COMPARISON OF 16S RIBOSOMAL DNA-SEQUENCES OF ALL XANTHOMONAS SPECIES

The phylogenetic relationships of all validly described species of the genus Xanthomonas and the type strain of Stenotrophomonas maltophilia were analyzed by sequencing and comparing 16S ribosomal DNAs (rDNAs). The two genera exhibited a mean sequence similarity value of 96.6%, c orresponding to differences at 50 nucleotide positions on average, The species of the genus Xanthomonas exhibited relatively high levels of overall sequence similarity; the mean similarity value was 98.2%, whic h corresponds to an average of 14 mutual nucleotide differences, Withi n the genus Xanthomonas, a group containing Xanthomonas albilineans, X anthomonas hyacinthi, Xanthomonas theicola, and Xanthomonas translucen s clustered apart from the main Xanthomonas core, whereas Xanthomonas sacchari formed a third phylogenetic lineage. Due to the very restrict ed variability in 16S rDNA sequences within the genus Xanthomonas, rDN A signatures that have possible diagnostic value for differentiating t he Xanthomonas species could not be determined with certainty, When se quence similarities were compared with DNA-DNA pairing data determined previously, there was only a limited correlation. This illustrates th e different resolving powers of the techniques for determining phyloge netic hierarchies and for species delineation.