Neighbor-joining tree for rDNA sequences types. Sequences corresponding to nucleotide positions 515 to 1492 of the E. coli sequence were used for calculations. Sulfolobus acidocaldarius is used as the outgroup. Accession numbers of the sequences retrieved from the databases are indicated in parentheses. The numbers at the branch nodes are bootstrap values (per 100 trials); only values greater than 50 are shown. The scale bar indicates 0.044 substitution per site.