In order to establish industrial production of 5'-inosmic acid (5'-IMP), a permeability mutant, KY13171, of Brevibacterium ammoniagenes, which accumulated 7 to 8 grams of 5'-IMP per liter and 4 to 6 grams of hypoxanthine (Hx) per liter (calculated as 5'-IMP), was improved by a genetical procedure. Further improved mutants were selected stepwise through repeating mutational work. The finally selected mutant. KY13369, accumulated 20 to 27 grams of 5'-IMP per liter, but not Hx.
Increased productivity of 5'-IMP and decreased productivity of Hx were not caused by the changes in 5'-IMP degrading activity, because these activities were not significantly different among the mutants. These results appear to indicate that the increased accumulation of 5'-IMP may be caused by the improvement in membrane permeability for 5'-IMP. However, the changes in phospholipid and fatty acid compositions were not enough to explain the increased permeability.