Evolutionary substitution of two amino acids in chloroplast SRP54 of higher plants cause its inability to bind SRP RNA

The chloroplast signal recognition particle (cpSRP) consists of a conserved 54 kDa subunit (cpSRP54) and a unique 43 kDa subunit (cpSRP43) but lacks SRP-RNA, an essential and universally conserved component of cytosolic SRPs. High sequence similarity exists between cpSRP54 and bacterial SRP54 except for a plant-specific C-terminal extension containing the cpSRP43-binding motif. We found that cpSRP54 of higher plants lacks the ability to bind SRP-RNA because of two amino acid substitutions within a region corresponding to the RNA binding domain of cytosolic SRP54, whereas the C-terminal extension does not affect RNA binding. Phylogenetic analysis revealed that these mutations occur in the cpSRP54 homologues of higher plants but not in most algae.