Purine catabolism in Escherichia coli and function of xanthine dehydrogenase in purine salvage

Escherichia coli is not known to utilize purines, other than adenine and ad enosine, as nitrogen sources. We reinvestigated purine catabolism because a computer analysis suggested several potential sigma(54)-dependent promoter s within a 23-gene fluster whose products have homology to purine catabolic enzymes, Our results did not provide conclusive evidence that the sigma(54 )-dependent promoters are active. Nonetheless, our results suggest that som e of the genes are metabolically significant. We found that even though sev eral purines did not support growth as the sole nitrogen source, they did s timulate growth with aspartate as the nitrogen source. Cells produced (CO2) -C-14 from minimal medium containing [C-14]adenine, which implies allantoin production. However, neither ammonia nor carbamoyl phosphate was produced, which implies that purine catabolism is incomplete and does not provide ni trogen during nitrogen-limited growth. We constructed strains with deletion s of two genes whose products might catalyze the first reaction of purine c atabolism. Deletion of one eliminated (CO2)-C-14, production from [C-14]ade nine, which implies that its product is necessary for xanthine dehydrogenas e activity, We changed the name of this gene to xdhA. The xdhA mutant grew faster with aspartate as a nitrogen source. The mutant also exhibited sensi tivity to adenine, which guanosine partially reversed, Adenine sensitivity has been previously associated with defective purine salvage resulting from impaired synthesis of guanine nucleotides from adenine, We propose that xa nthine dehydrogenase contributes to this purine interconversion.