PROCESS AND APPARATUS FOR SCISSION TREATMENT OF DEOXYRIBONUCLEIC ACID

Abstract: A process for the scission of a labelled DNA chemically modified base-selectively at the chemically modified posi­ tion to chemically determined DNA base sequence, which comprises modifying DNA with a chemical reagent, bringing the reaction solution into contact with an adsorbent to adsorb the chemically modified DNA, washing the adsorbent with a washing solution to remove the chemically modifying agent, eluting the adsorbed modified DNA with an eluent, concentrating the eluate, adding a scission reagent thereto, and heating the solution. This process is conducted using an apparatus in which a reactor (1) for retaining the DNA modification reaction solution, a liquid-transport pump (4), an adsorbent-containing apparatus (2), a cross valve (5), and a condenser (3) are mutually connected through pipes.

Description: Technical Field This invention relates to a procedure for chemically determining the base sequence of deoxyribonucleic acid and, in particular, its step of modifying radioactive phosphorus-labeled deoxyribonucleic acid base-selectively with chemical reagents and then cleaving the deoxyribonucleic acid strand at the position of the modified base. More particularly, it relates to a method for cleaving deoxyribonucleic acid by subjecting a reaction mixture containing deoxyribonucleic acid to a chemical modification reaction, separating the chemically modified deoxyribonucleic acid from the reaction mixture and then cleaving it at the position of the modified base and to apparatus for carrying out this method. Background Art Recently, a number of procedures for determining the base sequence of deoxyribonucleic acid (hereinafter referred to as DNA) directly have been developed. Among others, the chemical sequence determination procedure developed by Maxam and Gilbert is being widely used because it has the advantages of involving relatively simple experimental operations and comparing favorably in rapidity and accuracy with other determination procedures. Details of this procedure are described in Proc. Acad. Sci., Vol. 74, p. 560, 1977 and in Tanpakushitsu-Kakusan-Koso [Protein, Nucleic Acid and Enzyme (Japan)], Vol. 23, p. 182, 1978. Briefly stated, a DNA sample having one end labeled with radioactive phosphorus is prepared and the four bases constituting the DNA (i.e., guanine, adenine, thymine and cytosine which will hereinafter be referred to as G, A, T and C, respectively) are modified selectively with different chemical reagents. Then, with respect to each base, the DNA strand is cleaved at the position of the modified base and the resulting DNA fragments are subjected to gel electrophoresis in which the fragments are developed and separated in order of molecular chain length. By autoradiography, the developed and separated bands are recorded on an X-ray film and the bases corresponding to the bands on the X-ray film are identified to determine the base sequence of the original DNA sample. Broadly divided, the above-described procedure can be said to comprise the following four steps: (1) the step of labeling the 3' or 5' terminus of DNA with radioactive phosphorus; (2) the step of chemically modifying the labeled DNA in a base-selective manner and cleaving the DNA strand at the position of the modified base; (3) the step of developing the resulting DNA fragments by gel electrophoresis and recording the bands on an X-ray film; and (4) the step of analyzing the bands on the X-ray film to determine the base sequence. This Maxam-Gilbert procedure is being widely used because of its above-described advantages. However, since it has been conventional practice to carry out all of the foregoing steps by hand, determination of the base sequences of many DNA samples requires very troublesome operations and consumes much time. Therefore, it would be desirable to enhance the operating efficiency by mechanization of the steps. Among the above-described four steps, the present invention is directed to the second step, i.e., the step of chemically modifying the labeled DNA in a base-selective manner and cleaving the DNA strand at the position of the modified base, and is concerned with a method and apparatus for mechanizing this step to enhance the operating efficiency. Conventionally, the second step has been carried out by injecting a sample solution containing the labeled DNA into four tubes made of resin, subjecting them separately to chemical modification reactions specific for four bases (i.e., G, A, T and C) using their respective reagents and reaction conditions, adding ethanol to each tube, cooling the mixture to a temperature of -40 to -70°C, centrifuging it and sucking out the supernatant, washing the precipitate repeatedly (by adding ethanol thereto, centrifuging the mixture and sucking out the supernatant) to remove therefrom any excess reagents used for the chemical modification, adding an aqueous solution of a cleavage reagent (sodium hydroxide or piperidine) to the precipitate and then heating the resulting mixture to cleave the DNA strand at the position of the modified base. The present inventors have concluded that, among the above-described operations, those of adding ethanol to each tube, cooling the mixture and then centrifuging it constitute a difficulty in mechanizing this step. In order to overcome the difficulty, the present inventors have made intensive studies and have found that the adsorption technique can be used in place of the centrifugation one. The present invention has been completed on the basis of this discovery. A volatile eluent is used in the process of the present invention to elute the chemically modified deoxyribonucleic acid adsorbed on the adsorbent. As described hereinabove, in the chemical sequence determination procedure developed by Maxam and Gilbert, the concentrated solution of the chemically modified deoxyribonucleic acid is further treated by the following steps : (1) the step of cleaving the DNA strand at the position of the modified base in an alkaline medium, and (2) the step of developing the resulting DNA fragments by gel electrophoresis. In these steps, if the concentrated solution contains any chemical modification reagents or a solvent unnecessarily, the cleaving does not progress well or accurate development of gel electrophoresis cannot be expected, which causes an incorrect determination of the base sequence of DNA. In the present invention, these problems are overcome, since a volatile eluent is used and the resulting eluate is concentrated to vaporize the volatile eluent. Disclosure of the Invention According to the present invention, there is provided a method for cleaving deoxyribonucleic acid by subjecting a reaction mixture containing deoxyribonucleic acid to a chemical modification reaction, separating the chemically modified deoxyribonucleic acid from said reaction mixture, adding a cleavage reagent thereto and then heating the resulting mixture to cleave the deoxyribonucleic acid strand at the position of the modified base, characterized by said separation procedure comprising the steps of bringing said reaction mixture, which has undergone the chemical modification reaction, into contact with an adsorbent to adsorb the chemically modified deoxyribonucleic acid on said adsorbent, washing said adsorbent with a washing fluid to remove any chemical modification reagents from said adsorbent, bringing said adsorbent into contact with a volatile eluent to elute the chemically modified deoxyribonucleic acid adsorbent thereon, and then concentrating the resulting eluate. According to the present invention, there is also provided an apparatus for cleaving deoxyribonucleic acid by use of the above-described method, characterized by the combination of : (a) a reactor for subjecting a reaction mixture containing deoxyribonucleic acid to a chemical modification reaction, said reactor comprising washing means, which comprise a washing fluid tank and a two-way valve connected by a conduit to the reactor, to remove any chemical modification reagent from the adsorbent with a washing fluid, said reactor further comprising eluting means, which comprise an eluent tank and a two-way valve connected by a conduit to the reactor, for eluting chemically modified deoxyribonucleic acid from the adsorbent; (b) a transfer pump connected by a conduit to said reactor for bringing said reaction mixture, having undergone a chemical modification reaction, into contact with an adsorber; (c) an adsorber connected by a conduit to said transfer pump, said adsorber comprising the adsorbent for adsorbing and retaining the chemically modified deoxyribonucleic acid; (d) a concentrator connected by a conduit to said adsorber for concentrating deoxyribonucleic acid eluates received from the adsorber, said concentrator further comprising concentrating means which comprise a vacuum pump, a trap and a vacuum regulating valve connected by a conduit to said concentrator, and a vibrator, said concentrator further comprising cleavage reagent supplying means, which comprise an aqueous cleavage reagent solution tank and a two-way valve connected by a conduit to said concentrator, for adding cleavage reagent to the concentrator, said concentrator further comprising heating means for heating the cleavage reagent and deoxyribonucleic acid eluates, whereby the deoxyribonucleic acid is cleaved at the position of the modified base; and (e) a three-way valve which is operable to connect, during the adsorbing and the washing operations, by a conduit the adsorber to a waste liquid tank, and, during the eluting operation, to the concentrator. The above-described apparatus of the present invention can be very simply and mechanically operated by using an adsorber for adsorbing and retaining DNA from the reaction mixture having undergone a chemical modification, means for washing the adsorbed DNA to remove any reagents used for the chemical modification, means for eluting the adsorbed DNA and means for concentrating the resulting eluate, and the above-described method of the present invention can be carried out thereby. Brief Description of the Drawings Fig. 1 is a flow diagram illustrating a preferred embodiment of the apparatus of the present invention, and Fig. 2 is a diagram illustrating an electrophoretic autoradiogram of DNA cleaved in accordance with the present invention. Best Mode for Carrying Out the Invention In order to explain the method and apparatus of the present invention more fully, one embodiment thereof is described with reference to the accompanying drawings. In a reactor 1 shown in Fig. 1, a reaction vessel 1a is charged with radioactive phosphorus-labeled DNA, various reagents are added thereto, and the resulting reaction mixture is subjected to a chemical modification reaction at a predetermined temperature for a predetermined period of time. Thereafter, a transfer pump 4 is operated to pass the aforesaid reaction mixture through a DNA adsorber 2. The effluent from adsorber 2 is discharged into a waste liquid tank 6 via a three-way valve 5, while the chemically modified DNA is adsorbed on and retained by the adsorbent packed in adsorber 2. By opening a two-way valve 7, a predetermined amount of a washing fluid is poured into reaction vessel 1a from a washing fluid tank 8. Transfer pump 4 is operated again to conduct the washing fluid through transfer pump 4, DNA adsorber 2 and three-way valve 5 into waste liquid tank 6. Thus, any chemical modification reagents remained on the adsorbent within adsorber 2 are washed away. Then, by opening a two-way valve 9, a predetermined amount of an eluent is poured into reaction vessel 1a from an eluent tank 10. After the liquid outlet of three-way valve 5 is changed over to a concentrator 3, transfer pump 4 is operated again to conduct the eluent through transfer pump 4, DNA adsorber 2 and three-way valve 5 into a concentration vessel 3a. Thus, the chemically modified DNA adsorbed on and retained by the adsorbent within adsorber 2 is eluted and introduced into concentration vessel 3a. A vacuum pump 12 and a vibrator 13 are operated and a vacuum regulating valve 14 is opened to concentrate the eluate within concentration vessel 3a. The evaporated liquid is collected in a trap 11. After concentrator 3 is returned to atmospheric pressure, a two-way valve 15 is opened to pour a predetermined amount of an aqueous cleavage reagent solution from an aqueous cleavage reagent solution tank 16 into concentration vessel 3a. Then, concentration vessel 3a is heated at a predetermined temperature for a predetermined period of time to obtain base-selectively cleaved DNA fragments. As the adsorbent used in the present invention, there may be used both cation and anion exchangers. They include ones using polystyrene resin as the base material and ones using cellulose as the base material, and useful examples thereof are Dowex-X2, Dowex-50W (both are trade names; mfd. by Dow Chemical Co.) and AIEC-CM52, AIEC-DE52 (both are trade names; mfd. by Whatman Co.). Moreover, there may also be used nonionic porous resins such as Amberlite XAD-2 (trade name; mfd. by Rohm & Haas Co.), Diaion HP-10 (trade name; mfd. by Mitsubishi Chemical Industries, Limited) and Levacit OC1031 (trade name; mfd. by Bayer Co.). Furthermore, there may also be used fine-granular materials adapted for reversed phase partition which, for example, include packing materials for use in high-speed liquid chromatography, such as Microbonder Pack C18, Microbonder Pack CN and Microbonder Pack-Phenyl (all are trade names; mfd. by Waters Co.) The above-described adsorbent is usually used by packing it into a column made of glass, stainless steel or polystyrene. For example, it is preferable to use Setp Pack C-18 (trade name; mfd. by Waters Co.) or Bond Elute C-18 (mfd. by Analytichem-International Co.) which are available in the form of a packed column. As the eluent for eluting the DNA retained by the adsorbent, there is used an aqueous solution of a volatile acid or basic substance and preferably an aqueous solution of a volatile basic substance such as ammonia or piperazine, where the adsorbent comprises an ion exchanger. Where the adsorbent comprises a nonionic porous resin or a fine-granular material adapted for reversed phase partition, a mixture of water and a volatile, water-soluble organic solvent such as methanol, ethanol or acetonitrile is used. The reaction vessel is maintained at a predetermined temperature by means of a water bath or heater block. In consideration of the fact that the reaction vessel must be discarded after each analysis because of its contamination with radioactive phosphorus, it is preferable to use disposable vessels such as Eppendorf Tube 3810 (trade name; mfd. by Eppendorf Co.). The transfer pump serves to transfer the sample solution within the reaction vessel to the adsorber via a Teflon® or silicone tube, and may comprise a tube pump having small dead space. The concentrator preferably has a construction which is adapted to contain a vessel made of glass or synthetic resin for receiving the DNA solution eluted from the aforesaid adsorbent, is provided with a temperature control means for maintaining the vessel at a constant temperature, can be hermetically sealed so as to permit a reduced pressure to be established therein, and communicates with a ventilation system. Moreover, it is more convenient to provide the concentrator with means for permitting rotation, revolution, precession or vibration of the aforesaid vessel in order to concentrate the DNA solution smoothly under reduced pressure. As the washing fluid for washing and removing any chemical modification reagents from the adsorbent, there may be used purified water where the adsorbent comprises a cation or anion exchanger, and purified water or an aqueous solution containing not more than 10% of a water-soluble organic solvent where the adsorbent comprises a nonionic porous resin or a fine-granular material adapted for reversed phase partition. The method of the present invention, i.e. the sequence of steps for operating the apparatus of the present invention, can be readily automated by use of a sequencer. Moreover, it is also possible to carry out the selective cleavage of DNA for four bases simultaneously by installing four vessels 3a in concentrator 3 and using four reaction vessels, adsorbers and other units arranged in parallel. The apparatus of the present invention for carrying out the method of the present invention has a simple construction requiring no expensive deep freezer or high-speed centrifuge as contrasted with conventional methods, and permits automation at low cost. Example Synthetic linker α(5'-C-C-G-G-A-T-C-C-G-G-3') (Catalog No. L-1003; mfd. by Nippon Zeon Co., Ltd.) was labeled with radioactive phosphorus according to a procedure described in the literature (Tanpakushitsu-Kakusan-koso, Vol. 23, p. 182, 1978), injected into four 2-ml Eppendorf tubes (mfd. by Eppendorf Co.) in amounts of approximately 500,000 counts, and subjected to their respective chemical modification reactions. After the addition of a reaction stopper and then 1 ml of purified water, each tube was set in the reactor 1 of the apparatus illustrated in Fig. 1. Transfer pump 4 (comprising a tube pump, 1 ml/min.) was operated to pass the reaction mixture through adsorber 2 (containing the adsorbent Sep Pack® C-18; mfd. by Waters Co.), which was then washed with 2 ml of purified water. Using transfer pump 4, an eluent (comprising a 40% aqueous solution of ethanol) was likewise passed through adsorber 2. By opening three-way valve 5 to concentrator 3, the resulting eluate was collected in concentration vessel 3a. After concentrator 3 was heated to 90°C, vibrator 13, vacuum pump 12 and vacuum regulating valve 14 were operated to evaporate the eluate to dryness. Then, by opening two-way valve 15, 0.02 ml of a 0.1 N aqueous solution of caustic soda was introduced from aqueous cleavage reagent solution tank 16 (containing a 0.1 N aqueous solution of caustic soda) into concentration vessel 3a. This concentration vessel 3a was heated at 90°C for 30 minutes to complete the cleavage of the sample. The resulting fragments were subjected to electrophoresis using a 7 M urea-20% polyacrylamide gel and then to autoradiography. The results thus obtained are shown in Fig. 2. Analysis of this autoradiogram enables one to determine the base sequence of the sample accurately. In Fig. 2, the thick pattern seen at the right end represents an autoradiogram of the uncleaved sample.

Claims: A method for cleaving deoxyribonucleic acid by subjecting a reaction mixture containing deoxyribonucleic acid to a chemical modification reaction, separating the chemically modified deoxyribonucleic acid from said reaction mixture, adding a cleavage reagent thereto and then heating the resulting mixture to cleave the deoxyribonucleic acid strand at the position of the modified base, characterized by said separation procedure comprising the steps of bringing said reaction mixture, which has undergone the chemical modification reaction, into contact with an adsorbent to adsorb the chemically modified deoxyribonucleic acid on said adsorbent, washing said adsorbent with a washing fluid to remove any chemical modification reagents from said adsorbent, bringing said adsorbent into contact with a volatile eluent to elute the chemically modified deoxyribonucleic acid adsorbent thereon, and then concentrating the resulting eluate. An apparatus for cleaving deoxyribonucleic acid by use of the method according to claim 1, characterized by the combination of : (a) a reactor (1) for subjecting a reaction mixture containing deoxyribonucleic acid to a chemical modification reaction, said reactor comprising washing means, which comprise a washing fluid tank (8) and a two-way valve (7) connected by a conduit to the reactor, to remove any chemical modification reagent from the adsorbent with a washing fluid, said reactor further comprising eluting means, which comprise an eluent tank (10) and a two-way valve (9) connected by a conduit to the reactor, for eluting chemically modified deoxyribonucleic acid from the adsorbent; (b) a transfer pump (4) connected by a conduit to said reactor for bringing said reaction mixture, having undergone a chemical modification reaction, into contact with an adsorber; (c) an adsorber (2) connected by a conduit to said transfer pump, said adsorber comprising the adsorbent for adsorbing and retaining the chemically modified deoxyribonucleic acid; (d) a concentrator (3) connected by a conduit to said adsorber for concentrating deoxyribonucleic acid eluates received from the adsorber, said concentrator further comprising concentrating means which comprise a vacuum pump (12), a trap (11) and a vacuum regulating valve (14) connected by a conduit to said concentrator, and a vibrator (13), said concentrator further comprising cleavage reagent supplying means, which comprise an aqueous cleavage reagent solution tank (16) and a two-way valve (15) connected by a conduit to said concentrator, for adding cleavage reagent to the concentrator, said concentrator further comprising heating means for heating the cleavage reagent and deoxyribonucleic acid eluates, whereby the deoxyribonucleic acid is cleaved at the position of the modified base; and (e) a three-way valve (5) which is operable to connect, during the adsorbing and the washing operations, by a conduit the adsorber to a waste liquid tank, and, during the eluting operation, to the concentrator. Apparatus as claimed in claim 2, wherein said adsorber is packed with an adsorbent selected from the group consisting of ion exchangers, nonionic porous resins and fine-granular materials adapted for reversed phase partition. Apparatus as claimed in claim 2, wherein said concentrator is adapted to be capable of controlling the concentration vessel at an arbitrarily selected temperature and subatmospheric pressure and of permitting rotation, revolution, precession and vibration of said vessel. Apparatus as claimed in claim 2, wherein the vessels of said reactor and said concentrator and said adsorbent of said adsorber are adapted to be exchangeable.

Inventors: KUSUHARA, NOBUMI, MINAMI, TAMOTSU, ONOFUSA, MITSUO, SASAKI, TOSIHIRO
Applicants: MITSUI TOATSU CHEMICALS, INC.
Application year: 1984
Main classification: C07H21
IPC codes: C07H21