The biosynthesis of the imidazole ring of histidine.

The formation of 5-amino-1-ribosyl-4-imidazolecarboxamide 5’-phosphate by bacterial extracts incubated with adenosine triphosphate, an ATP-generating system, ribose-5-P, and glutamine has been reported by Love (1) and by Gots and Gollub (2). We have been able to confirm these observations; in addition we find that a derivative of ACP,1 rather than ACP itself, is formed when glutamine is omitted (3). There is no evidence for the participation of such a compound in the sequence of reactions responsible for the synthesis d.e noao of the purine precursor, ACP (4). This finding suggested that the observed formation of ACP from ATP, ribosed-P, and glutamine is the result of reactions other than those involved in the biosynthesis of purines. Such an interpretation is also supported by these additional observations: (a) Dialyzed bacterial extracts are able to produce ACP in the absence of added formate, COZ, and glycine, compounds which are all necessary for its synthesis de wo (2). (b) Added glycine-2-Cl4 is not incorporated into the ACP produced.2 (c) ACP is produced by extracts of a mutant which lacks the ability to convert 5-amino-1-ribosylimidazole 5’-phosphate to ACP. an essential reaction in the biosynthesis of purines (1). A possible source of the ACP produced under these circumstances is ATP itself. The hypothesis that ACP is indeed derived from ATP by removal of a carbon atom and a nitrogen atom was confirmed by the demonstration that added AMP-& C14, which equilibrates readily with ATP, was incorporated into the ACP without dilution. Furthermore, earlier experiments in this (5,6) and other laboratories (7,s) had shown that bacteria are capable of obtaining the N&2 portion of the imidazole ring of histidine from carbon 2 and an attached nitrogen atom of purine bases provided in the growth medium. The possibility therefore had to be considered that the conversion of ATP to ACP was involved in the biosynthesis of the imidazole ring of histidine. This assumption was substantiated by the finding that in addition to ACP a histidine precursor, n-erythro-imidazoleglycerol phosphate ester, is a product of these reactions. The nature of these enzymatic steps and the evidence for their role in histidine biosynthesis are the subject of thii paper.