AEM Accepts, published online ahead of print on 23 October 2009

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Appl. Environ. Microbiol. doi:10.1128/AEM.01214-09
Copyright (c) 2009, American Society for Microbiology and/or the Listed Authors/Institutions. All Rights Reserved.

Genomic Characterization of Intron-containing T7-like Phage phiL7 of Xanthomonas campestris

Chia-Ni Lee, Juey-Wen Lin, Shu-Fen Weng, and Yi-Hsiung Tseng^*

Institute of Microbiology, Immunology and Molecular Medicine, Tzu Chi University, Hualien 907, Taiwan; Institute of Biochemistry, National Chung Hsing University, Taichung 402, Taiwan; Institute of Molecular Biology, National Chung Hsing University, Taichung 402, Taiwan

^* To whom correspondence should be addressed. Email: yhtseng{at}mail.tcu.edu.tw.

The lytic phage phiL7, which morphologically belongs to the Siphoviridae family, infects Xanthomonas campestris pv. campestris (Xcc). Nucleotide sequence analysis has revealed that phiL7 contains a linear double-stranded 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 (Xoo) 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 Xoo phages: 1) phiL7 has a group I intron inserted in the DNA polymerase gene, the first such intron observed in Xanthomonas phages; 2) although infection of phiL7 exerted inhibition to the host RNA polymerase, similar to the situations in Xoo phages Xp10 and Xop411, sequence analysis did not identify a homologue of the Xp10 p7 that controls the shift from host RNAP to viral RNAP during transcription; and, 3) 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.