BIOSYNTHESIS OF PHYTOSIDEROPHORES, MUGINEIC ACIDS, ASSOCIATED WITH METHIONINE CYCLING

The biosynthesis of 2'-deoxymugineic acid, a key phytosiderophore, was examined in association with the putative methionine recycling pathwa y in the roots of wheat using labeling experiments and structural anal ysis, Feeding with D-[1-C-13]ribose did not result in C-13 enrichment of 2'-deoxymugineic acid, while D-[2-C-13]ribose resulted in C-13 enri chment at the C-4'', -1, -4' positions, and D-[5-C-13]ribose did in C- 1', -4, and -1'' positions of 2'-deoxymugineic acid, respectively. Fur thermore, two isotope-labeled intermediates of the methionine recyclin g pathway, 5-[5-H-2(2)]methylthioribose and 2-[1-C-13]keto-4-methylthi obutyric acid, were synthesized, and their incorporation into 2'-deoxy mugineic acids was investigated. Six deuterium atoms at the C-4, -1', and -1'' positions of 2'-deoxymugineic acid were observed after feedin g with 5-[5-H-2(2)]methylthioribose. Feeding with 2-[1-C-13]keto-4-met hylthiobutyric acid yielded 2'-deoxymugineic acid enriched with C-13 a t the C-4', -1, and -4'' positions. These results demonstrated for the first time that the biosynthesis of 2'-deoxymugineic acid is associat ed with the methionine recycling pathway. This association system func tions to recycle methionine required for continued synthesis of mugine ic acids in the roots of gramineous plants.