IDENTIFICATION OF THE EXO LOCI REQUIRED FOR EXOPOLYSACCHARIDE SYNTHESIS IN AGROBACTERIUM-RADIOBACTER NCIB 11883

We initiated a genetic analysis of Agrobacterium radiobacter NCIB 11883 with particular reference to the (exo) genes required for exopolysaccharide synthesis. Following mutagenesis with nitrosoguanidine, several exo mutant strains were isolated and several of the mutations were corrected by DNA cloned in a newly constructed cosmid library. Analysis of various complementing cosmids by genetic and physical criteria indicated that exo loci were quite widely dispersed in the bacterial genome. Certain exo mutations were corrected by different cosmids that shared no homologous DNA; possible explanations for this are presented. Using pho A fusions, it was shown that some exo genes were, or were closely linked to, genes that specified polypeptides associated with the bacterial cell surface. By introducing the cloned exo genes of Rhizobium meliloti it was found that only one out of thirty exo mutants of A. radiobacter was corrected by a defined exo locus of the former species; further analysis indicated that this particular exo gene corresponded to exoB of R. meliloti. Finally, it was found that several A. radiobacter exo mutants were non-mucoid on media with dicarboxylic acids as sole carbon source but appeared to be wild-type when sugars were the source of carbon.