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

Abstract

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.

Diversity and Plasticity of the Intracellular Plant Pathogen and Insect Symbiont “Candidatus Liberibacter asiaticus” as Revealed by Hypervariable Prophage Genes with Intragenic Tandem Repeats

“…While closing gaps for the "Ca. Liberibacter asiaticus" Psy2 genome, we found two prophage regions containing a large number of phage-related genes (16,54). Based on cloning and sequencing of a 3,513-bp DNA fragment (cloning ID pLJ108) from one of the prophage regions, one gene containing multiple nearly identical tandem repeats (NITRs) was identified and named as hyv I .…”

“…Liberibacter asiaticus" genome (16,54). Characterization of the variations of these prophages/ phages in the HLB bacteria may provide insight into their evolution and adaptation to host plants and insects.…”

“…Recent findings revealed two circular phage genomes, SC1 and SC2, which are present in the Florida "Ca. Liberibacter asiaticus" bacterial genome (54). The forms of the phage/prophage genomes, circular, linear, or integrated into the "Ca.…”

“…The forms of the phage/prophage genomes, circular, linear, or integrated into the "Ca. Liberibacter asiaticus" chromosome, varied with their different hosts (54). Since hyv I and hyv II were localized in two different prophage regions of the "Ca.…”

Detection and Characterization of Miniature Inverted-Repeat Transposable Elements in "Candidatus Liberibacter asiaticus"

“…No transposon was annotated in the complete sequences of three phages/prophages of "Ca. Liberibacter asiaticus," SC1, SC2, and FP2 (23,24). In sequence analyses, two characteristics are highly indicative of transposons: presence of TIRs and identification of a transposase gene (tps).…”