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Ribonucleic acid (RNA) isolated from Escherichia coli W3350 (F − , argE + C + B + H + ), in the absence of l -arginine, hybridizes with the separated leftward ( l ) and rightward ( r ) transcribing strands of the arginine transducing phage hφ80 dargE + C + B + H + ppc + imm λc I 857 deoxyribonucleic acid (DNA) with a ratio of 30:70, respectively. In the presence of l -arginine and its intermediates, l -ornithine and l -citrulline, RNA transcriptions from both the strands of the argECBH cluster were repressed. The derepressed RNA, when hybridized with the separated strands of hφ80 dargEC -I imm λ phage DNA (the arginine genes are inversely inserted in this phage), which has a deletion in gene E and extends to gene C of the argECBH cluster, showed no leftward transcription, whereas the rightward transcription was reduced to about 40% of that when the DNA carrying the entire ECBH cluster was used for hybridization. The hybridization results thus demonstrate that (i) the regulation of the argECBH gene cluster in E. coli is under transcriptional control, (ii) the orientation of transcription is divergent, (iii) E gene transcribes anticlockwise, whereas the rest of the genes, C, B , and H , transcribe clockwise, and (iv) the position of the promoter(s) and operator(s) is located between the E and C genes of the argECBH cluster.