XANTHINE METABOLISM IN BACILLUS-SUBTILIS - CHARACTERIZATION OF THE XPT-PBUX OPERON AND EVIDENCE FOR PURINE-CONTROLLED AND NITROGEN-CONTROLLED EXPRESSION OF GENES INVOLVED IN XANTHINE SALVAGE AND CATABOLISM

The xpt and pbuX genes from Bacillus subtilis were cloned, and their n ucleotide sequences were determined, The xpt gene encodes a specific x anthine phosphoribosyltransferase, and the pbuX gene encodes a xanthin e-specific purine permease, The genes have overlapping coding regions, and Northern (RNA) blot analysis indicated an operon organization, Th e translation of the second gene, pbuX, was strongly dependent on the translation of the first gene, xpt, Expression of the operon was repre ssed 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 terminatio n-antitermination control mechanism similar to the one suggested for t he regulation of the purine biosynthesis operon exists, The expression of the xpt-pbuX operon was reduced when hypoxanthine served as the so le nitrogen source, Under these conditions, the level of the hypoxanth ine- and xanthine-degrading enzyme, xanthine dehydrogenase, was induce d more than 80-fold, The xanthine dehydrogenase level was completely d erepressed in a glnA (glutamine synthetase) genetic background, Althou gh the regulation of the expression of the xpt-pbuX operon was found t o be affected by the nitrogen source, it was normal in a glnA mutant s train, This result suggests the existence of different signalling path ways for repression of the transcription of the xpt-pbuX operon and th e induction of xanthine dehydrogenase.