The regulation of glutamate and melibiose utilisation in Escherichia coli
Wild type cells of Escherichia coli K-12 are unable to grow on glutamate as the sole source of carbon. Glutamate-utilising mutants have been isolated previously and found to exhibit enhanced glutamate transport. Mutations at two loci gltS and gltR and possibly a third gltC were previously shown by other workers to mediate enhanced glutamate permeability. The gltR locus was suggested to be a negative regulatory for the gltS gene. A mutation at the gltS locus, the gltSo mutation increases the activity of a Na+-stimulated glutamate transport system GltI. The regulation of the GltI system was investigated by the isolation of MudX gene fusions which abolished Na+-stimulated glutamate transport. Two fusions with 'strong' and 'weak' B-galactosidase activities were isolated. These were both found to map at the gltS locus. Subsequent mapping exercises, suggested that the MudX fusions may be located in separate genes. Transcription of the 'strong' fusion was found to be impaired in the presence of the phs mutation. This is a mutation in the rpoA gene which encodes the -subunit of RNA polymerase. Glutamate-utilising suppressor mutants were isolated in both the MudX gene fusion strains. The suppressor mutations were found to map at the gltR locus. This was found to be the map location of a second glutamate transport system GltII. Thus, the gltR locus was found not to be the location of a negative regulator for the gltS gene. The melAB operon encodes the proteins for melibiose transport and utilisation. No regulatory locus has previously been reported for this transport system. The regulation of the melAB operon was investigated by cloning the melAB promoter into an lacZ expression vector. The region upstream from the melAB promoter was subsequently found to encode a trans-acting positive regulatory necessary for melAB expression. Transcription from the melAB promoter was also found to be impaired by the phs mutation. The results presented in this study substantiate previous observations that the phs mutation causes a generalised transcription defect.