Nucleosides as a carbon source in Bacillus subtilis: characterization of the drm-pupG operon

In Bacillus subtilis, nucleosides are readily taken up from the growth medi um and metabolized. The key enzymes in nucleoside catabolism are nucleoside phosphorylases, phosphopentomutase, and deoxyriboaldolase. The characteriz ation of two closely linked loci, drm and pupG, which encode phosphopentomu tase (Drm) and guanosine (inosine) phosphorylase (PupG), respectively, is r eported here. When expressed in Escherichia coli mutant backgrounds, drm an d pupG confer phosphopentomutase and purine-nucleoside phosphorylase activi ty. Northern blot and enzyme analyses showed that drm and pupG form a dicis tronic operon. Both enzymes are induced when nucleosides are present in the growth medium. Using mutants deficient in nucleoside catabolism, it was de monstrated that the low-molecular-mass effecters of this induction most lik ely were deoxyribose 5-phosphate and ribose 5-phosphate. Both Drm and PupG activity levels were higher when succinate rather than glucose served as th e carbon source, indicating that the expression of the operon is subject to catabolite repression. Primer extension analysis identified two transcript ion initiation signals upstream of drm; both were utilized in induced and n on-induced cells. The nucleoside-catabolizing system in B. subtilis serves to utilize the base for nucleotide synthesis while the pentose moiety serve s as the carbon source. When added alone, inosine barely supports growth of B. subtilis. This slow nucleoside catabolism contrasts with that of E. col i, which grows rapidly on a nucleoside as a carbon source. When inosine was added with succinate or deoxyribose, however, a significant increase in gr owth was observed in B. subtilis. The findings of this study therefore indi cate that the B. subtilis system for nucleoside catabolism differs greatly from the well-studied system in E. coli.