Insecticidal toxins
The insecticidal toxin complex (Tc) proteins were first purified from P. luminescens [61]. Tc homologues have also been described to be present in Yersinia spp. and in other insect-associated bacteria such as Serratia entomophila and Xenorhabdus nematophilus [62,63]. The respective genes encoding four high molecular weight toxin complexes are termed tca, tcb, tcc and tcd. Further experiments supported the hypothesis that TccC-like proteins might act as universal activators of, or chaperons for, different toxin proteins, while Tca-like and Tcd-like proteins contribute predominantly to the oral toxicity of bacterial supernatants [17]. It is speculated that the Tc toxins are active against different tissues within individual hosts, namely Tcb against hemocytes and Tcd and Tca against cells of the insect gut. In Y. enterocolitica, the insecticidal toxin genes are located on a distinct genomic island termed tc-PAIYe of 21 kb, and are low-temperature induced [7]. Similar islands in which regulatory genes are followed by three tca genes, phage-related genes and one or two tccC genes, are present in the genomes of Y. pseudotuberculosis IP32953 and Y. pestis KIM. In P. luminescens, the insecticidal genes are organized in the tcd island harbouring nine tcd- and tcc-like genes and several non tc-like genes, while further nine tcc-like genes are scattered over the chromosome [24]. The reason for the over-represence of tc-like genes in the P. luminescens genome might reflect the different strategies followed by both bacteria within insects, namely the rapid killing for exploiting the victim as a food source in case of P. luminescens, and infection of and persistance within the invertebrate host as possibly preferred by Y. enterocolitica.