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The Complete Genome Sequence of ‘Candidatus Liberibacter americanus’, Associated with Citrus Huanglongbing

¹Departamento Científico–Fundecitrus, Araraquara, SP, 14807-040, Brazil; ²Plant Pathology Department, University of Florida, Gainesville 32611, U.S.A.; ³Departamento de Bioquímica, Instituto de Química, Universidade de São Paulo, São Paulo, SP, 05508-000, Brazil; ⁴Faculdade de Computação, Universidade Federal de Mato Grosso do Sul, Campo Grande, MS, 79070-900, Brazil; ⁵Laboratório de Bioquímica Fitopatológica, Instituto Biológico, São Paulo, SP, 04014-002, Brazil; ⁶ICBR University of Florida, Gainesville 32610, U.S.A; ⁷UMR-1332 Biologie du Fruit et Pathologie, INRA, Université Bordeaux Ségalen, Villenave d'Ornon, France

Liberibacter spp. form a Rhizobiaceae clade of phloem-limited pathogens of limited host range. Two obligately parasitic species have been sequenced: ‘Candidatus Liberibacter asiaticus’, which causes citrus huanglongbing (HLB) worldwide, and ‘Ca. L. solanacearum’, which causes potato “zebra chip” disease. A third (proposed) species, Liberibacter crescens, was isolated from mountain papaya, grown in axenic culture, and sequenced. In an effort to identify common host determinants, the complete genomic DNA sequence of a second HLB species, ‘Ca. L. americanus’ strain ‘São Paulo’ was determined. The circular genome of 1,195,201 bp had an average 31.12% GC content and 983 predicted protein encoding genes, 800 (81.4%) of which had a predicted function. There were 658 genes common to all sequenced Liberibacter spp. and only 8 genes common to ‘Ca. L. americanus’ and ‘Ca. L. asiaticus’ but not found in ‘Ca. L. solanacearum’. Surprisingly, most of the lipopolysaccharide biosynthetic genes were missing from the ‘Ca. L. americanus’ genome, as well as OmpA and a key regulator of flagellin, all indicating a ‘Ca. L. americanus’ strategy of avoiding production of major pathogen-associated molecular patterns present in ‘Ca. L. asiaticus’ and ‘Ca. L. solanacearum’. As with ‘Ca. L. asiaticus’, one of two ‘Ca. L. americanus’ prophages replicated as an excision plasmid and carried potential lysogenic conversion genes that appeared fragmentary or degenerated in ‘Ca. L. solanacearum’.