Taxonomic significance of 2,4-diaminobutyric acid isomers in the cell wall peptidoglycan of actinomycetes and reclassification of Clavibacter toxicus as Rathayibacter toxicus comb. nov.

An HPLC procedure which separates D- and L-amino acid isomers was applied to an analysis of peptidoglycan of 2,4-diaminobutyric acid (DAB)-containing actinomycetes. The cell wall peptidoglycans of species of the genera Agromyces, Clavibacter and Rathayibacter contain DAB and have been differentiated principally by their menaquinone profile. These peptidoglycans are known to be identical in structure, all being of the B2 gamma type, possessing both D- and L-DAB. The type strains of all the subspecies of Clavibacter michiganesis have D- and L-DAB in almost equal proportions in their cell wall peptidoglycan as previously reported. In contrast, the type strains of Clavibacter toxicus and all valid species of the genera Agromyces and Rathayibacter contain the L-isomer of DAB almost exclusively. This characteristic is in good agreement with phylogenetic analyses based on 16S rDNA sequences and menaquinone profiles. On the basis of these data, the transfer of Clavibacter toxicus to the genus Rathayibacter as Rathayibacter toxicus comb. nov. is proposed. The isomer profile of DAB is shown to be a good taxonomic marker to differentiate these genera.