A novel system for sucrose uptake pathway in xanthomonas phytopathogen : supC is the intracellular sucrose-hydrolyzing enzyme of the system

Abstract

In the genome of a phytopathogen Xanthomonas axonopodis pv. citri that has been recently sequenced, there is a cluster of three structural genes designated suc1, fyuA and xac3490, respectively. This gene cluster may deserve a special attention from various contextual grounds with respect to a novel system for sucrose utilization pathway (SUP) hitherto unknown for any microorganism. The present report is a part of research concerning how X. axonopodis pv. glycine (X.a.g.), a bacterium causative of soybean pustule disease, uptakes and metabolizes sucrose as the major carbon source in its natural habitats. An X.a.g. gene, designated supC, was found to be highly homologous to xac3490 and to be closely situated together with two structural genes that also show a high homology to the suc1 and fyuA genes, respectively. In order to functionally characterize supC, molecular biological manipulations and biochemical measurements were performed. That is, (1) supC was overexpressed in a recombinant E. coli, and the gene product was purified and measured for sucrose-processing activity; and (2) a supC mutant was constructed by a transposon insertion and subjected to biological assay for the cellular activity for sucrose metabolism. The results obtained clearly demonstrate that the supC gene encodes an enzyme that is solely responsible for intracellular hydrolysis of sucrose in X.a.g. as the first step of catabolic pathway of the natural carbon source.