Mutational analysis of Asp51 of Anabaena azollae glutamine synthetase. D51E mutation confers resistance to the active site inhibitors L-methionine-DL-sulfoximine and phosphinothricin

The role of Asp51 in the catalytic activity of glutamine synthetase from th e cyanobacterium Anabaena azollae has been analyzed. Five mutant enzymes. D 51S, D51A, D51E, D51N and D51R, were constructed by site-directed mutagenes is and characterized. Asp51 appears to participate in the binding of ammoni um ion, as affinity for this substrate was affected in all cases, although it varied according to the charge and/or size of the amino-acid residue, de creasing in the order Glu > Asn > Ser > Ala. The replacement of Asp51 by Gl u (D51E) conferred besides a high resistance to the herbicides L-methionine -DL-sulfoximine and phosphinothricin, as a result of a decreased phosphoryl ation ability.