The inserted fragment includes a transposase gene and five truncated ORFs (Fa–Fe) that share sequence similarity to tail fiber genes. In P2 phage, insertions commonly occur in the fun(Z) gene location (Nilsson & Haggard-Ljungquist, 2007). Mobilization of the inserted sequences in the respective strains may have been facilitated by the transposases encoded in the inserted

element and by pairs of direct and inverted repeats identified in this region (Fig. 2 and Table S4). Both xnp1 and xbp1 encode the CI repressor rather than a C-type repressor selleckchem typically found in P2 phage. Induction with mitomycin C suggests that the formation of ssDNA-RecA nucleoprotein complexes is likely to be involved in the regulation of xenorhabdicin production. xnp1 and xbp1 also contain a dinI gene that is not usually found in P2-type phage. DinI is involved in the stabilization of ssDNA-RecA complexes (Lusetti et al., 2004). Typical P2-type lysis genes are not present in xnp1 or xbp1; however, both contain a conserved enp gene that encodes a putative

endolysin. Neither locus contains a holin gene homolog. A lambdoid-like holin gene had previously been identified in X. nematophila Cell Cycle inhibitor F1 that may facilitate secretion of endolysin into the periplasm (Brillard et al., 2003). Alternatively, the holin gene (hol-1) from the xnp2 and xbp2 loci (data not shown) may provide holin lysis timing function. Similar to other phage systems, it is Erastin in vitro also possible that the endolysin protein may accumulate in the cytoplasm until it leaks out and causes damage to the cell wall (Garrett et al., 1981; Young, 2002). The main fiber proteins, XnpH1 (728 amino acids) of X. nematophila and XbpH1 (872 amino acids) of X. bovienii, are mosaic structures in which the N-terminal, middle, and C-terminal regions display distinct patterns

of sequence conservation. The first 213 residues of the N-terminus of these fiber proteins share 93% sequence identity (Fig. 4a, blue boxes). The high level of sequence identity correlates with this region of the protein being involved in fiber assembly (Haggard-Ljungquist et al., 1992). The middle region of XnpH1 between amino acids 402 and 509 (Fig. 4a, lavender box) is 80% identical to the N-terminal 108 residues of Fa. In addition, the 520–728 region of XnpH1 is 46% identical to Fc (not shown). It is of interest to note that Fb, Fd, and Fe comprise a second group of truncated fiber genes that do not share sequence similarity to the C-terminal region of XnpH1 but are similar to each other (Fig. 4b). The middle region of XbpH1 between amino acids 368 and 577 (Fig. 4a, dark pink) is 100% identical to the N-terminal 210 residues of Fh (Fh-N). The C-terminal region of XnpH1 and XbpH1 each contain sequences that are highly similar to a truncated fiber gene in the opposing genome.