Structural role for a conserved region in the CTP synthetase glutamine amide transfer domain

Site-directed mutations were introduced into a conserved region of the Escherichia coli CTP synthetase glutamine amide transfer domain. The amino acid replacements, valine 349 to serine, glycine 351 to alanine, glycine 352 to proline, and glycine 352 to cysteine, all increased the lability of CTP synthetase. The proline 352 replacement abolished the capacity to form the covalent glutaminyl-cysteine 379 catalytic intermediate, thus preventing glutamine amide transfer function; NH3-dependent CTP synthetase activity was retained. In CTP synthetase (serine 349), both glutamine and NH3-dependent activities were increased approximately 30% relative to that of the wild type. CTP synthetase mutants alanine 351 and cysteine 352 were not overproduced because of apparent instability and proteolytic degradation. We conclude that the conserved region between residues 346 and 355 in the CTP synthetase glutamine amide transfer domain has an important structural role.