cis-Dominant, glutamine synthetase constitutive mutations of Escherichia coli independent of activation by the glnG and glnF products

Summary Mutants resistant to 80 M L-methionine-DL-sulfoximine (MS) were isolated on glucose-minimal 15 mM NH₊⁴medium plates from Escherichia coli cells which were hypersensitive to this concentration of the analogue by virtue of their harboring glnG mutations. MS-resistant mutants derived from strain MX902 carried, in addition to its glnG74::Tn5 allele, mutations tightly linked to glnA, as shown by P1-mediated transduction experiments. One particular allele, gln-76, which suppressed the MS-sensitivity conferred by glnG74::Tn5 but not its Ntr^– phenotype (inability to transport and utilize compounds such as arginine or proline as the only nitrogen sources), was shown to allow constitutive expression of glutamine synthetase in the absence not only of a functional glnG product but also of a functional glnF product. This behavior was found to be cis-dominant in complementation experiments with F'14 merogenotes. In an otherwise wild-type genetic background as in MX929 (gln-76 glnA⁺ glnL⁺ glnG⁺ glnF⁺), however, normal activation, mediated by the glnG and glnF products was preferred over that mediated by gln-76.