Expression of the guanine operon of Escherichia coli as analyzed by bacteriophage lambda induced mutations

Studies were made on two guanine-requiring strains of Escherichia coli isolated independently as a result of insertion of prophage gamma into one of the structural genes of the guanine operon. These mutants do not exhibit any detectable guaB function but express the guaA function constitutively at a low level, presumably due to transcription from the pI promoter on the prophage. Various types of plaque-forming gua-transducing phages were generated from these lysogens. The approximate location and the mode of substitution of the gua genes in the phage genome were determined. These results clearly indicate that the gene guaB is located closer to the operator-promoter region of the gua operon than is guaA, and the gene order is "operator"-guaB-guaA.