Episodic sitewise positive selection on the signal recognition particle protein Ffh in Actinobacteria

In this work, we report a case of episodic sitewise positive selection acting on the highly conserved SRP protein Ffh in Actinobacteria. An elevated non-synonymous to synonymous mutation ratio (ω) was observed along the non-terminal branches of the species tree, which contained 11 Actinobacteria species, where positively selected residues were frequently observed within the signal sequence-binding domain. Together with the estimated lineage-specific ω ratios for several core components in the major protein translocation systems, our data suggest that this positive selection might be partially driven by the diversification of signal sequences.