Abstract
Inovirus XacF1 (7325 nucleotides) is integrated into the genome of Xanthomonas citri pv. citri (Xcc) strains at the host dif site (attB) by the host XerC/D recombination system. The XacF1 attP sequence is located within the coding region of ORF12, a possible phage regulator. After integration, this open reading frame (ORF) is split into two pieces on the host genome. We examined dynamic integration/excision of XacF1 in Xcc strain MAFF 301080 and found that the integration started at 4 h postinfection (p.i.) and peaked at 12 h p.i. Thereafter, the ratio of integrated to free forms remained constant, suggesting equilibrium of integration and excision of XacF1 in the host genome. However, the integrated state became very unstable following a 5′‐deletion of ORF12 in XacF1, suggesting that ORF12 plays a key role in the integration cycle of XacF1 in Xcc strains.
Highlights
Asian citrus canker (ACC), caused by Xanthomonas citri pv. citri, Xcc, is one of the most serious diseases in citrus-producing areas of the tropical and subtropical countries [1,2]
A deletion mutant of XacF1 (ΔXacF1) that lacks an entire region of ORF12 could infect the host cells with forming plaques, but did not give any consistent integration bands in PCR
It is known that XacF1 can integrate into the host genome by site-specific recombination by the host XerC/D system [6]
Summary
Asian citrus canker (ACC), caused by Xanthomonas citri pv. citri, Xcc (syn., Xanthomona saxonopodis pv. citri and Xanthomonas campestris pv. citri), is one of the most serious diseases in citrus-producing areas of the tropical and subtropical countries [1,2]. A filamentous bacteriophage (inovirus) XacF1 infecting Xcc strains was isolated and characterized [6] This phage encodes 13 open reading frames (ORF) and its genome is 7325 nucleotides in size (Fig. S1). After infection by XacF1, the bacterial host cells showed several cultural and physiological changes including lower levels of EPS production, Abbreviations ACC, Asian citrus canker; EPS, exopolysaccharide; MM, minimal medium; NA, nutrient agar; NB, nutrient broth; ORF, open reading frame; p.i., postinfection; Xcc, Xanthomonas citri pv. Integration of phage XacF1 in Xanthomonas citri reduced motility, slower growth rate, and a dramatic reduction in the virulence [6] This virulence-lowering effect of XacF1 infection suggests that XacF1 could be used as a biocontrol agent against citrus canker pathogens. Because phage infection effects on the host (such as the virulence-lowering effect) may change depending on the phage states, namely a free-replicating state or a prophage state [8], we are interested to characterize the dynamics of XacF1 infection and integration in the Xcc host cells
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