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Ca. Liberibacter asiaticus’ Carries an Excision Plasmid Prophage and a Chromosomally Integrated Prophage That Becomes Lytic in Plant Infections

April 2011 , Volume 24 , Number  4
Pages  458 - 468

Shujian Zhang,1 Zomary Flores-Cruz,1 Lijuan Zhou,2 Byung-Ho Kang,3 Laura A. Fleites,1 Mark D. Gooch,4 Nelson A. Wulff,5 Michael J. Davis,6 Yong-Ping Duan,7 and Dean W. Gabriel1

1Plant Pathology Dept., University of Florida, Gainesville, FL 32611, U.S.A.; 2Indian River Research and Education Center, University of Florida, Ft. Pierce, FL 34945, U.S.A.; 3Microbiology and Cell Science Dept., University of Florida, Gainesville, FL 32611, U.S.A.; 4Division of Plant Industry, Florida Department of Agriculture and Consumer Services, Gainesville, FL 32608, U.S.A.; 5Departamento Científico, Fundecitrus, Araraquara, SP 14807-040, Brazil; 6Citrus Research and Education Center, University of Florida, Lake Alfred, FL 33850, U.S.A.; 7Horticultural Research Laboratory, USDA-ARS, Fort Pierce, FL 34945, U.S.A.

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Accepted 14 December 2010.

Huanglongbing (HLB), also known as citrus greening, is a lethal disease of citrus caused by several species of ‘Candidatus Liberibacter’, a psyllid-transmitted, phloem-limited, alpha proteobacteria. ‘Ca. Liberibacter asiaticus’ is widespread in Florida citrus. The recently published ‘Ca. L. asiaticus’ psy62 genome, derived from a psyllid, revealed a prophage-like region of DNA in the genome, but phage have not been associated with ‘Ca. L. asiaticus’ to date. In the present study, shotgun sequencing and a fosmid DNA library of curated ‘Ca. L. asiaticus’ UF506, originally derived from citrus symptomatic for HLB, revealed two largely homologous, circular phage genomes, SC1 and SC2. SC2 encoded putative adhesin and peroxidase genes that had not previously been identified in ‘Ca. L. asiaticus’ and which may be involved in lysogenic conversion. SC2 also appeared to lack lytic cycle genes and replicated as a prophage excision plasmid, in addition to being found integrated in tandem with SC1 in the UF506 chromosome. By contrast, SC1 carried suspected lytic cycle genes and was found in nonintegrated, lytic cycle forms only in planta. Phage particles associated with ‘Ca. L. asiaticus’ were found in the phloem of infected periwinkles by transmission electron microscopy. In psyllids, both SC1 and SC2 were found only as prophage.

© 2011 The American Phytopathological Society