The use of metabolic selection markers has advanced stable cell line generation, increasing productivity while simultaneously eliminating the need for antibiotic reagents. This study explores the potential of bacterially derived glutamine synthetases (GS) as a novel generation of metabolic selection markers to further enhance CHO cell culture performance. GS-I proteins were extracted from the genomes of enterobacterial and actinomycetes species. Three of these enzymes demonstrated functionality when stably transfected into GS-deficient CHO cells, leading to a 3- to 4-fold increase in antibody titer compared to endogenous GS from Cricetulus griseus. This study indicates that the functionality of bacterial GS enzymes in mammalian cells is determined by solvent accessibility and the geometry of the catalytic binding pocket. Dysfunctional variants showed a less accessible bifunnel. Bacterial GS were evaluated for their bioprocess performance leading to superior stable pool and clone performance. Transcriptome analysis further revealed that regulatory cellular mechanisms were decoupled in a cross-species set-up, reinforcing the suitability of repurposing bacterial enzymes as selection markers in mammalian cell lines. By modulating the selection stringency, an increase in expression performance was achieved without impairing the bioprocess behavior or long-term cell line stability.