Gamma-glutamyl transpeptidase from two plant growth promoting rhizosphere fluorescent pseudomonads

Glutathione is the most abundant non-protein thiol compound present in many cells. Because this molecule is involved in many physiological processes, each cell maintains a critical level of glutathione. Gamma-glutamyl transpeptidase (GGT, E.C.2.3.2.2) is the key enzyme involved in the glutathione cycle. In the present study, GGT was isolated from two plant growth promoting rhizosphere isolates, Pseudomonas protegens strain Pf-5 and Pseudomonas fluorescens strain PfT-1. GGT in these strains is located in the periplasm and possessed good hydrolytic activity at pH 8.0. Strains Pf-5 and PfT-1 showed maximum enzyme activity when grown at 30–35 °C. The ggt gene from both the strains was cloned in pGEM-T cloning vector and sequenced. Subsequently, GGT expressed in Escherichia coli BL21(DE3) using the pET-28a(+) expression vector was purified and characterized. The enzymes are active in a wide range of pH and some divalent cations significantly enhanced the hydrolytic activity. These enzymes showed higher thermal stability as compared to those of other mesophilic strains, as they retained ~50 % of activity at 50 °C even after 12 h of incubation. The enzymes could also tolerate up to 3.0 M NaCl.