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Journal of Bacteriology, February 2001, p. 1248-1258, Vol. 183, No. 4
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.4.1248-1258.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.

Genetic Characterization of a Sinorhizobium meliloti Chromosomal Region Involved in Lipopolysaccharide Biosynthesis

Antonio Lagares,1,2,* Daniela F. Hozbor,1 Karsten Niehaus,2 Augusto J. L. Pich Otero,1 Jens Lorenzen,2 Walter Arnold,2 and Alfred Pühler2

Instituto de Bioquímica y Biología Molecular, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, 1900 La Plata, Argentina,1 and Lehrstuhl für Genetik, Fakultät für Biologie, Universität Bielefeld, D-33501 Bielefeld, Germany2

Received 17 August 2000/Accepted 11 November 2000

The genetic characterization of a 5.5-kb chromosomal region of Sinorhizobium meliloti 2011 that contains lpsB, a gene required for the normal development of symbiosis with Medicago spp., is presented. The nucleotide sequence of this DNA fragment revealed the presence of six genes: greA and lpsB, transcribed in the forward direction; and lpsE, lpsD, lpsC, and lrp, transcribed in the reverse direction. Except for lpsB, none of the lps genes were relevant for nodulation and nitrogen fixation. Analysis of the transcriptional organization of lpsB showed that greA and lpsB are part of separate transcriptional units, which is in agreement with the finding of a DNA stretch homologous to a "nonnitrogen" promoter consensus sequence between greA and lpsB. The opposite orientation of lpsB with respect to its first downstream coding sequence, lpsE, indicated that the altered LPS and the defective symbiosis of lpsB mutants are both consequences of a primary nonpolar defect in a single gene. Global sequence comparisons revealed that the greA-lpsB and lrp genes of S. meliloti have a genetic organization similar to that of their homologous loci in R. leguminosarum bv. viciae. In particular, high sequence similarity was found between the translation product of lpsB and a core-related biosynthetic mannosyltransferase of R. leguminosarum bv. viciae encoded by the lpcC gene. The functional relationship between these two genes was demonstrated in genetic complementation experiments in which the S. meliloti lpsB gene restored the wild-type LPS phenotype when introduced into lpcC mutants of R. leguminosarum. These results support the view that S. meliloti lpsB also encodes a mannosyltransferase that participates in the biosynthesis of the LPS core. Evidence is provided for the presence of other lpsB-homologous sequences in several members of the family Rhizobiaceae.

* Corresponding author. Mailing address: Instituto de Bioquímica y Biología Molecular, Facultad de Ciencias Exactas, Universidad Nacional de La Plata, calles 47 y 115, 1900 La Plata, Argentina. Phone: 54-221-4250497, ext. 31 or 32. Fax: 54-221-4244854. E-mail: lagares{at}biol.unlp.edu.ar.

Journal of Bacteriology, February 2001, p. 1248-1258, Vol. 183, No. 4
0021-9193/01/$04.00+0   DOI: 10.1128/JB.183.4.1248-1258.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.


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Copyright © 2001 by the American Society for Microbiology. All rights reserved.