Extraction of bacterial chromosomal DNAs was performed
by modifications of the methods of Marmur (1961), Saito
& Miura (1963) and Ezaki et al. (1983). DNA base compositions
were determined by the method of Tamaoka &
Komagata (1984). DNA–DNA hybridizations were carried
out by the photobiotin-labelling method using microplate
wells, as described by Ezaki et al. (1989). Levels of DNA–
DNA hybridization were determined colorimetrically
(Verlander, 1992). Colour intensity was measured as A450
on a VersaMax microplate reader (Molecular Devices).