Effect of ammonium ions on spiramycin biosynthesis in Streptomyces ambofaciens

The effect of ammonium on growth and spiramycin biosynthesis in Streptomyces ambofaciens cultured on a chemically defined medium was studied. Spiramycin biosynthesis was better in the presence of valine and isoleucine than in the presence of ammonium. This production was reduced in the presence of excess ammonium (100 mM). The addition of catabolic intermediates of valine and isoleucine reserved the negative effect of ammonium. Valine dehydrogenase (VDH), the enzyme responsible for valine, leucine and isoleucine catabolism, was repressed when excess ammonium was present in the medium. This repression was approximately 25% when the ammonium concentration was increased from 50 to 100 mM. In addition to the repression of VDH biosynthesis, ammonium inhibited the activity of this enzyme. This inhibition was 45 and 65% in the presence of 50 and 100 mM ammonium, respectively.