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Summary: Four independent porphobilinogen-accumulating mutants of Salmonella typhimurium lt2 were isolated by selecting for dwarf colony formation on neomycin agar media. Cell-free extracts of the parent strain, but not of the mutants, were able to convert 5-aminolaevulinic acid or porphobilinogen to porphyrins. The results indicated that the mutants were deficient in uroporphyrinogen I synthase (EC.4.3.1.8) activity: these are the first mutants of this type reported in S. typhimurium lt2. Mapping of the hemC locus (for uroporphyrinogen I synthase) by F-mediated conjugation and by P22-mediated transduction showed the gene sequence ilvEDAC-hemC-cya-metE.
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