Seventy-seven reference and field strains from different geographic origins and hosts representing the six recognized species and their different biovars were analysed for diversity of their genes encoding the major 25 and 36 kDa outer-membrane proteins (OMPs) by PCR-RFLP. The 25 kDa OMP is encoded by a single gene () whereas two closely related genes ( and ) encode and potentially express the 36 kDa OMP. Analysis of PCR products of the gene digested with nine restriction enzymes revealed two species-specific markers, i.e. the absence of the RV site in all strains and an ~ 50 bp deletion at the 3′ terminal end of the gene in all strains. Analysis of PCR products of the and genes digested with 13 restriction enzymes indicated a greater diversity than the gene among the six species and within the and species. Greater polymorphism was also detected for the than for the gene, especially in which seemed to carry two similar (but not identical) copies of instead of one copy each of and for the other species as was previously suggested by Ficht (1990; 4, 1135-1142). Results of PCR-RFLP indicated that distinction can be made between species and some of their biovars, except between and bv. 3 and 4, on the basis of the size and diversity of their major OMP genes, and that it could be of importance for diagnostic, epidemiological and evolutionary study purposes.


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