THE RHIZOBIUM-MELILOTI EXOK GENE AND PRSD PRSE/EXSH GENES ARE COMPONENTS OF INDEPENDENT DEGRADATIVE PATHWAYS WHICH CONTRIBUTE TO PRODUCTIONOF LOW-MOLECULAR-WEIGHT SUCCINOGLYCAN/

When grown on medium supplemented with the succinoglycan-binding dye, Calcofluor, and visualized under UV light, colonies of Rhizobium melil oti (Sinorhizobium meliloti) exoK mutants produce a fluorescent halo w ith a delayed onset relative to wild-type colonies. By conducting tran sposon mutagenesis of exoK mutants of R. meliloti and screening for co lonies with even more severe delays in production of these fluorescent halos, we identified three genes, designated prsD, prsE, and exsH, wh ich are required for the eventual production of fluorescent halos by e xoK colonies. Nucleotide sequence indicates that the prsD and prsE gen es encode homologues of ABC transporters and membrane fusion proteins of Type I secretion systems, respectively, whereas exsH encodes a homo logue of endo-1,3-1,4-beta-glycanases with glycine-rich nonameric repe ats typical of proteins secreted by Type I secretion systems. The exoK gene and the prsD/prsE/exsH genes were shown to be components of inde pendent pathways for production of extracellular succinoglycan degradi ng activities and for production of low-molecular-weight succinoglycan by R. meliloti. Based on these results, we propose that ExsH is a suc cinoglycan depolymerase secreted by a Type I secretion system composed of PrsD and PrsE, and that the ExsH and ExoK glycanases contribute to production of low-molecular-weight succinoglycan.