The Evolutionary Origin Of Chloroplast Transit Peptide

Proteins targeted to chloroplast usually have chloroplast transit peptide(cTP) at their N-terminals.Since chloroplast is the endosymbiotic ruminant of Cyanobacteria,a natural consideration is made that cTPs may originate from Cyanobacteria.However,a number of proteins targeted to chloroplast do not seem to be homologs of Cyanobacteria.The observation that chloroplast transit peptides can target proteins to nonplant mitochondria raise the possibility that the chloroplast transit peptide may originate from the mitochondrial presequence.What is the evolutionary origin of chloroplast transit peptides? This is an old but unsolved question.We investigated the appearance of cTP-like sequences in 7 protein sequence datasets across three domains of life,including Cyanobacteria,Alpha purple proteobacteria,Slime molds, aquificales and thermatogales,archaea,as well as chloroplast and plant mitochondria genomes. The results indicate that,in Cyanobacteria,alpha purple proteobacteria,Slime molds and archaea, the cTP-like sequences were over-represented;while in chloroplast and plant mitochondria genomes,the cTP-like sequences were under-represented;and in ancient bacteria(aquificales and thermatogales),the occurrence of cTP-like sequences is indistinguishable from that of the randomized dataset.Our results suggest that chloroplast transit peptides presumably have two evolutionary origins:Bacteria(cyanobacterial and alpha purple proteobacterial) and Achaea.Moreover, chloroplast transit peptides may have appeared early at the divergence of archaea and eukaryotes.