IDENTIFICATION AND CHARACTERIZATION OF GENES (XAPA, XAPB, AND XAPR) INVOLVED IN XANTHOSINE CATABOLISM IN ESCHERICHIA-COLI

We have characterized four genes from the 52-min region on the Escheri chia coli linkage map. Three of these genes are directly involved in t he metabolism of xanthosine, whereas the function of the fourth gene i s unknown. One of the genes (xapA) encodes xanthosine phosphorylase. T he second gene, named xapB, encodes a polypeptide that shows strong si milarity to the nucleoside transport protein NupG, The genes xapA and xapB are located clockwise of a gene identified as xapR, which encodes a positive regulator belonging to the LysR family and is required for the expression of xapA and xapB. The genes xapA and xapB form an oper on, and their expression was strictly dependent on the presence of bot h the XapR protein and the inducer xanthosine. Expression of tile xapR gene is constitutive and not autoregulated, unlike the case for many other LysR family proteins. In minicells, the XapB polypeptide was fou nd primarily in the membrane fraction, indicating that XapB is a trans port protein like NupG and is involved in the transport of xanthosine.