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Abstract
Cell extracts of Streptomyces tendae grown in nikkomycin production media contained an enzyme (HisAT) that transaminated l-histidine as the sole amino substrate with pyruvate as the amino group acceptor. HisAT was purified about 190-fold from the crude extract of S. tendae. The enzyme was determined by gel filtration and SDS-PAGE to be a homodimer with a subunit molecular mass of approximately 45 kDa. The aminotransferase had maximum activity at pH 7·0 and 37 °C. The enzyme was highly specific for l-histidine; pyruvate, 2-oxobutyrate, 2-oxovalerate and 2-oxocaproate were used as keto acceptors to about the same extent. The reaction mechanism was ping-pong. The K m values for l-histidine and pyruvate, determined from Lineweaver-Burk plots, were 25 mm and 10 mm, respectively. Neither cell extracts of non-producing S. tendae mutants nor extracts of Streptomyces lividans, a species that does not synthesize nikkomycins, showed transaminating activity with a narrow substrate specificity for l-histidine as the amino donor. This strongly suggests that the formation of HisAT is essential for nikkomycin production.
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