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Journal of Bacteriology, October 1998, p. 5432-5436, Vol. 180, No. 20
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.

Different Phenotypic Classes of Sinorhizobium meliloti Mutants Defective in Synthesis of K Antigen

Gordon R. O. Campbell,1 Bradley L. Reuhs,2 and Graham C. Walker*

Department of Biology, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139,1 and Complex Carbohydrate Research Center, University of Georgia, Athens, Georgia 30602-47122

Received 22 May 1998/Accepted 3 August 1998

For Sinorhizobium meliloti (also known as Rhizobium meliloti) AK631 to establish effective symbiosis with alfalfa, it must be able to synthesize a symbiotically active form of its K antigen, a capsular polysaccharide containing a Kdo (3-deoxy-D-manno-octulosonic acid) derivative. Previously isolated mutants defective in the synthesis of K antigen are resistant to bacteriophage phi 16-3. By screening ca. 100,000 Tn5-mutagenized R. meliloti bacteria for resistance to bacteriophage phi 16-3, we isolated 119 mutants, 31 of which could not be complemented by genes previously identified as being required for K-antigen synthesis. Of these 31 new mutants, 13 were symbiotically defective and lacked the K antigen. Through genetic and phenotypic analyses, we have grouped these mutants into four distinct classes. Although all of these mutants lack the K antigen, many also have altered lipopolysaccharides (LPS), suggesting that the biochemical pathways for the synthesis of K antigen and LPS have common enzymatic steps. In addition, we have found that these and other classes of K-antigen-defective mutants of S. meliloti AK631 exhibit unique patterns of sensitivities to phage strains to which the parental strain was resistant. Our studies have identified new classes of genes required for both the synthesis of K antigen and the symbiotic proficiency of S. meliloti AK631. Some of these classes of genes also play a role in LPS synthesis.

* Corresponding author. Mailing address: Department of Biology, Massachusetts Institute of Technology, Cambridge, MA 02139. Phone: (617) 253-6716. Fax: (617) 253-2643. E-mail: gwalker{at}mit.edu.

Journal of Bacteriology, October 1998, p. 5432-5436, Vol. 180, No. 20
0021-9193/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.


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