Extracts from K-12 contained two distinct enzymes capable of catalysing the phosphorylation of hydroxymethylpyrimidine (HMP) to HMP monophosphate: pyridoxine kinase (EC and an enzyme that has not previously been genetically analysed, HMP kinase (EC Two distinct genes, and specify the activities of the former and latter enzymes, respectively. The inactivation of both genes by independent mutations in the same cell resulted in the complete loss of HMP kinase activity. Experiments with a series of strains that carry mutations in and revealed that the ability of the double mutant () to utilize HMP in thiamin pyrophosphate biosynthesis was restored by introducing the wild-type allele corresponding to the mutation. The locus was mapped on the chromosome near the and loci, which govern the activities of phosphomethylpyrimidine kinase (EC and hydroxyethylthiazole kinase (EC, respectively.


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