The purF locus codes for the first enzyme, glutamine phosphoribosylpyrophosphate amidotransferase, of the purine biosynthetic pathway. A strain of Escherichia coli K-12 was isolated in which the lac structural genes were fused to the control region of the purF locus. This purF-lac fusion was shown to respond to purine-specific regulatory signals. A plaque-forming lambda transducing phage bearing this purF-lac fusion was isolated. This phage was used to genetically determine the direction of transcription for the pufF locus by two independent means. Results from both methods agreed that the direction of transcription of the purF locus was clockwise on the standard Escherichia coli K-12 genetic map.