Thiophene biosynthesis in Tagetes roots: molecular versus metabolic regulation

Abstract

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 first and second heterocyclic ring by addition of reduced sulfur to two acetylenic groups of tridecapentaynene. Two presumptive intermediates in the sequence of reactions were isolated from a mutant of T. erecta with an altered thiophene spectrum. The compounds were purified by HPLC and identified by GC-MS and ¹H-NMR. One of them proved to be a monothiophene, the other a methylated bithienyl. The position of these thiophenes in the biosynthetic route was clarified by studying their conversion by Tagetes tissues. After formation of the monothiophene, the pathway branches. One branch leads to the major bithienyls in these species whereas the end product of the other branch are methylated forms which are only minor components in Tagetes. Substitutions at the vinyl end of the molecule presumably are the last reactions of both routes.

Because of the importance of sulfur addition in thiophene biosynthesis, its regulation was studied by varying the sulfur supply to isolated Tagetes patula roots. A forty-fold reduction in sulfate concentration in the medium relative to the standard level had no effect on root growth and development but severely slowed down thiophene accumulation. When roots cultured at low sulfate were returned to standard conditions, the rate of thiophene biosynthesis slowly increased. The rise was completely blocked by the transcription inhibitor cordycepin. It was inferred that the incorporation of sulfur is mainly regulated at the molecular level.