Gibberellin, a plant hormone, has been produce industrially by Gibberellin fujikuroi. Continuous strain improvement of industrial microorganism is an essential role in the commercial success. The aim of this work was to improve gibberellic acid yield by mutagenesis of Gibberellin fujikuroi strain C by ultraviolet light and NTG using TLC and HPLC as primary and secondary screening, respectively. Correlation between morphological change of mutant strains and efficiency of GA(subscript 3) production was also studies. UV-28 was a mutant obtained by UV irradiation of spores of Gibberellin fujikuroi strain C. Maximum yield of GA(subscript 3) produce by UV4-28 was 5.1% higher than that by the original strain while the content of red pigment in fermentation borth was lower. Treatment of UV4-28 with NTG resulted in strain 08-19 capable to produce GA(subscript 3) at 722 mg/1 which was 32.2% higher than that by strain C. Strain N6-3, N7-54 and N9-34 were obtained by further treatment of strain 08-19 with NTG. GA(subscript 3) produce by these strains were 850, 852, and 884 mg/1, respectively which were 55.6, 56 and 62% higher than that by strain C, respectively. Cultivation of N9-34 in 5-L fermentor gave miximum GA(subscript 3) yield of 983 mg/1 on day 13 which was 11% higher than that shake flask. The present works also found that morphological can be used for primary selection. Most mutants having similar morphology to that of the original strain gave higher GA3 yield than those with morphological changes.
