Magnesium sensitivity of l-aspartate: 2-oxoglutarate aminotransferase in Escherichia coli

Abstract

Strains 619 and 961, derived from Escherichia coli K12, show growth inhibition by 0.4 M Mg 2+. The inhibition is overcome by l-aspartate. The related strain 977 exhibits Mg 2+, which arose concurrently with a mutation to argR − . Transduction experiments indicated that the genetic determinant for Mg 2+ sensitivity is closely linked to, but not identical with, argR. Since aspartate reverses the inhibition, aspartate aminotransferase ( l-aspartate:2-oxoglutarate aminotransferase, EC 2.6.1.1), an enzyme of aspartate synthesis, was examined. Incubation of 961 in a growth medium containing Mg 2+ leads to a sharp decrease in the specific activity of aspartate aminotransferase in 2 h, followed by a gradual increase. In similar experiments with 977, a greatly reduced drop and a much sharper rise occur. Extracted aspartate aminotransferase is virtually insensitive to Mg 2−. Extracts of Mg 2+-treated 961, on incubation at 37°C, show nearly full restoration of aspartate aminotransferase activity within several hours. The reversible inactivation appears to affect the finished enzyme, in the absence of protein synthesis, as demonstrated in cell suspension experiments. Under these conditions, the inactivating effect of Mg 2+ is enhanced by aspartate. Tests with growing cells suggest that aspartate may repress aspartate aminotransferase formation. Growth of 961 in the presence of 0.4 M Mg 2+ plus aspartate leads to relatively elevated levels of aspartate aminotransferase activity. In this case, on extractiion, no appreciable restoratiion occurs, and in cell suspension experiments, the aspartate aminotransferase shows no Mg 2+ sensitivity. The aminotransferase from Mg 2+ plus aspartate-grown cells and the normal enzyme are similar in affinity for aspartate, sedimentation behaviour, and inactivation at 60°C, but differ in their response to heat treatment at 50°C. It is thought that the two enzymes may be specified by the same gene, but may differ structurally, as a function of availability of Mg 2+ during their formation.