Microbial transformation of ent-kaurenoic acid and its 15-hydroxy derivatives by the SG138 mutant of Gibberella fujikuroi

Feeding experiments with ent-kaurenoic acid (4), 15 alpha -hydroxy-ent-kaur enoic acid (5), 15 beta -hydroxy-ent-kaurenoic acid (6), and mixtures of 4 plus 5 and 4 plus 6 were conducted using the SG138 mutant of Gibberella fuj ikuroi, to gain information about the phenotype of this unique strain. The biotransformation of 5 gave 7 beta ,15 alpha -dihydroxykaurenolide (9) and 7 beta ,15 alpha -dihydroxy-ent-kaurenoic acid (13). The incubation of 6 pr oduced 7 beta ,15 beta -dihydroxy-ent-kaurenoic acid (7) and 7 beta ,15 bet a -dihydroxykaurenolide (10). No 15-hydroxylated gibberellins were detected in any of these experiments. The results indicated that a hydroxy group at C-15 does not inhibit 7 beta -hydroxylase activity but in the SG138 strain obstructs the enzymatic ring-B contraction of ent-kaurenoids to gibberelli ns. Exogenous 4 stimulated both the excretion of ent-kaurene and the fungal metabolism of 5 and 6.