AROMATIC BIOSYNTHESIS

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Abstract
Participation of p-aminobenzoic acid [PABA] in the synthesis of vit. B12 is shown by the sparing effect of B12 on the PABA requirement of Escherichia coli mutants. Since the molar requirement of E. coli for B12 is only 2% of its requirement for PABA, the latter might conceivably participate in B12 synthesis either as a catalyst or as a precursor. Origin of the benzene ring of B12 from PABA would readily account for the failure of certain mutants blocked early in the synthesis of aromatic metabolites to require B12, since the quintuple supplement provided these strains contains PABA. Methionine and B12 spare PABA to a similar extent. Hence the relation of PABA to methionine synthesis appears to depend on the role of PABA in the formation of B12, which in turn takes part in the formation of methionine from homocysteine. Pteroylglutamic acid [PGA] and citrovorum factor cannot substitute for PABA in E. coli, even in the presence of B12. The reported ability of PGA to replace thymine as a partial substitute for PABA was not confirmed.