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

The pathway for histidine biosynthesis in Salmonella typhimurium, elucidated largely through the efforts of Dr. B. N. Ames and co-workers,1-5 is shown in Figure 1. The figure also shows that portion of the pathway for purine biosynthesis which involves the further metabolism of 4-amino-5-imidazole carboxamide ribonucleotide (phosphoribosyl-AIC).6 Dr. Phillip Hartman and his colleagues have shown that the structural genes for the enzymes catalyzing the reactions of the histidine pathway are localized in a small segment of the bacterial chromosome, the histidine operon.7' 8 The phenomenon of coordinate derepression 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 histidine available to the organism. Evidence presented by Martin10 indicates that the histidine operon is transcribed into a polycistronic messenger-RNA, and studies on polarity in the histidine operon7' 11,12 suggest that this messenger-RNA is translated from the end which