Also known as gamma-glutamylcysteine synthetase and gamma-ECS (EC:126.96.36.199). This enzyme catalyzes the first and rate limiting step in de novo glutathione biosynthesis. Members of this family are found in archaea, bacteria and plants. May and Leaver discuss the possible evolutionary origins of glutamate-cysteine ligase enzymes in different organisms and suggest that it evolved independently in different eukaryotes, from an ancestral bacterial enzyme. They also state that Arabidopsis thaliana gamma-glutamylcysteine synthetase is structurally unrelated to mammalian, yeast and Escherichia coli homologs. In plants, there are separate cytosolic and chloroplast forms of the enzyme.