SUMMARY: synthesizes pyrimidines using a pathway similar to that established in Soluble and bound forms of dihydro-orotate dehydrogenase are present. Both forms are active in 2,6-dichloro-phenolindophenol reduction assays; that the particulate enzyme normally links to oxygen via a cytochrome system was shown in studies of a haem-requiring mutant.

Activities of the enzymes of pyrimidine synthesis are six- to 20-fold greater in anaerobically or semi-anaerobically grown bacteria than in those grown in air, unless uracil is present. Aerobic cultures rapidly accumulate dihydroorotate in the medium after transfer to anaerobic conditions; derepression of the dehydrogenase also ensues.

These findings show that the requirement for uracil displayed by when growing anaerobically is due to its inability to dehydrogenate dihydroorotate in the absence of oxygen: they cannot explain the ability of to grow anaerobically without uracil.


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