Regulation of valine catabolism by ammonium inStreptomyces ambofaciens, producer of spiramycin

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Abstract
In Streptomyces ambofaciens, valine favored spiramycin biosynthesis by supplying aglycone precursors. The kinetics of valine consumption and isobutyrate production showed that isobutyrate accumulated in the cell during the growth phase, was excreted in the stationary phase, and then was reassimilated during spiramycin production. When valine was in excess, its deamination led to high ammonium excretion and to a significant drop in spiramycin production. We demonstrated that ammonium ions were the cause of the negative effect. Addition of a chelator agent, Ca3(PO4)2, improved spiramycin production by sixfold. In contrast, addition of ammonium, between 0 and 48 h, severely reduced spiramycin production. The negative effect of ammonium was reversed by addition of a catabolic intermediate of valine, isobutyrate. In addition to stimulating the specific growth rate, ammonium ions slowed down valine catabolism: the specific valine uptake rate, excretion, and reassimilation of isobutyrate were lowered by the pulse of ammonium. Our study showed that in addition to valine dehydrogenase, which provided the nitrogen necessary to the cell, ammonium ions repressed ketoisovalerate dehydrogenase, which introduced valine as carbon, energy, and aglycone precursor sources. However, valine dehydrogenase and ketoisovalerate dehydrogenase did not constitute the principal enzymatic targets of the negative effect of ammonium in spiramycin production.Key words: spiramycin, Streptomyces ambofaciens, valine catabolism, ammonium.