The Threonine‐Sensitive Homoserine Dehydrogenase and Aspartokinase Activities of Escherichia coli 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 55000 as opposed to 86000 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 47000). 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.