, Volume 179, Issue 2, pp 331-340

A second purine nucleoside phosphorylase in Escherichia coli K-12

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Enzyme purification studies have indicated the existence in E. coli of only a single species of purine nucleoside phosphorylase. Strains mutated in the gene (deoD) specifying this enzyme cannot grow on purine nucleosides such as inosine, adenosine and guanosine as carbon source. We have selected secondary-site revertants of a deoD strain by plating on adenosine or inosine as carbon source and we have shown that the site of the mutation enabling growth on adenosine or inosine in these revertants, termed xapR, lies between nupC and ptsI at 51 min, almost exactly 180° from the deoD gene on the E. coli chromosome. In some xapR mutants there is a constitutive synthesis of a second purine nucleoside phosphorylase; in other xapR mutants, this enzyme is induced by inosine. The properties of this enzyme in the xapR mutants are very similar to that of xanthosine phosphorylase found in wild-type cells induced with xanthosine, and we thus consider that xapR mutants are altered in the regulation of xanthosine phosphorylase.

From an xapR strain mutants were isolated which lacked this second purine nucleoside phosphorylase. The site of this mutation, xap, was 90% co-transducible with xapR. Such strains could not grow on xanthosine as sole carbon source. The rate of mutation to xapR was very low (3x10-8). Also studies with an F-prime covering the xapR + gene revealed that xapR2 was partially dominant to xapR +. We therefore suggest that the xapR + gene product is an inducer protein for the gene specifying xanthosine phosphorylase, which is inactive until converted by the inducer into a form able to switch on the operon, i.e. there is positive control of the xap gene.

Communicated by G. O'Donovan