Decrease de novo purine synthesis B.
The de novo pathway leading to the synthesis of AMP and GMP begins with the transfer of an amido group from glutamine to PRPP (). Since PRPP is used for the both de novo and salvage synthesis of purine and pyrimidine nucleotides as well as for the synthesis of NAD, histidine and tryptophan, any stress that alters PRPP availability affects multiple pathways. Given the essential nature of PRPP, all free-living organisms contain at least one gene encoding PRPP synthetase (PRS; EC 220.127.116.11) (). Recently four PRS cDNAs were isolated from Arabidopsis by functional complementation of an Escherichia coli mutant lacking PRS activity (Genbank accessions X83764 (PRS1), X92974 (PRS2), AJ012406 (PRS3), AJ012407 (PRS4)) (). A fifth PRS sequence that predicts a protein with high amino acid sequence homology to the other Arabidopsis PRSs, particularly with PRS1 and 2 was located on chromosome II and designated PRS5 (). Kinetic characterization of the gene products of PRS1-4 indicated that PRS3 and 4 represent a novel class of PRSs since their activities are independent of inorganic phosphate (Pi). A phylogenetic comparison of 45 putative PRS amino acid sequences from 31 organisms also suggested that PRS3 and 4 are a divergent family, distinct from any of the other PRS sequences (). The subcellular localization of the Arabidopsis PRSs has not yet been determined but may resemble the compartmentation of spinach PRSs that have been localized to the cytosol, chloroplast and mitochondria ().
Purine and pyrimidine metabolism.
Synthesis of Purine Ribonucleotides
The enzymes that salvage adenine (Ade) and adenosine (Ado) have been studied to a greater extent than other purine enzymes, in part because the salvage cycle is thought to contribute also to the metabolism of cytokinins (CKs; ). Since CK bases/nucleosides are proposed to be the active form of this growth regulator, metabolism by the salvage cycle enzymes could affect the level of active hormone in a plant cell. Although progress has been made on characterizing adenine salvage metabolism, it remains unclear as to whether plant cells utilize these enzymes for CK interconversion in vivo. Further genetic analysis coupled with more sensitive measurements of the CK constituents in relevant mutants will be necessary to resolve this issue.
Synthesis of Pyrimidine Ribonucleotides
Scavennec J, Maraninchi D, Gastaut JA,Carcassonne Y and Cailla HL: Purine and pyrimidine ribonucleosidemonophosphate patterns of peripheral blood and bone marrow cells inhuman acute leukemias. Cancer Res. 42:1326–1330. 1982.