Relationship of 16S rRNA sequence similarity to DNA hybridization in prokaryotes.

The relationship between 16S rRNA sequence similarity (S) and the extent of DNA hybridization (D) was well described by the equation In(-InD) = 0.53 In(-InS)+2.201 when D was determined by either the S1 nuclease or membrane filter methods. When the presence of nonultrametric rRNA sequences and differences between genera or families were controlled, this relationship accounted for 78% of the variability of D given S, and it was possible to estimate the distribution of D from S with a known precision. Thus, DS>> 0.95, D decreased from 1.0 to 0.15; and within the range 0.95 >S> 0.90, D decreased from 0.15 to 0.06. Lastly, at least some of the rRNA sequences from about one-third of the taxa examined had nonultrametric properties where S was much lower than expected from the value of D. For these taxa, S was a poor indicator of relatedness for closely related strains. Thus, the ultrametric properties of rRNA sequences should be tested before making taxonomic or phylogenetic conclusions based upon S.