Phylogeny of Sym plasmids of rhizobia by PCR-based sequencing of a nodC segment.

To understand the host specificity of rhizobia and the relationship between the evolution of Sym plasmids and that of host plants, we determined partial nodC sequences of 10 representative rhizobium strains and then constructed an evolutionary tree for the deduced amino acid sequences with four published sequences. These coding sequences yield a phylogenetic tree similar to that for leghemoglobin of host plants, suggesting that the evolution of common nodulation genes may be linked to host legume evolution and speciation.