SOME FACTORS AFFECTING THE EXCRETION OF OROTIC ACID BY MUTANTS OF AEROBACTER AEROGENES

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Abstract
Three mutants of Aerobacter aerogenes. strain 1033, all of which excrete orotic acid were isolated. Mutant P-12 was obtained by UV irradiation and requires uracil or cytosine, or the nucleosides or nucleotides of these bases for growth. It responds but poorly to thymine. Mutant 5/582 was obtained by the UV irradiation of strain P-12 and has the same growth requirements as the latter strain and in addition requires histidine for growth. Both of these strains accumulate large amounts of orotic acid (208 nag per liter). Mutant C-O is a back-mutant obtained from mutant P-12. It has no growth requirements beyond those of the wild strain but accumulates orotic acid although in much smaller amounts (13 mg per liter) than the other 2 mutants. The greater the amount of uracil given to strain P-12, the less orotic acid was formed. The formation of orotic acid by this strain occurred only subsequent to growth. Similar results were obtained with uridine and uridylic acid, except that with low levels of the nucleotide growth was slow and orotic acid was formed. Experiments with the back-mutant C-O and the double-mutant 5/582 clearly showed that inhibition of orotic acid formation was due not to growth but to the presence of uracil in the culture medium. It was shown also that uracil did not suppress formation of an adaptive enzyme which might convert a precursor "x" of orotic acid to orotic acid but that the enzyme was always present and that uracil inhibited its activity.