Amino-3-cyano-5-methyl-6-methoxypyrazine 4-Oxide (11). A solution of 0.50 g of 2-amino-3-cyano-5-methylpyrazine 1,4-dioxide in 20 ml of dry methanol containing 0.43 g of sodium methoxide was heated under reflux overnight. The resulting brownish-red solution was evaporated to about half its volume under reduced pressure, cooled, and filtered. The collected solid was crystallized from acetic acid (charcoal) to give 0.25 g (46%) of colorless crystals: mp >235°dec; nmr (CF3COOH) 2.07 (3, s, C5-CH3), 3.71 (3, s, OCHs).2-Amino-3-cyano-5-methyl-6-guanidinopyrazine 4-Oxide (12). A mixture of 0.20 g of 2-amino-3-cyano-5-methyI-6-methoxypyrazine 4-oxide, 0.32 g of guanidine hydrochloride, and 0.23 g of sodium methoxide in 12 ml of dry methanol was heated under reflux for 14 hr. The yellow solid which had separated was collected by filtration and crystallized from glacial acetic acid. The resulting acetate salt of 12 was stirred with aqueous sodium bicarbonate to give a colorless solid which was crystallized from aqueous DMF: yield 0.13 g (57%); mp >280°dec; ir 2210 cm-1 (CN).

The photodimerization of 1.3-dimethyluracil in a variety of organic solvents is described. The reactions lead to the formation of the four possible cyclobutane-type dimers, the relative amounts depending on the solvent employed. The structures and stereochemistry of the dimers are described and a triplet state is proposed as their precursor.

An enzymatic activity that inserts purines into depurinated DNA was found in a soluble enzyme extract of Escherichia coli. This activity brings about the insertion of adenine and guanine into the appropriate apurinic sites in double-stranded DNA by using the corresponding deoxyribonucleoside triphosphates as the purine donors. Magnesium ions are required for this activity, it is inhibited by caffeine, and it does not act on depurinated single-stranded DNA. The insertion activity described here may represent a step in a repair mechanism, "base-insertion repair," whereby apurinic sites (which may occur in double-stranded DNA either due to the removal of damaged purines with specific glycosylases or by spontaneous depurination) are directly filled with the correct missing purine base.Damage to DNA in cells frequently takes the form of the removal of purines from the sugar-phosphate backbone, which leads to depurinated DNA (apDNA). Depurination of DNA may occur via one of three known pathways. These are spontaneous hydrolysis of the purine-sugar glycosylic bond (1), enhanced spontaneous hydrolysis of alkylated purines in DNA due to labilization of the glycosylic bond caused by the alkylation (e.g., the depurination of 3-methyladenine or 7-methylguanine) (2, 3), and the removal of alkylated purines from DNA catalyzed by specific glycosylases as was shown, for example, for 3-methyladenine or 0-6-methylguanine residues in DNA (4-6).According to current views, the damage caused by depurination is repaired by excision repair (5, 7). This pathway is initiated by an endonuclease specific for apurinic sites in DNA which causes a single-strand break in the DNA at the vicinity of the apurinic site. Subsequently, excision, polymerization, and ligation steps occur, leading to restoration of the DNA integrity. Endonucleases specific for apurinic sites have been purified from various sources (5,(8)(9)(10)(11)(12)(13), and the total repair of apurinic sites in DNA in vitro has been demonstrated with bacterial (14) and human enzymes (15).In this manuscript we present evidence for the existence of an enzymatic activity in a soluble enzyme extract of Escherichia coli that directly and specifically inserts the correct missing adenine or guanine into the appropriate apurinic sites in calf thymus and PM2 DNAs. This activity may represent a mechanism, "base-insertion repair," for repair of apurinic sites in DNA.MATERIALS AND METHODS [8-3H] by removal of the triphosphate moiety with bacterial alkaline phosphatase, followed by acid hydrolysis in 2% HCI for 15 min at 1000C. The resulting base and deoxyribose were separated either by high-voltage paper electrophoresis on Whatman no. 3MM paper, at pH 1.9 and 2500 V for 25 min, or by chromatography on Whatman no. 1 paper in n-butanol/water/concentrated ammonia (86:13:1, vol/vol). The paper was cut into 1-cm strips and radioactivity was measured. Ninety seven percent of the initial radioactivity was found in the spots corresponding to the base and the sugar. Thirty percent of the 14C l...

Abstract— Photochemical and γ‐ray‐induced reactions of purines with amines are described. These lead to the appropriate 8‐substituted purines. The photoproducts were obtained in yields of up to 100% by the use of a variety of photosensitizers, including peroxides. A free radical mechanism is proposed for these reactions. Such reactions may be relevant to the problem of the nature of DNA‐protein cross‐links induced by ultraviolet radiation.

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