text: In bacteria, HMP-P arises by conversion of the purine biosynthetic precursor 5-aminoimidazole ribotide (AIR) through the action of enzymes such as phosphomethylpyrimidine synthase, a member of the radical SAM superfamily. Studies using isotopically labelled AIR have shown which atoms carry into the product. Mechanisms by which this occurs are not yet known with certainty. 
  In yeasts, HMP-P is derived from metabolites of histidine and pyridoxine. Some of these transformations appear to be catalyzed by radical SAM enzymes. Isotopically labelled precursors have been used to investigate this biogenesis. Mechanisms of the transformations are unknown. 
  In Salmonella, HMP-P can be derived independently of purine biogenesis when AICAR is available.  
  In algae, thiamine forms and precursors are scavenged by uptake from water of exogenous products from other organisms. In higher plants, thiamine biogenesis resembles that of bacteria. In some circumstances, thiamine forms and precursors may be obtained through symbiotic relationships with microorganisms in the soil. 
  Genes relevant for transformations in the biogenesis of HMP-P, HET-P, and TPP have been identified in various organisms and some of the proteins resulting from their expression have been characterized. Biosynthesis of TPP is feedback inhibited through actions of a riboswitch.
  Research is ongoing towards understanding biochemistry involved and towards facilitating technologies of socioeconomic value for supply of thiamine in various forms.