BIOTECHNOLOGY LETTERS Volume 17 No.1 (January 1995) p.31-34 Received 14th December THE INDUCTION OF VALINE DEHYDROCENASE ACTIVITY FROM STREPTOMYCES BY L-VALINE IS NOT REPRESSED BY AMMONIUM Kien Trung Nguyen l , lieu Thi Nguyen, and Wadishv Behal Institute of Microbiology, Academy of Sciences of the Czech Republic, VideAska 1083,142 20 Prague 4, Czech Republic. SUMMARY Activity of valine dehydrogenases (VDH) from Stmptoomyces aureofaciens and S. fradiae is strongly induced by L-valine even in the presence of 25mM NH,+. When added into 16 h-old cultures growing with 100mM NH,+, L-valine induced the synthesis of VDH. The results indicate that Streptomyces can utilize L-valine in the presence of NH,‘, and the induction of VDH activity by L-valine is not repressed by NH,+. INTRODUCTION Valine catabolism is the major source providing n-butyrate, propionate and methylmalonate units for the biosynthesis of macrolide and polyether antibiotics in StrePtomyces (Omura et al., 1983; PospfXiI et al., 1983; Omura et al., 1986; Tang et al., 1994). Activity of valine dehydrogenase (VDH), the first enzyme of the valine catabolic pathway is strongly induced by valine but not by ammonium (Pristley and Robinson, 1989; Navarrete et al., 19901, except S. aweofaciens (VanCurovd d a/., 1988). Ammonium ion inhibits the induction of VDH activity by L-valine in slowly growing S. coelicolor cells (Navarrete et al., 1990), and represses VDH activity in S. fradiae cells growing in ammonium containing medium, resulting in a limited supply of tylosin units (Omura et a1.,1983; 1986). However, it is unclear whether ammonium ion represses VDH activity in Streptomyces cells actively growing in valine-supplemented medium, which is usually used for tylosin and monensin production (Navarrete et al., 1990). In the present study, we studied the effect of ammonia on the induction of VDH activity by L-valine in S. aureofaciens and S. fradiae cells exponentially growing in a chemically defined medium. 31