L-Glutamate Enhances the Expression of Thermus Maltogenic Amylase in Escherichia coli

Escherichia coli BL21 (DE3) transformed with a thermostable Thermus maltogenic amylase (ThMA), isolated from a Gram-negative bacterium Thermus strain IM6501, grew well and efficiently produced ThMA in a complex medium but not in a chemically defined medium (DM). By supplementing L-glutamate to DM medium, both the specific growth rate and ThMA expression significantly increased. Alterations in the cellular responses of recombinant E. coli to L-glutamate were analyzed at the protein level by two-dimensional gel electrophoresis and mass spectrometry. The ppGpp synthase (RelA) was significantly reduced in cells grown with L-glutamate and was consistent with the low level of ppGpp, an indicator of stringent response. On the other hand, protein chain elongation factor (EF-Tu) and manganese-containing superoxide dismutase (MnSOD), which protects cells against oxidative damage, was significantly elevated by L-glutamate supplementation. These results indicate that L-glutamate enhances ThMA expression and increases the E. coli growth rate not only by overcoming the stringent response but also by increasing the synthesis of EF-Tu and MnSOD.