SUMMARY: Approximately 500 vitamin B6 auxotrophs were isolated from 18 independent cultures of strain CR63. None grew in minimal medium supplemented with 2′-hydroxy-pyridoxine. Eighteen auxotrophs which had arisen independently were further characterized. All of them were defective in vitamin B6 synthesis rather than in an aminotransferase involved in vitamin B6 utilization. Two different phenotypes were recognized: ‘Oxidase’ mutants which grew only when supplied with pyridoxal or pyridoxal 5′-phosphate and ‘Pre Pn’ mutants which would also grow with pyridoxine or pyridoxine phosphate. ‘Oxidase’ mutants were confined to a single linkage group, but data from interrupted mating experiments established that ‘Pre Pn’ mutants fall into two linkage groups which are possibly identical to and . All mutations in the region were allelic rather than located in two closely linked genes.


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