Variability and functional role of chromosomal sequences in 16SrI-B subgroup phytoplasmas including aster yellows and related strains

AIMS: Partial genetic characterization of several chromosomal regions on 35 16SrI-B phytoplasma strains maintained in periwinkle and collected in different geographical areas from plants of diverse species. METHODS AND RESULTS: Genes coding for ribosomal protein rpL22, elongation factor EF-Tu and random cloned sequences amplified with primers AY19p/m, G35p/m and BB88F1/R1 after RFLP analyses showed a high degree of polymorphism among the strains studied. The ribosomal protein (rp) subgroups B and K, and an undescribed subgroup designated N, were identified. Amplicons obtained with primers AY19p/m and BB88F1/R1, revealed a high and a low degree of polymorphism, respectively. CONCLUSIONS: A probable spacer role could be attributed to the AY19p/m sequence and a possible coding function to the BB88F1/R1 sequence. No relationship was found among genetic polymorphisms, identified by statistical analyses, and epidemiological or biological parameters. SIGNIFICANCE AND IMPACT OF THE STUDY: The analyses of five different genomic sequences of the 35 strains belonging to subgroup 16SrI-B allowed a finer distinction among them, confirming that the polymorphism level of 16S rDNA is too low to be adopted as unique parameter for classification.