The Threonine-Sensitive Homoserine Dehydrogenase and Aspartokinase Activities of <em>Eschericia coli</em> K12.

Aspartokinase I-homoserine dehydrogenase I from Escherichia coli K 12 was subjected to mild proteolysis. A fragment carrying only the desensitized homoserine dehydrogenase activity was purified. It is a dimer having a subunit molecular weight of 55 000 as opposed to 86 000 for the subunit of the native tetrameric enzyme. On the other hand, a threonine sensitive aspartokinase devoid of homoserine dehydrogenase activity was extracted from the mutant Gif 108. The purified protein is shown to have subunits shorter than those of the wild-type enzyme (molecular weight 47 000). It was shown that the two catalytic activities of aspartokinase I-homoserine dehydrogenase I are sequentially distributed on the single polypeptide chain: the aspartokinase activity is located in the amino-terminal section and the homoserine dehydrogenase in the carboxyl-terminal section. The results are discussed in relation to the configuration of the native enzyme and with the possible origin of the bifunctional protein. [ABSTRACT FROM AUTHOR]