XANTHOMONAS-CAMPESTRIS PV. CAMPESTRIS GUM MUTANTS - EFFECTS ON XANTHAN BIOSYNTHESIS AND PLANT VIRULENCE

Xanthan is an industrially important exopolysaccharide produced by the phytopathogenic, gram-negative bacterium Xanthomonas campestris pv. c ampestris. It is composed of polymerized pentasaccharide repeating uni ts which are assembled by the sequential addition of glucose-1-phospha te, glucose, mannose, glucuronic acid, and mannose on a polyprenol pho sphate carrier (L. Ielpi, R. O. Couso, and M. A. Dankert, J. Bacteriol . 175:2490-2500, 1993), A cluster of 12 genes in a region designated x psI or gum has been suggested to encode proteins involved in the synth esis and polymerization of the lipid intermediate. However, no experim ental evidence supporting this suggestion has been published, In this work, from the biochemical analysis of a defined set of X. campestris gum mutants, we report experimental data for assigning functions to th e products of the gum genes, We also show that the first step in the a ssembly of the lipid-linked intermediate is severely affected by the c ombination of certain gum and non-gum mutations. In addition, we provi de evidence that the C-terminal domain of the gumD gene product is suf ficient for its glucosyl-1-phosphate transferase activity. Finally, we found that alterations in the later stages of xanthan biosynthesis re duce the aggressiveness of X. campestris against the plant.