Guanine-plus-cytosine content, hybridization percentages, and EcoRI restriction enzyme profiles of spiroplasmal DNA.

The guanine-plus-cytosine (G + C) content of spiroplasmal DNA was calculated from the melting temperature determined spectrophotometrically and the buoyant density determined by equilibrium density gradient centrifugation in CsCl. Only two ranges of G + C values were found: 25-27 mol% and 29-32 mol%. The DNA of the following spiroplasmas has 25-27 mol% G + C: Spiroplasma citri (serogroup I-1); the spiroplasmas pathogenic to the honeybee (KC3, BC3, and B63; serogroup I-2); the corn stunt strain (E275; serogroup I-3); the tick strain 277F (serogroup I-4); the drosophila strain (serogroup II); and one group of flower spiroplasmas (serogroup III). The DNA of a second group of flower spiroplasmas (serogroup IV) and the SMCA strain (serogroup V) has a G + C content of 29-31 mol/. The classification of flower spiroplasmas into two groups on the basis of G + C content agrees well with the groupings based on serologic and protein analysis. Spiroplasmas isolated from honeybees in Morocco (B13) or froghoppers in Corsica (L89) have 29-31 mol% G + C, a value that corroborates the relatedness of these strains and the flower spiroplasmas of serogroup IV found by serologic analysis. Reannealing experiments between the vivo-labeled DNA of S. citri and unlabeled DNA of other spiroplasmas gave the following percentages of hybridization: 64% with honeybee spiroplasma DNA, 49% with corn stunt spiroplasma DNA, and 19% with tick spiroplasma 277F DNA; no significant hybridization was observed with DNA of any other spiroplasma. The taxonomic position of the tick spiroplasma 277F within serogroup I was confirmed by hybridization experiments involving [3H]DNA of this strain. The value of polyacrylamide gel analysis of DNA fragments produced by the action of EcoRI restriction enzyme on DNAs from various spiroplasmas is also discussed.