Genomic Characterization of the Intron-Containing T7-Like Phage phiL7 ofXanthomonas campestris

Abstract

The lytic phage phiL7, which morphologically belongs to the Siphoviridae family, infects Xanthomonas campestris pv. campestris. Nucleotide sequence analysis has revealed that phiL7 contains a linear doublestranded DNA genome (44,080 bp, 56% G؉C) with a 3-protruding cos site (5-TTACCGGAC-3) and 59 possible genes. Among the deduced proteins, 32 have homologs with known functions and 18 show no database similarities; moreover, the genes encoding these 18 proteins mostly have varying G؉C contents and form clusters dispersed along the genome. Only 39 genes have sequences related (27% to 78%) to those of sequenced genes of X. oryzae pv. oryzae phages, although the genome size and architecture of these Xanthomonas phages are similar. These findings suggest that phiL7 acquired genes by horizontal transfer, followed by evolution via various types of mutations. Major differences were found between phiL7 and the X. oryzae pv. oryzae phages: (i) phiL7 has a group I intron inserted in the DNA polymerase gene, the first such intron observed in Xanthomonas phages; (ii) although infection of phiL7 exerted inhibition to the host RNA polymerase, similar to the situations in X. oryzae pv. oryzae phages Xp10 and Xop411, sequence analysis did not identify a homologue of the Xp10 p7 that controls the shift from host RNA polymerase (RNAP) to viral RNAP during transcription; and (iii) phiL7 lacks the tail fiber protein gene that exhibits domain duplications thought to be important for host range determination in OP1, and sequence analysis suggested that p20 (tail protein III) instead has the potential to play this role.The genus Xanthomonas in the gamma subdivision of the Proteobacteria is a diverse and economically important group of bacterial plant pathogens consisting of 27 species (35), causing tremendous loss in agriculture worldwide. Individual species of this genus comprise multiple pathogenic variants (pathovars) and collectively cause diseases on at least 124 monocot species and 268 dicot species: for example, Xanthomonas campestris pv. campestris causes black rot in crucifers, X. campestris pv. vesicatoria causes spot disease in peppers and tomatoes, X. axonopodis pv. citri causes citrus canker, and X. oryzae pv. oryzae causes bacterial leaf blight on rice plants (17). In addition, members of the genus Xanthomonas commonly produce great amounts of an exopolysaccharide (xanthan gum) and X. campestris pv. campestris has been the organism of choice in industry for production of xanthan, which is useful in food, agriculture, industry and cosmetics (55).For controlling bacterial plant diseases, chemical control with antibiotics and copper compounds has been the standard (31, 46). However, many agrochemicals are toxic and may pose significant environmental and health risks (14). Therefore, there has been a resurgence of interest recently in using bacteriophages to replace the currently prevalent chemical measures, in which phages can be used effectively as part of integrated disease management strategies (12,15,16,32). Although pha...

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“…The Xanthomonas phage phiL7 is a lytic phage, which has a long tail and belongs to the Siphoviridae family. It infects the plant pathogen Xanthomonas campestris [ 48 ].…”

CRISPR elements provide a new framework for the genealogy of the citrus canker pathogen Xanthomonas citri pv. citri

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