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

摘要

The genetic characterization of a 5.5-kb chromosomal region ofSinorhizobium meliloti 2011 that contains lpsB, a gene required for the normal development of symbiosis withMedicago spp., is presented. The nucleotide sequence of this DNA fragment revealed the presence of six genes: greAand lpsB, transcribed in the forward direction; andlpsE, lpsD, lpsC, and lrp, transcribed in the reverse direction. Except for lpsB, none of thelps genes were relevant for nodulation and nitrogen fixation. Analysis of the transcriptional organization oflpsB 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 andlrp 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 otherlpsB-homologous sequences in several members of the familyRhizobiaceae.