reactions are performed in different tubes. For each reaction, 10 to 20 u of enzyme is used. 
     
         ______________________________________Not I Reaction buffer______________________________________   6    mM Tris-HCI (pH 7.9)  6 mM MgCl.sub.2  150 mM NaCl  1 mM DTT  0.5 u Not I/μl reaction buffer______________________________________ 
    
     Incubate at 37° C., 3 h (final volume 20 μl). 
     Then the other cDNA ends and the plasmid are prepared with the other enzyme. 
     
         ______________________________________Sse 8387 I Reaction buffer______________________________________   10  mM Tris-HCl (pH 7.5)  80 mM KCI  7 mM β-Mercaptoethanol  0.5 u Sse 8387 I/μl reaction buffer______________________________________ 
    
     Incubate at 37° C., 3 h (final volume 50 μl). 
     Extract, Precipitate 
     Add 50 μl of phenol:chloroform:isoamyl ethanol (25:24:1) and vortex thoroughly. Centrifuge at 14,000×g for 5 min. Transfer the upper layer to a fresh 1.5 ml microcentrifuge tube. Add 12.5 μl of 7.5 M NH 4  OAc, 5 μl yeast tRNA (GIBCO BRL, Life Technologies), and 150 μl of absolute ethanol (-20° C.). Centrifuge at 14,000×g for 20 min. Remove the supernatant. 
     Wash the pellet with 800 μl of 70% ethanol (-20° C.) and centrifuge. Dry the samples, one tube having cDNA with cohesive termini and one tube having vector with cohesive termini. 
     Example 10 
     Ligation of All of the cDNA to the Plasmid Vector (pSPORT 1) 
     
         ______________________________________Reaction buffer VIII______________________________________50     mM Tris-HCI (pH 7.6)  5 mM MgCl.sub.2  0.5 mM ATP  5 mM DTT  500 μg/ml Bovine Serum Albumin (Fraction 5; Sigma)  2.5 μg/ml plasmid pSPORT 1, Not I/Sse 8387 I-cut cDNA  25 u/ml T4 DNA Ligase______________________________________ 
    
     Incubate at 4° C., overnight. 
     
         ______________________________________Example of a 20 μl reaction:______________________________________  11.5 μl water  2 μl 10X Reaction Buffer VIII  5 μl (all) cDNA  0.5 μg/2 μl vector  1 μl T4 Ligase______________________________________ 
    
     Example 11 
     E. coli Ligated cDNA Transformation by Electroporation 
     Combine 20 μl ligation reaction (standard recipes may be found in protocol manuals) 5 μl yeast tRNA, 12.5 μl 7.5 M NH 4  OAc, and 75 μl 100% ethanol (-20° C.). Vortex the mixture. Centrifuge at 14,000×g, 20 min, at room temperature. Wash the pellet with 800 μl of 70% ethanol (-20° C.). Centrifuge at 14,000×g, 5 min, at room temperature, twice. Dry the ligated sample. 
     Add 3 μl of sterile, distilled water to the dried pellet. Add 1 μl of the ligated cDNA/pSPORT 1 to 50 μl of electrotransformable cells such as ELECTROMAX DH1OB™ cells, and electroporate. Other methods of transformation may be used, but electroporation is preferred because it is the most efficient. Efficient transformation of ELECTROMAX DH1OB™ Cells requires a field strength of ≠16.6 kV/cm and a pulse length of 4 to 5 ms [Life Technologies, ELECTROMAX DH1OB™ Cells, Cat. No.: 18290-015]. Add 1 ml of S.O.C. medium [described in the Life Technologies product sheet] to the electroporated cells and incubate them at 37° C. for 1 h with vigorous aeration. 
     Plate the cells on LB plates containing 100 μg/ml Ampicilin, incubate the plates overnight at 37° C. 
     Example 12 
     Sequence Confirmation of Full Length Target Molecule 
     1. Plasmid Purification by using Wizard Lambda Preps: DNA Purification System [Cat. No.: A7290. Promega] 
     Culture a single bacteria clone that contains a constructed plasmid in 5 ml LB plus Ampicilin (100 ug/ml) at 37° C. for 16-18 h with vigorous aeration. Centrifuge the cultures at 4000×g for 5 min. Dissolve the bacteria pellet in 500 μl of Cell Resuspension Buffer (50 mM Tris-HCl (pH7.5); 10 mM EDTA; 100 μg/ml RNase A). Add 500 μl of Cell Lysing Solution. Keep at room temperature for 5 min (0.2 M NaOH and 1% SDS). Add 500 μl of Neutralization Solution. Keep on ice for 15 min (1.32 M Potassium Acetate). Centrifuge the sample at 14,000×g for 15 min. Mix the supernatant with 800 μl of Resine Buffer. Recipes for the three previously mentioned solutions and buffers can be found in Promega materials and in the Book of Molecular Cloning (as Solutions I, II, and III). Keep at room temperature for 5 min. 
     Load the mixture on the column. Wash the column with 3 ml of Column Washing Buffer (80 mM Potasium acetate; 8.3 mM Tris-HCl (pH 7.5); 40 μM EDTA; 55% ethanol). Dry the column by centrifugation at 14,000×g for 1 min. Elute the plasmid DNA with 50 μl distilled water by the centrifugation. Check the concentration with standard methods, such as a spectrophotometer. 
     2. Sequencing Analysis of Both Ends of Full Length Target cDNA 
     A preferred sequencing tool is the Perkin-Elmer Applied Biosystems 377 Prism DNA Sequencer. In fluorescent sequencing, the extended and terminated DNA molecules are labeled with fluorescent dyes, rather than with radioactivity. Reactions are electrophoresed through modified acrylamide gels using quartz plates. As the molecules pass through a region near the bottom of the gel (36 cm from the well), a laser passes over the gel, exciting the dyes and detecting their emissions. The data is interpreted by the computer and the base sequence is determined. The data received is in the form of an electropherogram. The primers are T7 (5&#39; TA ATA CGA CTC ACT ATA GGG 3&#39;, SEQ ID NO:3), and SP6 (5&#39; GAT TTA GGT GAC ACT ATA G 3&#39;, SEQ ID NO:4). All references cited in this patent application are herein incorporated by reference to the same extent as though each individual reference had been incorporated. 
     
         __________________________________________________________________________#             SEQUENCE LISTING   - -  - - &lt;160&gt; NUMBER OF SEQ ID NOS: 4   - - &lt;210&gt; SEQ ID NO 1  &lt;211&gt; LENGTH: 39  &lt;212&gt; TYPE: DNA  &lt;213&gt; ORGANISM: murine  &lt;220&gt; FEATURE:  &lt;221&gt; NAME/KEY: primer.sub.-- bind  &lt;222&gt; LOCATION: (1)..(20)  &lt;223&gt; OTHER INFORMATION: DNA primer  &lt;220&gt; FEATURE:  &lt;223&gt; OTHER INFORMATION: Description of Artificial - #Sequence: primer  - - &lt;400&gt; SEQUENCE: 1  - - atgttctgag cggccgccct tttttttttt ttttttttt      - # - #    39  - -  - - &lt;210&gt; SEQ ID NO 2 &lt;211&gt; LENGTH: 37 &lt;212&gt; TYPE: DNA &lt;213&gt; ORGANISM: murine &lt;220&gt; FEATURE: &lt;223&gt; OTHER INFORMATION: Description of Artificial - #Sequence: primer &lt;220&gt; FEATURE: &lt;221&gt; NAME/KEY: primer.sub.-- bind &lt;222&gt; LOCATION: (1)..(22)  - - &lt;400&gt; SEQUENCE: 2  - - agtatgcgaa cctgcaggga aggggggggg ggggggg      - #- #      37   - -  - - &lt;210&gt; SEQ ID NO 3  &lt;211&gt; LENGTH: 20  &lt;212&gt; TYPE: DNA  &lt;213&gt; ORGANISM: murine  &lt;220&gt; FEATURE:  &lt;223&gt; OTHER INFORMATION: Description of Artificial - #Sequence: primer &lt;220&gt; FEATURE: &lt;221&gt; NAME/KEY: primer.sub.-- bind &lt;222&gt; LOCATION: (1)..(20)  - - &lt;400&gt; SEQUENCE: 3  - - taatacgact cactataggg            - #                  - #  - # 20  - -  - - &lt;210&gt; SEQ ID NO 4 &lt;211&gt; LENGTH: 19 &lt;212&gt; TYPE: DNA &lt;213&gt; ORGANISM: murine &lt;220&gt; FEATURE: &lt;223&gt; OTHER INFORMATION: Description of Artificial - #Sequence: primer &lt;220&gt; FEATURE: &lt;221&gt; NAME/KEY: primer.sub.-- bind &lt;222&gt; LOCATION: (1)..(19)  - - &lt;400&gt; SEQUENCE: 4  - - gatttaggtg acactatag             - #                  - #  - # 19__________________________________________________________________________