Newly identified relatives of botulinum neurotoxins shed light on their molecular evolution

The evolution of bacterial toxins is a central question to understanding the origins of human pathogens and infectious disease. Through genomic data mining, we traced the evolution of the deadliest known toxin family, clostridial neurotoxins, comprised of tetanus and botulinum neurotoxins (BoNT). We identified numerous uncharacterized lineages of BoNT-related genes in environmental species outside ofClostridium, revealing insights into their molecular ancestry. Phylogenetic analysis pinpointed a sister lineage of BoNT-like toxins in the gram-negative organism,Chryseobacterium piperi, that exhibit distant homology at the sequence level but preserve overall domain architecture. Resequencing and assembly of theC. piperigenome confirmed the presence of BoNT-like proteins encoded within two toxin-rich gene clusters. AC. piperiBoNT-like protein was validated as a novel toxin that induced necrotic cell death in human kidney cells. Mutagenesis of the putative active site abolished toxicity and indicated a zinc metalloprotease-dependent mechanism. TheC. piperitoxin did not cleave common SNARE substrates of BoNTs, indicating that BoNTs have diverged from related families in substrate specificity. The new lineages of BoNT-like toxins identified by computational methods represent evolutionary missing links, and suggest an origin of clostridial neurotoxins from ancestral toxins present in environmental bacteria.Significance statementThe origins of bacterial toxins that cause human disease is a key question in our understanding of pathogen evolution. To explore this question, we searched genomes for evolutionary relatives of the deadliest biological toxins known to science, botulinum neurotoxins. Genomic and phylogenetic analysis revealed a group of toxins in theChryseobacterium piperigenome that are a sister lineage to botulinum toxins. Genome sequencing of this organism confirmed the presence of toxin-rich gene clusters, and a predictedC. piperitoxin was shown to induce necrotic cell death in human cells. These newly predicted toxins are missing links in our understanding of botulinum neurotoxin evolution, revealing its origins from an ancestral family of toxins that may be widespread in the environment.