Mutation affecting the charging reaction of alanyl-tRNA synthetase from Escherichia coli K 10

Summary

A temperature-sensitive mutant of Escherichia coli was identified as having an altered alanyl-tRNA synthetase. Specific activity of wild type and mutant cell-free extracts showed no difference in the hydroxamate assay; the charging activity, however, was more than 10 fold lower for mutant extract protein. Wild type alanyl-tRNA synthetase has been purified 344 fold, the mutant enzyme was enriched 45 fold. With these preparations the following results were obtained:

Sedimentation analysis in sucrose gradients indicates a molecular weight of the mutant enzyme of half the size of the wild type enzyme. Analytical gel filtration yields an approximate size for the native enzyme of 165000 and for the mutant enzyme material of 95,000. The mutant alanyl-tRNA synthetase differs from the wild type enzyme by a 10 fold increase in the k _mfor tRNA; no true difference in the k _m-values for the other substrates was detected. Temperature studies indicate an unusual low temperature-optimum for the charging reaction of both enzymes, whereas hydroxamate fromation capacity increases linearly up to almost 50°C. High temperature treatment of the native enzyme selectively affects the aminoacylation reaction but not the activation step; no effect of such treatment of the mutant enzyme was detected. It is proposed that the mutation causes the enzyme to dissociate and that the resulting subunits possess and altered tRNA binding site.