Isolation and characterization of regulatory mutations affecting the expression of the guaBA operon of Escherichia coli K-12.

We isolated strains of Escherichia coli K 12 in which the lac structural genes were fused to the structural genes of the guaBA operon. These strains were used to isolate regulatory mutations that increased the expression of the guaBA operon under normal repressing conditions as compared to the wild type parental fusion strain. Three classes of guaBA specific regulatory mutations were identified. Class I regulatory mutations were trans-acting and unlinked to the guaBA operon as shown by bacteriophage P1 transduction. Class II regulatory mutations were tightly linked to the guaBA operon, cis-dominant to the wild type allele in a cis-trans analysis and were regarded as control region mutations. Class III regulatory mutations were tightly linked to the guaBA operon and trans-recessive to the wild type allele in a cis-trans analysis. We have designated the locus responsible for the class III regulatory mutations as guaR. The guaR locus is tightly linked and was mapped to the counterclockwise side of the guaBA operon. The guaR locus is proposed to specify a trans acting regulatory element involved in the regulation of the guaBA operon.