Evolution of codon and amino acid usage in bacterial protein toxins.

Toxin proteins are secreted by most pathogens as an integral part of pathogenic mechanism(s). The toxins act by either damaging the host cell membrane (for example, pore-forming toxins and RTX toxins) or by modulation of important cellular pathways (for example, inhibition of protein translation by ribosome-inactivating proteins). The mechanism of action of these toxins provides the pathogen with strategies for adaptation in the unfavorable host environment. Though, secreted by different pathogenic species, the protein toxins seem to share common features that allow the protein to bind to specific molecules and enter the host cell. Earlier studies have suggested role of several events like horizontal gene transfer and insertion-deletion mutations in evolution of protein toxins. The present study involving 125 bacterial protein toxins secreted by 49 pathogenic bacteria focuses on the role and constraints of the bacterial genome on evolution of codon and amino acid usage in respective bacterial protein toxins. We compare the nucleotide composition, codon and dinucleotide usage trends between different classes of bacterial protein toxins and between individual toxins and the parent bacterial genome expressing the toxin(s).