Cloning and characterization of the gene cluster for palatinose metabolismfrom the phytopathogenic bacterium Erwinia rhapontici

Erwinia rhapontici is able to convert sucrose into isomaltulose (palatinose , 6-O-alpha -D-glucopyranosyl-D-fructose) and trehalulose (1-O-alpha -D-glu copyranosyl-D-fructose) by the activity of a sucrose isomerase, These sucro se isomers cannot be metabolized by plant cells and most other organisms an d therefore are possibly advantageous for the pathogen, This view is suppor ted by the observation that in vitro yeast invertase activity can be inhibi ted by palatinose, thus preventing sucrose consumption. Due to the lack of genetic information, the role of sucrose isomers in pathogenicity has not b een evaluated, Here we describe for the first time the cloning and characte rization of the palatinose (pal) genes from Erwinia rhapontici, To this end , a 15-kb chromosomal DNA fragment containing nine complete open reading fr ames (ORFs) was cloned, The pal gene products of Erwinia rhapontici were sh own to be homologous to proteins involved in uptake and metabolism of vario us sugars from other microorganisms, The palE, palF, palG, palH, palK, palQ , and palZ genes were oriented divergently with respect to the palR and pal l genes, and sequence analysis suggested that the first set of genes consti tutes an operon, Northern blot analysis of RNA extracted from bacteria grow n under various conditions implies that the expression of the palI gene and the palEFGNKQZ genes is oppositely regulated at the transcriptional level. Genes involved in palatinose uptake and metabolism are down regulated by s ucrose and activated by palatinose, Palatinose activation is inhibited by s ucrose, Functional expression of palI and palQ in Escherichia call revealed sucrose isomerase and palatinase activity, respectively.