Summary: To elucidate the repression mechanism of ammonium ions on the biosynthesis of tylosin in NRRL 2702, enzyme activities involved in the metabolism of the aspartate family of amino acids were evaluated in relation to the ammonium ion concentration and tylosin production. It was found that aspartate aminotransferase was essential for both cell growth and tylosin production. However, both threonine dehydratase and valine dehydrogenase were repressed by supplemented ammonium ions at concentrations higher than 50 mm. Threonine dehydratase was purified from cell-free extracts by acetone precipitation, ion-exchange chromatography and gel filtration, and its molecular mass was estimated to be 67200 Da. The optimum pH and temperature for threonine dehydratase activity were 7·5 and 25 °C, respectively, and the value for threonine under these optimum conditions was 21 mm. The inhibition pattern of ammonium ions on the activity of threonine dehydratase appeared to be a mixed type.


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