ISOLATION AND CHARACTERIZATION OF AN ESCHERICHIA-COLI SERYL-TRANSFER-RNA SYNTHETASE MUTANT WITH A LARGE INCREASE IN K-M FOR SERINE

A mutant of Escherichia coli resistant to serine hydroxamate which has a large increase in K-m for serine of seryl-tRNA synthetase is descri bed. The mutant serS gene was cloned and sequenced and was found to co ntain a single-base-pair mutation, resulting in the substitution of th e residue alanine 262 by valine in motif 2. The methyl side chain of a lanine 262 is not exposed at the active site, and molecular modeling i ndicated that replacement of alanine 262 by valine does not significan tly affect the configuration of amino acids at the active site. This f inding suggests that the residue at this position may be involved in a conformational change (possibly induced by ATP binding) which is nece ssary for optimal binding of the cognate amino acid.