Insect Control: Biological and Synthetic Agents - Index of

toxicity to insects. Oral insecticidal activity against
another lepidopteran, Pieris brassicae, was used to
identify tc gene homologs from X. nematophilus
PMF1296 (Morgan et al., 2001). In this study, a
cosmid library was made in Escherichia coli and
individual cosmids were screened for oral activity.
Two overlapping cosmids with oral activity were recovered
and one of these was completely sequenced.
Transposon mutagenesis of the cosmids was then
carried out to discover which open reading frames
were involved in oral toxicity (Morgan et al., 2001).
The genes encoding insecticidal activity were termed
Xenorhabdus protein toxins (xpt) and insertions
within the large gene xptA1 were associated with a
loss of oral activity (Morgan et al., 2001). The predicted
amino acid sequences of the xpt genes
show high similarity to the tc genes of Photorhabdus
and their closest homologs have been indicated in a
revised map of the open reading frames found in the
cosmid (Figure 2). The orally toxic Xenorhabdus
cosmid therefore contains two tcdA homologs
transcribed in opposite directions (xptA1 and
xpta2) with tcaC (xptC1) andtccC (xptB1) homologs
between them. Expression of one tcdA-like
gene (xptA1) alone in E. coli was sufficient to reproduce
oral activity in E. coli (Morgan et al., 2001).
This data suggests that the two different genera of
9: Insecticidal Toxins from Photorhabdus and Xenorhabdus 315
Figure 1 The toxin complex a (tca) locus from three different Photorhabdus strains highlighting the three different color-coded
elements tcaAB, tcaC, and tccC (see key). Note that this locus is only complete in strain W14 (in which it confers oral toxicity) and that
in TT01 the locus is deleted from the equivalent location in the genome and found as a partially complete copy elsewhere (see text
for discussion).
nematode symbionts use similar toxin complex-like
genes.
9.2.1.3. Genomic organization of Photorhabdus tc
genes Extended sequencing of the tcd locus in
strain W14 revealed that clusters of tcdA-like
genes were present in the Photorhabdus W14
genome (Figure 3). Further, these multiple tcdA
and tcdB-like genes are inserted next to an AspV
tRNA in a potential pathogenicity island (Waterfield
et al., 2002). This island is a region of DNA inserted
in putative ‘‘core’’ sequence, i.e., sequence similar to
core sequences in E. coli and Yersinia pestis. The island
contains multiple copies of tccC-like sequences,
which may promote recombination, and also enteric
repetitive intergenic consensus (ERIC) sequences
(Waterfield et al., 2002). The region also contains
numerous directly duplicated open reading frames
and a fragment of a tcdA-like gene, suggesting that
it may be prone to rearrangement. Finally, a comparison
of pathogenicity islands or phage inserted at
the Asp tRNA in other related bacteria shows similar
genomic islands carrying other toxin genes and rhs
elements in place of tccC-like elements (Figure 4). All
these factors combine to support the concept that
the tcd-island is a pathogenicity island. Moreover it
is interesting to note that this island is adjacent to a