Abstract: In order to verify the function of genD2, a recombinant plasmid pDB303 derivated from pKC1139 was constructed for blocking-up genD2. pDB303 was introduced into Micromonospora purpurea G1008 by conjugation. A desired mutant strain GD238 was obtained by PCR analysis. Antibiotics were isolated from the fermentation broth of GD238, the parent strain G1008 as the control. The result of mass spactral analysis indicated that the mutant strain GD238 only produce gentamicinA2 and A2e instead of gentamicinC complex, and that genD2 might be responsible for dehydrogenation at C-3″of garosamine.