An antimicrobial origin of transit peptides accounts for early endosymbiotic events.

Primary endosymbiosis, which gave rise to mitochondria or chloroplasts, required successful targeting of a number of proteins from the host cytosol to the endosymbiotic organelles. A survey of studies published in separate fields of biological research over the past 40 years argues for an antimicrobial origin of targeting peptides. It is proposed that mitochondria and chloroplast derive from microbes that developed a resistance strategy to antimicrobial peptides that consisted in their rapid internalization and proteolytic disposal by microbial peptidases.