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.