Identification of divergent botulinum neurotoxin homologs in Paeniclostridium ghonii

Abstract

Botulinum neurotoxins (BoNTs) are the most potent family of toxins known to science. Bioinformatic studies in recent years have revealed that they are members of a broader toxin family, with an increasing number of divergent homologs identified in genomes of organisms outside of the Clostridium genus. Here, we report the identification of two putative divergent BoNT-like homologs in the genomes of two strains of Paeniclostridium ghonii. We designated them PG-toxin 1 (PGT1) and PG-toxin 2 (PGT2), which share ~54% protein sequence identity. Unlike any other known BoNT homologs, PGT1 and PGT2 are composed of two separate subunits encoded on two neighboring genes: one encoding the protease domain (light chain, LC) with a conserved HExxH motif, and the second encoding the heavy-chain (HC) containing the putative translocation domain and receptor-binding domain. Phylogenetic analysis of both the LC and HC reveal that it is a divergent member of the lineage of BoNT that also includes BoNT/X, BoNT/En and the insecticidal PMP1. The gene clusters harboring PGT1 and PGT2 also include a putative insecticidal delta-endotoxin, Cry8Ea1, as well as putative endolysin and bacteriocin genes that may facilitate lytic toxin secretion, suggesting a possibility that this gene cluster might serve an insecticidal purpose.