Nancy Rotzel
Department of Biology
Marquette University

Rhizobium etli and Phaseolus vulgaris enter into symbiosis when R. etli infect the roots of P. vulgaris to form nitrogen fixing nodules. The normal amount of the complete R. etli lipopolysaccharide (LPS), including the terminal O-antigen portion, has been found to be necessary for the successful infection of the legume.

One illustration of this conclusion if R. etli mutant CE166, which does not affect P. vulgaris. Strain CE166 carries a mutation that prevents the synthesis of N-acettylquinovosamine (QuiNAc), which is normally found as a single sugar reside in R. etli LPS. The LPS of this mutant strain lacks this residue, but it produces a complete LPS molecule that otherwise has normal sugar content. However, probably as a secondary consequence of the absence of this residue, the mutant strain is deficient in the relative amount of complete O-antigen-containing lipopolysaccharide. It was found previously that a long stretch of cloned DNA, called the lps alpha genetic region, could suppress the O-antigen deficiency when present in multiple copies in mutant CE166, even though the mutation of CE166 is not in this lps alpha region. This suppression results in a greater relative amount of complete LPS, but the LPS still lacks QuiNAc. Concommitantly, the ability to infect P. vulgaris is partially restored. 

To determine the genetic basis of this suppression, multiple copies of the different portions of the lps alpha genetic region were transformed into strain CE166. The lps alpha locus has lps genes that span at least 20 kb of the contiguous DNA. Analysis of recombinant bacteria carrying deletions of this DNA have suggested that an 8.5 kb stretch of DNA is sufficient for suppression. Furthermore, a gene at one end of this DNA appears to be required, although it has not been excluded yet that other genes may be required as well.