THIOPHENE BIOSYNTHESIS IN TAGETES ROOTS - MOLECULAR VERSUS METABOLIC-REGULATION

The biosynthesis of thiophenes from polyacetylenes and its regulation were studied in seedlings and transformed roots of Tagetes erecta and T. patula. The key steps in the conversion are the closure of the firs t and second heterocyclic ring by addition of reduced sulfur to two ac etylenic groups of tridecapentaynene. Two presumptive intermediates in the sequence of reactions were isolated from a mutant of T. erecta wi th an altered thiophene spectrum. The compounds were purified by HPLC and identified by GC-MS and H-1-NMR. One of them proved to be a monoth iophene, the other a methylated bithienyl. The position of these thiop henes in the biosynthetic route was clarified by studying their conver sion by Tagetes tissues. After formation of the monothiophene, the pat hway branches. One branch leads to the major bithienyls in these speci es whereas the end product of the other branch are methylated forms wh ich are only minor components in Tagetes. Substitutions at the vinyl e nd of the molecule presumably are the last reactions of both routes. B ecause of the importance of sulfur addition in thiophene biosynthesis, its regulation was studied by varying the sulfur supply to isolated T agetes patula roots. A forty-fold reduction in sulfate concentration i n the medium relative to the standard level had no effect on root grow th and development but severely slowed down thiophene accumulation. Wh en roots cultured at low sulfate were returned to standard conditions, the rate of thiophene biosynthesis slowly increased. The rise was com pletely blocked by the transcription inhibitor cordycepin. It was infe rred that the incorporation of sulfur is mainly regulated at the molec ular level.