A novel Sinorhizobium meliloti operon encodes an alpha-glucosidase and a periplasmic-binding-protein-dependent transport system for alpha-glucosides

The most abundant carbon source transported into legume root nodules is pho tosynthetically produced sucrose. yet the importance of its metabolism by r hizobia in planta is not Set known. To identify genes involved in sucrose u ptake and hydrolysis, we screened a Sinorhizobium meliloti genomic library and discovered a segment of S. meliloti DNA which allows Ralstonia eutropha to grow on the alpha-glucosides sucrose, maltose, and trehalose. Tn5 mutag enesis localized the required genes to a 6.8-kb region containing fire open reading frames which were named agl, for alpha-glucoside utilization. Four of these (aglE, aglF, aglG, and aglK) appear to encode a periplasmic-bindi ng-protein-dependent sugar transport system, and one (aglA) appears to enco de an alpha-glucosidase with homology to family 13 of glycosyl hydrolases. Cosmid-borne ngl genes permit uptake of radio-labeled sucrose into R. eutro pha cells. Analysis of the properties of agl mutants suggests that S. melil oti possesses at least one additional alpha-glucosidase as well as a lower- affinity transport system for cu-glucosides. It is possible that the Fix(+) phenotype of agl mutants on alfalfa is due to these additional functions. Loci found by DNA sequencing to be adjacent to aglEFGAK include a probable regulatory gene (aglR), zwf and edd, which encode the first two enzymes of the Entner-Doudoroff pathway pgl, which shows homology to a gene encoding a putative phosphogluconolactonase, and a novel Rhizobium-specific repeat cl ement.