ABSTRACT
In bacteria, fungi, and plants, which can assimilate inorganic nitrogen, glutamine synthesis is usually the first stage in the assimilation of ammonium into organic nitrogen compounds, whereas in animals, which exist in a relative surplus of nitrogen, the primary function of enzyme glutamine synthetase (GS) in intermediary metabolism is to regulate the relative fluxes of carbon and nitrogen. In mammals, plasma concentrations of glutamine, at about 0.5mM, are higher than that of any other amino acid. The cloning of a GS gene can be greatly facilitated by gene amplification, especially if no partially homologous GS gene is available for use as a probe. The Chinese hamster GS gene was able to be cloned directly, without going through a cDNA intermediate, by utilizing a CHO cell line with a 1000-fold amplification of GS gene expression. The ease of selection of GS gene amplification in certain cell types has led to its exploitation in the expression of foreign genes in mammalian cells.
