Xanthine metabolism in Bacillus subtilis: characterization of the xpt-pbuX operon and evidence for purine- and nitrogen-controlled expression of genes involved in xanthine salvage and catabolism

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Abstract
The xpt and pbuX genes from Bacillus subtilis were cloned, and their nucleotide sequences were determined. The xpt gene encodes a specific xanthine phosphoribosyltransferase, and the pbuX gene encodes a xanthine-specific purine permease. The genes have overlapping coding regions, and Northern (RNA) blot analysis indicated an operon organization. The translation of the second gene, pbuX, was strongly dependent on the translation of the first gene, xpt. Expression of the operon was repressed by purines, and the effector molecules appear to be hypoxanthine and guanine. When hypoxanthine and guanine were added together, a 160-fold repression was observed. The regulation of expression was at the level of transcription, and we propose that a transcription termination-antitermination control mechanism similar to the one suggested for the regulation of the purine biosynthesis operon exists. The expression of the xpt-pbuX operon was reduced when hypoxanthine served as the sole nitrogen source. Under these conditions, the level of the hypoxanthine- and xanthine-degrading enzyme, xanthine dehydrogenase, was induced more than 80-fold. The xanthine dehydrogenase level was completely derepressed in a glnA (glutamine synthetase) genetic background. Although the regulation of the expression of the xpt-pbuX operon was found to be affected by the nitrogen source, it was normal in a glnA mutant strain. This result suggests the existence of different signalling pathways for repression of the transcription of the xpt-pbuX operon and the induction of xanthine dehydrogenase.