EVIDENCE FOR A ROLE FOR THE GUMB AND GUMC GENE-PRODUCTS IN THE FORMATION OF XANTHAN FROM ITS PENTASACCHARIDE REPEATING UNIT BY XANTHOMONAS-CAMPESTRIS

The biosynthesis of the extracellular polysaccharide xanthan in Xantho monas campestris pv. campestris is directed by a cluster of 12 genes, gumB-gumM. Several xanthan-deficient mutants of the wild-type strain 8 004 have previously been described which carry Tn5 insertions in this region of the chromosome. Here it is shown that the transposon inserti on in one of these mutants, strain 8397, is located 15 bp upstream of the translational start site of the gumB gene. EDTA-treated cells of s train 8397 were able to synthesize the lipid-linked pentasaccharide re peating unit of xanthan from the three nucleotide sugar donors (UDP-gl ucose, GDP-mannose and UDP-glucuronic acid) but were unable to polymer ize the pentasaccharide into mature xanthan. A subclone of the gum gen e cluster carrying gumB and gumC restored xanthan production to strain 8397 to levels approximately 28% of the wild-type. In contrast, subcl ones carrying gumB or gumC alone were not effective. These results are discussed with reference to previous speculations, based on computer analysis, that gumB and gumC are both involved in the translocation of xanthan across the bacterial membranes.