RESEARCH N early 20% of the world's cultivated area is aff ected by salinity (Sairam and Tyagi, 2004). Salinity is caused by rock weathering, wind-transported materials, poor-quality irrigation water, and seawater intrusion onto land. The total saline area of North, Central, and South America is 15.8, 2.0, and 129.0 million ha, respectively (Rengasamy, 2006). Overall health of soybean [Glycine max (L.) Merr.] plants and seed yield are aff ected by saline conditions. The threshold salinity for soybean is 5.0 dS m-1 (Chinnusamy et al., 2005). Salt damage in soybean results from the accumulation of chloride in stems and leaves, which causes leaf necrosis; reduces greenness in leaves, plant biomass, plant height, leafl et size, seed yield, seed quality, and fi eld seed emergence; and increases plant mortality (Able and MacKenzie, 1964;
While investigating the effects of structural analogues of thymine on the thymine-requiring strain of E&cherichia coli 15 T we noticed the presence in the bacterial deoxyribonucleic acid of a new base which we have identified as 6-methylaminopurine. Although 6-methylaminopurine is usually present in small amounts (2% of the adenine) in the deoxyribonucleic acid of E. coli 15 Tits relative proportion increases considerably during growth of the bacteria in thymine-deficient conditions, obtained either by maintaining the thymine concentration in the medium at a low level, or by the addition of thymine antagonists such as 5-aminouracil or 2-thiothymine. Examination of a number of deoxyribonucleic acids has shown that 6-methylaminopurine also occurs in small proportions as a constituent of deoxyribonucleic acid from several bacteria and bacterial viruses. This paper describes the identification of 6-methylaminopurine, its deoxyribonucleoside and deoxyribonucleotide from deoxyribonucleic acid of E. coli 15 T, and the occurrence of the base in other deoxyribonucleic acids. Brief accounts of this work have appeared previously (Dunn & Smith, 1955a, b). MATERIALS 5-Aminoura¢il was obtained as a gift from Imperial Chemical Industries Ltd. and 2-thiothymine from Genatosan Products. 5-Methylaminouracil was prepared according to Johnson & Matsuo (1919) and 5-dimethylaminouracil according to Wheeler & Jamieson (1904). 6-Methylaminopurine, 6-methylpurine, and 8-methylpurine were obtained from Dr D. J. Brown, 2-methyladenine from Dr D. M. Brown and 6-dimethylaminopurine from the American Cyanamid Co. 5-Amino-4-methyluracil. This was prepared from 4methyluracil (L. Light and Co.) according to Behrend (1885). The product gave a single ultraviolet-absorbing spot in solvents 1-5. 5-Amino-l-methyluracil. 1-Methyluracil was prepared by essentially the method which has since been described by Brown, Hoerger & Mason (1955a). It was crystallized from acetone and its spectrum was identical with that of 1-methyluracil (Shugar & Fox, 1952). The 1-methyluracil was converted into the 5-bromo derivative by treatment with Br. in CS2 (Wheeler & Merriam, 1903), and this was crystallized from water. 5-Bromo-l-methyluracil was converted into 5-amino-1-methyluracil by heating with excess of NH3 in a sealed tube at 1500 for 2 hr. (Wheeler & Johnson, 1904). This was not crystallized, but paper chromatography showed that there was only one major ultraviolet-absorbing product. Spectral data were obtained from chromatographically purified samples. 4-Amino-5-metthyluracil. This was prepared by a method essentially the same as that described by Bergmann & Johnson (1933), but the intermediate methylcyanacetyl urea was not isolated. Instead, the conditions used by Conrad (1905) for preparing 4-aminouracil were applied. 4-Amino-5-methyluracil separated out as the red sodium salt. After washing this with ethanol the free base was released by the addition of HCI and was crystallized from water at pH 7. As the product contained a small proportion of 4-aminouracil...
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