| Abstract: | Inosine-producing cultures were found among mutants resistant to 6-mercaptoguanine (6MG) derived from a 5'-inosinic acid (IMP)-producing strain, KY 13102, of Brevibacterium ammoniagenes. Inosine-producing ability was very frequent among the mutants resistant to a low concentration (10 to 50 mug/ml) of 6MG. The accumulation of inosine by strain KY 13714 was stimulated by a low concentration of adenine (25 mg/liter) but was depressed by high levels of adenine. The accumulation by strain KY 13714 was not inhibited by manganese ion but instead was stimulated by its excess, in contrast to IMP accumulation by KY 13102. Addition of hypoxanthine at an early stage of cultivation accelerated inosine accumulation. Furthermore, on addition of hypoxanthine and of a surface-activating agent after 48 hr of cultivation, the simultaneous accumulation of IMP and inosine was observed. A 9.3-mg amount of inosine per ml accumulated after 4 days of cultivation at 30 C. The inosine-producing mutant did not differ from the IMP-producing strain either in 5' purine nucleotide degradation or in IMP formation from hypoxanthine. However, it was found to be completely devoid of purine nucleoside-degrading activity. The conversion of IMP accumulation to inosine can be explained by the lack of nucleosidedegrading activity. The relationship between deficiency of nucleoside-degrading activity and resistance to low levels of 6MG is discussed, and a new mechanism for 6MG resistance is presented. |
