Phosphomethylpyrimidine synthase (, thiC (gene)) is an enzyme with systematic name 5-amino-1-(5-phospho-<small>D</small>-ribosyl)imidazole formate-lyase (decarboxylating, 4-amino-2-methyl-5-phosphomethylpyrimidine-forming). This enzyme catalyses the following chemical reaction
This enzyme binds a 4Fe-4S cluster.
The starting material is 5-aminoimidazole ribotide, which undergoes a rearrangement reaction via radical intermediates which incorporate the blue, green and red fragments shown into the product.