The activation domain of class I aminoacyl-tRNA synthetases, which contains the Rossmann fold and the signature sequences HIGH and KMSKS, is generally split into two halves by the connective peptides (CP1, CP2) whose amino acid sequences are idiosyncratic. CP1 has been shown to participate in the binding of tRNA as well as the editing of the reaction intermediate aminoacyl-AMP or the aminoacyl-tRNA. No function has been assigned to CP2. The amino acid sequence of Acidithiobacillus ferrooxidans TrpRS was predicted from the genome sequence. Protein sequence alignments revealed that A. ferrooxidans TrpRS contains a 70 amino acids long CP2 that is not found in any other bacterial TrpRS. However, a CP2 in the same relative position was found in the predicted sequence of several archaeal TrpRSs. A. ferrooxidans TrpRS is functional in vivo in Escherichia coli. A deletion mutant of A. ferrooxidans trpS lacking the coding region of CP2 was constructed. The in vivo activity of the mutant TrpRS in E. coli, as well as the kinetic parameters of the in vitro activation of tryptophan by ATP, were not altered by the deletion. However, the K m value for tRNA was seven-fold higher upon deletion, reducing the efficiency of aminoacylation. Structural modeling suggests that CP2 binds to the inner corner of the L shape of tRNA.
