FR-900520 and FR-900523, novel immunosuppressants isolated from a Streptomyces. I. Taxonomy of the producing strain.

A new subspecies of the genus Streptomyces, the proposed name of which is Streptomyces hygroscopicus subsp. yakushimaensis subsp. nov., is described. Soil isolate, strain No. 7238, produces the novel immunosuppressants, FR-900520 and FR-900523. The organism is characterized by its gray aerial mycelium color, hygroscopic spore mass and spiral spore chains with warty or spiny spore surfaces. It is nonchromogenic. Strain No. 7238 shows characteristics most closely related to Streptomyces antimycoticus and S. hygroscopicus, although there are differences in physiological characteristics and carbohydrate utilization. In terms of morphological characteristics, strain No. 7238 is different from S. antimycoticus, but resembles S. hygroscopicus. The differences are not sufficient to establish a new species. It would be most suitable to designate strain No. 7238 as a new subspecies within the species of S. hygroscopicus.