Allows the formation of correctly charged Gln-tRNA(Gln) through the transamidation of misacylated Glu-tRNA(Gln) in the mitochondria. The reaction takes place in the presence of glutamine and ATP through an activated gamma-phospho-Glu-tRNA(Gln). ATP + H2O + L-glutamine + L-glutamyl-tRNA(Gln) = ADP + H(+) + L-glutamate + L-glutaminyl-tRNA(Gln) + phosphate Subunit of the heterotrimeric GatCAB amidotransferase (AdT) complex, composed of A (QRSL1), B (GATB) and C (GATC) subunits. Predominantly expressed in tissues characterized by high rates of oxidative phosphorylation (OxPhos), including muscle and heart. The disease may be caused by variants affecting the gene represented in this entry. Belongs to the GatB/GatE family. GatB subfamily.