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J Bacteriol, April 1998, p. 1607-1617, Vol. 180, No. 7
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Xanthomonas campestris pv. campestris gum Mutants: Effects on Xanthan Biosynthesis and Plant Virulence

Federico Katzen,1,2 Diego U. Ferreiro,1 Cristian G. Oddo,1 M. Verónica Ielmini,1 Anke Becker,2 Alfred Pühler,2 and Luis Ielpi1,*

Instituto de Investigaciones Bioquímicas Fundación Campomar, Facultad de Ciencias Exactas y Naturales, UBA, and CONICET, (1405) Buenos Aires, Argentina,1 and Lehrstuhl für Genetik, Universität Bielefeld, D-33501 Bielefeld, Germany2

Received 17 October 1997/Accepted 24 January 1998

Xanthan is an industrially important exopolysaccharide produced by the phytopathogenic, gram-negative bacterium Xanthomonas campestris 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 xpsI or gum 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 X. campestris gum mutants, we report experimental data for assigning functions to the products of the gum genes. We also show that the first step in the assembly of the lipid-linked intermediate is severely affected by the combination of certain gum and non-gum mutations. In addition, we provide evidence that the C-terminal domain of the gumD 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 X. campestris against the plant.

* Corresponding author. Mailing address: Instituto de Investigaciones Bioquímicas Fundación Campomar, Patricias Argentinas 435, (1405) Buenos Aires, Argentina. Phone: 54(1) 863-4011/19. Fax: 54(1) 865-2246. E-mail: lielpi{at}iris.iib.uba.ar.




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