-!- The enzyme catalyzes the penultimate reaction in thiamine de novo biosynthesis, condensing the pyrimidine and thiazole components. -!- The enzyme is thought to accept the product of EC 2.8.1.10 as its substrate. -!- However, it has been shown that in some bacteria, such as Bacillus subtilis, an additional enzyme, EC 5.3.99.10 converts that compound into its tautomer 2-(2-carboxy-4-methylthiazol-5-yl)ethyl phosphate, and that it is the latter that serves as the substrate for the synthase. -!- In addition to this activity, the enzyme participates in a salvage pathway, acting on 4-methyl-5-(2-phosphono-oxyethyl)thiazole, which is produced from thiamine degradation products. -!- In yeast this activity is found in a bifunctional enzyme and in the plant Arabidopsis thaliana the activity is part of a trifunctional enzyme.