Glutamine synthetase (GS; EC 6.3.1.2, L-glutamate: ammonia ligase) is an essential enzyme in nitrogen assimilation. It catalyzes glutamine synthesis using glutamate and ammonium with ATP hydrolysis. Four forms of GSs have been described in literature. These enzyme types are discriminated based on their primary and quaternary structures. GS-encoding genes are believed to be of the oldest functioning genes studied, and its evolutionary history was explored in classic studies in the 90s. Here, we evaluated GS-homologous sequences from the three life domains to revisit their origins and evolutionary history. There are clear examples of ancient duplications and interdomain horizontal gene transfers. We present GS-encoding genes as one multigenic family that comprises three distinct groups. Our findings are presented in light of two main hypotheses for GS origins and evolutions, and we argue in favor of gene duplications giving rise to the three genes in the Last Universal Common Ancestral. Type I family is the most diverse one, presenting a subgroup of polyamine metabolizing enzymes, besides many examples of noncatalytic GS homologs. Many instances of gene loss, duplication, and transfer have occurred after life diversification, contributing to GS complex evolutionary history.
Keywords: Gene evolution; glnA; glnII; glnN; glnT; nitrogen.
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