PHYLOGENETIC ANALYSIS OF THE GENERA RHODOCOCCUS AND NOCARDIA AND EVIDENCE FOR THE EVOLUTIONARY ORIGIN OF THE GENUS NOCARDIA FROM WITHIN THERADIATION OF RHODOCOCCUS SPECIES

16S rDNA analysis was performed on 32 strains of 26 species of the gen era Rhodococcus and Nocardia in order to investigate the phylogenetic structure of these genera within the radiation of other mycolic-acid-c ontaining genera such as Corynebacterium, Dietzia, Gordona, Mycobacter ium and Tsukamurella. The genus Rhodococcus shows a complex structure, consisting of six phylogenetically equidistant lineages. The genus No cardia does not appear to be a sister taxon of Rhodococcus but branche s off from within the radiation of Rhodococcus; thus its species can b e considered to be derived from a Rhodococcus ancestor. The main known phenotypic feature that separates Nocardia species from those of Rhod ococcus appears to be the presence of a cyclic component in menaquinon e of the MK-8(H-4) type.