Abstract: Provided are compositions comprising polynucleotides encoding glutamine synthetase (GS) polypeptides having improved properties, such as increased enzymatic activity and/or increased thermostability. Also provided are recombinant DNA constructs, plants, plant cells, seed, grain comprising the polynucleotides. Additionally, various methods of employing the polynucleotides in plants, such as methods for increasing GS activity in a plant, methods for increase seed protein content in a seed of a plant, and methods for increasing yield of a plant, are also provided herein.