Nonidentity of the aspartate and the aromatic aminotransferase components of transaminase A in Escherichia coli

Abstract

Tyrosine, added to the growth medium of a strain of Escherichia coli K-12 lacking transaminase B, repressed the tyrosine, phenylalanine, and tryptophan aminotransferase activities while leaving the aspartate aminotransferase activity unchanged. This suggested that the aspartate and the aromatic aminotransferase activities, previously believed to reside in the same protein, viz. transaminase A, are actually nonidentical. Further experiments showed that, upon incubation at 55 C, the aspartate aminotransferase of crude extracts was almost completely stable, whereas the tyrosine and phenylalanine activities were rapidly inactivated. Apoenzyme formation was faster, and apoenzyme degradation proceeded more slowly with aspartate aminotransferase than with tyrosine aminotransferase. Electrophoresis in polyacrylamide gels separated the aminotransferases. A more rapidly moving band contained tyrosine, phenylalanine, and tryptophan aminotransferases, and a slower band contained aspartate aminotransferase. A mutant of E. coli K-12 with low levels of aspartate aminotransferase exhibited unchanged levels of tyrosine aminotransferase. Thus, transaminase A appears to be made up of at least two proteins: one of broad specificity whose synthesis is repressed by tyrosine and another, specific for aspartate, which is not subject to repression by amino acids. The apparent molecular weights of both the aspartate and the aromatic aminotransferases, determined by gel filtration, were about 100,000.