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J. Biochem, 1990, Vol. 108, No. 1 116-121
© 1990 Japanese Biochemical Society

research-article

Temperature-Sensitive Mutant of Bacillus subtilis Glutamine Synthetase Obtained by Random Mutation

Yoshio Nakano and Kinuko Kimura

Laboratory of Biochemistry, College of Science, Rikkyo (St. Paul's) University Toshima-ku, Tokyo 171

Random mutations were introduced into the B. subtills glutamine synthetase gene by using nitrous acid, and a high temperature-sensitive mutant was selected. DNA sequencing of the restriction fragment containing the mutation revealed a single base-pair change resulting in the substitution of Leu 318 with Phe. The mutant enzyme was purified, and its kinetic and physical properties were characterized. The Mg2+-dependent activity and Mg2+ plus Mn2+-dependent activity of the mutant were less than 5% of those of the wild-type at 37%C, and these activities decreased above 15%C, whereas the Mn2+-dependent activity was nearly normal. Affinity of the mutant enzyme for glutamate was extremely decreased although the Km values for NH3 or ATP were almost the same as those of the wild-type. The mutant enzyme was more susceptible than the wild-type enzyme to digestion with chymotrypsin in the presence of glutamate, ATP, and Mg2+, although addition of glutamate, ATP, and Mn2+ completely protected both enzymes. These results and circular dichroism analyses suggested that Leu 318 is at the glutamate-binding site and that the substitution of Leu 318 for Phe reduces the ability of the enzyme to form the enzyme-substrate complex, probably supported by Mg2+.


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