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The pathway for histidinle biosyinthesis in Salmonella typhimurium, elucidated largely through the efforts o:f Dr. B. N. Ames and co-workers, 1-5 is shown in Figure 1. The figure also shows that portioin of the pathway for puriine biosynthesis which involves the further metabolism of 4-amino-5-imidazole carboxamide riboinucleotide (phosphoribosyl-AIC).6 Dr. Phillip Hartmain and his colleagues have shown that the structural genes for the enzymes catalyziing the reactioins of the histidine pathway are localized in a small segment of the bacterial chromosome, the histidine operon.7' 8 The phenomenoni of coordiniate derepressioin of the histidine enzymes was originally described by Ames and Garry, 9 who found that the histidine operon functions as a single unit in response to the level of histidiine available to the organism. Evidence presented by Martin10 inidicates that the histidinie operon is transcribed into a polycistroiiic messenger-RNA, anid studies oIn polarity in the histidine operon7' 11,12 suggest that this messenger-RNA is trainslated from the end which

proceedings of the national academy of sciences of the united states of america

Chain initiation in a polycistronic message: sequential versus simultaneous derepression of the enzymes for histidine biosynthesis in Salmonella typhimurium.