Functional diversification of the twin-arginine translocation pathway mediates the emergence of novel ecological adaptations.

Microorganisms occupy a myriad of ecological niches that show an astonishing diversity. The molecular mechanisms underlying microbes' ecological diversity remain a fundamental conundrum in evolutionary biology. Evidence points to that the secretion of a particular set of proteins mediates microbes' interaction with the environment. Several systems are involved in this secretion, including the Sec secretion system and the Tat pathway. Shifts in the functions of proteins from the secretion systems may condition the set of secreted proteins and can, therefore, mediate adaptations to new ecological niches. In this manuscript, we have investigated processes of functional divergence (FD)-a term used here to refer to the emergence of novel functions by the modification of ancestral ones-of Tat pathway proteins using a large set of microbes with different lifestyles. The application of a novel approach to identify FD allowed us to distinguish molecular changes in the three Tat proteins among different groups of archaea and bacteria. We found these changes as well as the composition of secreted proteins to be correlated with differences in microbe's lifestyles. We identified major signatures of FD in halophilic and thermophilic archaea as well as in pathogenic bacteria. The location of amino acids affected by FD in functionally important domains of Tat proteins made it possible to find the link between the molecular changes in Tat, the set of secreted proteins and the environmental features of the microbes. We present evidence that links specific molecular changes in secretion mediating proteins of microbes to their ecological adaptations.