Biosynthesis of thiamin under anaerobic conditions in Saccharomyces cerevisiae.

We studied the biosynthetic route of thiamin in Saccharomyces cerevisiae to see whether the route differed under aerobic and anaerobic conditions. Histidine and pyridoxine are the precursors of the pyrimidine moiety of thiamin under aerobic conditions. Formate is incorporated into the pyrimidine via histidine. The incorporation of [13C]formate and [5'-(2)H2]pyridoxine into the pyrimidine was examined under anaerobic conditions. The labels from [13C]formate and [5'-(2)H2]pyridoxine were not incorporated into the pyrimidine under anaerobic conditions, indicating that the biosynthetic pathway of the pyrimidine differed from that under aerobic conditions. On the other hand, [15N]glycine was incorporated into the thiazole under both anaerobic and aerobic conditions. The biosynthetic pathway of the thiazole was therefore unaltered by the O2 concentration.