Molecular characterization of a mosaic locus in the genome of 'Candidatus Liberibacter asiaticus'

A genomic region (CLIBASIA_05640 to CLIBASIA_05650) of 'Ca. L. asiaticus' showing hyper-sequence variation or locus mosaicism was identified and investigated using 262 bacterial strains (188 from China and 74 from Florida). Based on the characteristic electrophoretic profiles of PCR amplicons generated by a specific primer set, eight electrophoretic types (E-types) were identified, six E-types (A, B, C, D, E, and F) in China and four E-types (A, C, G, and H) in Florida. The 'Ca. L. asiaticus' strains from China consisted predominately of E-type A (71.3%) and E-type B (19.7%). In contrast, the 'Ca. L. asiaticus' strains from Florida was predominated by E-type G (82.4%). Diversity of 'Ca. L. asiaticus' in China was also evidenced. Strains from the high altitude Yunnan Province consisted of five E-types with E-type B being the majority (62.8%), whereas strains from the low altitude coastal Guangdong Province consisted of only two E-types with E-type A as the majority (97.0%). Sequence analyses revealed that variation of DNA amplicons was due to insertion/deletion events at CLIBASIA_05650 and the downstream intergenic region.This study demonstrated the genomic mosaicism of 'Ca. L. asiaticus' resulted from active DNA insertion/deletion activities. Analyses of strain variation depicted the significant inter- and intra-continent diversity of 'Ca. L. asiaticus'.Huanglongbing (HLB) is a destructive disease of citrus production worldwide. All known commercial citrus cultivars are susceptible to HLB. The disease was first noted in Chaoshan area in Guangdong Province of the People's Republic of China in the late of 1800s [1] and is currently distributed in 10 citrus producing provinces in South China. HLB is now established in Sao Paulo of Brazil [2] and Florida of the United States [3] where it poses a great threat to the citrus industry. The disease is associated with three species of non-culturable, phloem-limited, α-Proteobacteria: 'Candidatus Liberibacter asiaticus', 'Ca.