Summary: The biosynthesis of the extracellular polysaccharide xanthan in pv. is directed by a cluster of 12 genes, Several xanthan-deficient mutants of the wild-type strain 8004 have previously been described which carry Tn5 insertions in this region of the chromosome. Here it is shown that the transposon insertion in one of these mutants, strain 8397, is located 15 bp upstream of the translational start site of the gene. EDTA-treated cells of strain 8397 were able to synthesize the lipid-linked pentasaccharide repeating unit of xanthan from the three nucleotide sugar donors (UDP-glucose, GDP-mannose and UDP-glucuronic acid) but were unable to polymerize the pentasaccharide into mature xanthan. A subclone of the gene cluster carrying and restored xanthan production to strain 8397 to levels approximately 28% of the wild-type. In contrast, subclones carrying or alone were not effective. These results are discussed with reference to previous speculations, based on computer analysis, that and are both involved in the translocation of xanthan across the bacterial membranes.


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