Proposal of a Twin Aarginine Translocator System-Mediated Constraint against Loss of ATP Synthase Genes from Nonphotosynthetic Plastid Genomes

Organisms with nonphotosynthetic plastids often retain genomes; their gene contents provide clues as to the functions of these organelles. Yet the functional roles of some retained genes -- such as those coding for ATP synthase -- remain mysterious. In this study, we report the complete plastid genome and transcriptome data of a nonphotosynthetic diatom and propose that its ATP synthase genes may function in ATP hydrolysis to maintain a proton gradient between thylakoids and stroma, required by the twin arginine translocator (Tat) system for translocation of particular proteins into thylakoids. Given the correlated retention of ATP synthase genes and genes for the Tat system in distantly related nonphotosynthetic plastids, we suggest that this Tat-related role for ATP synthase was a key constraint during parallel loss of photosynthesis in multiple independent lineages of algae/plants.