Comparison of sequence analysis of 16S-23S rDNA spacer regions, AFLP analysis and DNA-DNA hybridizations in Bradyrhizobium

The sequences of the 16S-23S rDNA intergenic spacer region of 62 strains of Bradyrhizobium, including representatives of the three valid species, were determined. The majority of strains had a single rRNA operon type and all contained a tRNA(Ala) and a tRNA(Ile) gene. Analysis of the sequence data produced groupings in line with previously obtained AFLP data. DNA-DNA hybridizations were performed to determine to what extent spacer sequences and AFLP profiles reflected the overall genomic similarities. Strains belonging to the same AFLP group, and strains with spacer sequences diverging less than 4%, were found to belong to the same genospecies. More remote relationships (DNA homology levels of 40-60 %) between species were reflected in the spacer sequence analysis, but not in the AFLP analysis. For the genus Bradyrhizobium, 16S-23S rDNA spacer sequence analysis provides taxonomic information similar, but not always identical to that obtained by DNA-DNA hybridizations. Our results indicate that this genus consists of a group of four highly related genospecies (Bradyrhizobium japonicum, Bradyrhizobium liaoningense and two other genospecies) and at least three other genospecies, one of which is Bradyrhizobium elkanii.