Responsible for the formation of the pyrimidine heterocycle in the thiamine biosynthesis pathway. Catalyzes the formation of hydroxymethylpyrimidine phosphate (HMP-P) from histidine and pyridoxal phosphate (PLP). The protein uses PLP and the active site histidine to form HMP-P, generating an inactive enzyme. The enzyme can only undergo a single turnover, which suggests it is a suicide enzyme. 2 Fe(3+) + 4 H2O + L-histidyl-[4-amino-5-hydroxymethyl-2-methylpyrimidine phosphate synthase] + N(6)-(pyridoxal phosphate)-L-lysyl-[4-amino-5-hydroxymethyl-2-methylpyrimidine phosphate synthase] = (2S)-2-amino-5-hydroxy-4-oxopentanoyl-[4-amino-5-hydroxymethyl-2-methylpyrimidine phosphate synthase] + 3-oxopropanoate + 4-amino-2-methyl-5-(phosphooxymethyl)pyrimidine + 2 Fe(2+) + 2 H(+) + L-lysyl-[4-amino-5-hydroxymethyl-2-methylpyrimidine phosphate synthase] Cofactor biosynthesis; thiamine diphosphate biosynthesis. Homodimer. Belongs to the NMT1/THI5 family.