<i>Xanthomonas campestris</i> pv. campestris <i>gum</i> Mutants: Effects on Xanthan Biosynthesis and Plant Virulence

<jats:title>ABSTRACT</jats:title> <jats:p> Xanthan is an industrially important exopolysaccharide produced by the phytopathogenic, gram-negative bacterium <jats:italic>Xanthomonas campestris</jats:italic> pv. campestris. It is composed of polymerized pentasaccharide repeating units which are assembled by the sequential addition of glucose-1-phosphate, glucose, mannose, glucuronic acid, and mannose on a polyprenol phosphate 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 <jats:italic>xpsI</jats:italic> or <jats:italic>gum</jats:italic> has been suggested to encode proteins involved in the synthesis and polymerization of the lipid intermediate. However, no experimental evidence supporting this suggestion has been published. In this work, from the biochemical analysis of a defined set of <jats:italic>X. campestris gum</jats:italic> mutants, we report experimental data for assigning functions to the products of the <jats:italic>gum</jats:italic> genes. We also show that the first step in the assembly of the lipid-linked intermediate is severely affected by the combination of certain <jats:italic>gum</jats:italic> and non- <jats:italic>gum</jats:italic> mutations. In addition, we provide evidence that the C-terminal domain of the <jats:italic>gumD</jats:italic> gene product is sufficient for its glucosyl-1-phosphate transferase activity. Finally, we found that alterations in the later stages of xanthan biosynthesis reduce the aggressiveness of <jats:italic>X. campestris</jats:italic> against the plant. </jats:p>