Comparative Analysis of argK-tox Clusters and Their Flanking Regions in Phaseolotoxin-Producing Pseudomonas syringae Pathovars Hiroyuki Genka, 1 Tomoya Baba, 2 Masataka Tsuda, 1 Shigehiko Kanaya, 3 Hirotada Mori, 3 Takanobu Yoshida, 4 Masako Tsujimoto Noguchi, 4 Kenichi Tsuchiya, 5 Hiroyuki Sawada 4 * 1 Graduate School of Life Sciences, Tohoku University, Sendai, Japan 2 Institute for Advanced Biosciences, Keio University, Tsuruoka, Yamagata, Japan 3 Nara Institute of Science and Technology, Ikoma, Nara, Japan 4 National Institute for Agro-Environmental Sciences, 3-1-3 Kannondai, Tsukuba, Ibaraki 305-8604, Japan 5 National Agriculture Research Center for Western Region, Zentsuji, Kagawa, Japan Received: 13 November 2005 / Accepted: 27 April 2006 [Reviewing Editor: Dr. Debashish Bhattacharya] Abstract. DNA fragments containing argK-tox clusters and their flanking regions were cloned from the chromosomes of Pseudomonas syringae pathovar (pv.) actinidiae strain KW-11 (ACT) and P. syringae pv. phaseolicola strain MAFF 302282 (PHA), and then their sequences were determined. Comparative analysis of these sequences and the sequences of P. syringae pv. tomato DC3000 (TOM) (Buell et al., Proc Natl Acad Sci USA 100:10181–10186, 2003) and pv. syringae B728a (SYR) (Feil et al., Proc Natl Acad Sci USA 102:11064–11069, 2005) revealed that the chromosomal backbone regions of ACT and TOM shared a high similarity to each other but presented a low similarity to those of PHA and SYR. Nevertheless, almost-identical DNA regions of about 38 kb were confirmed to be present on the chromosomes of both ACT and PHA, which we named ‘‘tox islands.’’ The facts that the GC content of such tox islands was 6% lower than that of the chromosomal backbone regions of P. syringae, and that argK-tox clusters, which are considered to be of exogenous origin based on our previous studies (Sawada et al., J Mol Evol 54:437–457, 2002), were confirmed to be contained within the tox islands, suggested that the tox islands were an exogenous, mobile genetic element inserted into the chromo- somes of P. syringae strains. It was also predicted that the tox islands integrated site-specifically into the homologous sites of the chromosomes of ACT and PHA in the same direction, respectively, wherein 34 common gene coding sequences (CDSs) existed. Furthermore, at the left end of the tox is- lands were three CDSs, which encoded polypeptides and had similarities to the members of the tyrosine recombinase family, suggesting that these putative site-specific recombinases were involved in the recent horizontal transfer of tox islands. Key words: Pseudomonas syringae — Pathovar phaseolicola — Pathovar actinidiae — Comparative analysis — Phaseolotoxin — argK-tox cluster — Horizontal transfer — Mobile genetic element — tox island — Tyrosine recombinase Introduction The Pseudomonas syringae complex is known to comprise phytopathogenic bacteria inhabiting the surface of various plants, which are genetically diverse and classified into more than 60 pathovars (pv.) on the basis of their pathogenicity and host specificities (Bradbury 1986; Rudolph 1995; information found at the web site of the International Society for Plant Pathology [www.isppweb.org/names_bacterial. asp]). *Present address: National Institute of Agrobiological Sciences, 2-1-2 Kannondai, Tsukuba, Ibaraki 305-8602, Japan Correspondence to: Hiroyuki Sawada; email: sawada@affrc.go.jp J Mol Evol (2006) 63:401–414 DOI: 10.1007/s00239-005-0271-4