In , nucleosides are readily taken up from the growth medium and metabolized. The key enzymes in nucleoside catabolism are nucleoside phosphorylases, phosphopentomutase, and deoxyriboaldolase. The characterization of two closely linked loci, and , which encode phosphopentomutase (Drm) and guanosine (inosine) phosphorylase (PupG), respectively, is reported here. When expressed in mutant backgrounds, and confer phosphopentomutase and purine-nucleoside phosphorylase activity. Northern blot and enzyme analyses showed that and form a dicistronic operon. Both enzymes are induced when nucleosides are present in the growth medium. Using mutants deficient in nucleoside catabolism, it was demonstrated that the low-molecular-mass effectors of this induction most likely were deoxyribose 5-phosphate and ribose 5-phosphate. Both Drm and PupG activity levels were higher when succinate rather than glucose served as the carbon source, indicating that the expression of the operon is subject to catabolite repression. Primer extension analysis identified two transcription initiation signals upstream of ; both were utilized in induced and non-induced cells. The nucleoside-catabolizing system in serves to utilize the base for nucleotide synthesis while the pentose moiety serves as the carbon source. When added alone, inosine barely supports growth of . This slow nucleoside catabolism contrasts with that of , which grows rapidly on a nucleoside as a carbon source. When inosine was added with succinate or deoxyribose, however, a significant increase in growth was observed in . The findings of this study therefore indicate that the system for nucleoside catabolism differs greatly from the well-studied system in .


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