Expression of three plant glutamine synthetase cDNA in Escherichia coli

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Abstract
Three cDNA clones encoding the closely related glutamine synthetase (GS) .alpha., .beta. and .gamma. polypeptides of Phaseolus vulgaris (French bean) were recombinantly expressed in Escherichia coli. The GS expression plasmids correctly synthesised the recombinant .alpha., .beta. and .gamma. polypeptides which then assembled into catalytically active homo-octameric isoenzymes. These isoenzymes behaved similarly to their native homologoues on ion-exchange and gel-filtration chromatography. Furthermore, the .alpha. and .gamma. isoenzymes complemented a GS(glnA)-deficient mutant, thus demonstrating their physiological activity in E. coli. Differences were observed between the three recombinant GS plasmids in their quantitative expression of the GS polypeptides and their ability to complement the E. coli mutant. These differences were correlated to the degree of solubility of the polypeptide, which was observed to be independent on the temperature of expression. The production of active GS isoenzymes in E. coli facilitates the isolation and characterisation of the individual P. vulgaris homo-octameric GS isoenzymes.