RHIZOBIUM-HUAUTLENSE SP. NOV., A SYMBIONT OF SESBANIA-HERBACEA THAT HAS A CLOSE PHYLOGENETIC RELATIONSHIP WITH RHIZOBIUM-GALEGAE

The nitrogen-fixing rhizobial symbionts of Sesbania herbacea growing i n the nature reserve at the Sierra de Huautla, Mexico, were isolated a nd characterized, All 104 isolates together with the type strain for R hizobium galegae, HAMBI 540(T), had similar 16S rRNA genes as revealed by PCR-RFLP analysis. Similarity in the sequences of the 16S rRNA gen es placed the isolates on a phylogenetic branch shared with R. galegae . Among 66 randomly selected isolates, three closely related electroph oretic alloenzyme types (ETs) were identified, which were distinct fro m 10 ETs distinguished among 23 strains of R. galegae. A new species R hizobium huautlense, represented by the Sesbania isolate S02(T), is pr oposed based upon low estimates of DNA relatedness between our chosen type strain and the type strains for the other species, the dissimilar ity of the nucleotide sequence of the 16S rRNA genes, and their distin ct ETs compared with R. galegae. The description of R. huautlense is s ignificant because in the reconstruction of the phylogeny of R. huautl ense there was a shift in the node of the branch of Agrobacterium viti s relative to that of R. galegae. The revised phylogenetic tree would tend to indicate common ancestry between R. galegae and Rhizobium legu minosarum.