THE 32-KILOBASE EXP GENE-CLUSTER OF RHIZOBIUM-MELILOTI DIRECTING THE BIOSYNTHESIS OF GALACTOGLUCAN - GENETIC ORGANIZATION AND PROPERTIES OFTHE ENCODED GENE-PRODUCTS

Proteins directing the biosynthesis of galactoglucan (exopolysaccharid e II) in Rhizobium meliloti Rm2011 are encoded by the exp genes, Seque nce analysis of a 32-kb DNA fragment of megaplasmid 2 containing the e xp gene cluster identified previously (J. Glazebrook and G. C. Walker, Cell 56:661-672, 1989) revealed the presence of 25 open reading frame s, Homologies of the deduced exp gene products to proteins of known fu nction suggested that the exp genes encoded four proteins involved in the biosynthesis of dTDP-glucose and dTDP-rhamnose, six glycosyltransf erases, an ABC transporter complex homologous to the subfamily of pept ide and protein export complexes, and a protein homologous to Rhizobiu m NodO proteins. In addition, homologies of three Esp proteins to tran scriptional regulators, methyltransferases, and periplasmic binding pr oteins were found, The positions of 26 Tn5 insertions in the exp gene cluster mere determined, thus allowing the previously described geneti c map to be correlated with the sequence. Operon analysis revealed tha t the exp gene cluster consists of five complementation groups. In com parison to the wild-type background, all exp complementation groups we re transcribed at a substantially elevated level in the regulatory muc R mutant.