Escherichia coli twin arginine (Tat) mutant translocases possessing relaxed signal peptide recognition specificities

The twin arginine (Tat) secretion pathway allows the translocation of folded proteins across the cytoplasmic membrane of bacteria. Tat-specific signal peptides contain a characteristic amino acid motif ((S/T)RRXFLK) including two highly conserved consecutive arginine residues that are thought to be involved in the recognition of the signal peptides by the Tat translocase. Here, we have analyzed the specificity of Tat signal peptide recognition by using a genetic approach. Replacement of the two arginine residues in a Tat-specific precursor protein by lysine-glutamine resulted in an export-defective mutant precursor that was no longer accepted by the wild-type translocase. Selection for restored export allowed for the isolation of Tat translocases possessing single mutations in either the amino-terminal domain of TatB or the first cytosolic domain of TatC. The mutant Tat translocases still efficiently accepted the unaltered precursor protein, indicating that the substrate specificity of the translocases was not strictly changed; rather, the translocases showed an increased tolerance toward variations of the amino acids occupying the positions of the twin arginine residues in the consensus motif of a Tat signal peptide.