Functional redundancy in the nonspecific RNA binding domain of a class I tRNA synthetase.

The sequence of a 228-amino acid nonspecific RNA binding domain appended to the N terminus of a eukaryote tRNA synthetase is shown here to have two lysine-rich clusters (LRCs) that are functionally significant in vivo and in vitro. These two LRCs have unrelated sequences and are separated by a spacer of over 100 amino acids. By using a sensitive test for function in vivo, each LRC is shown to be sufficient in the absence of the other. This sufficiency requires fusion of the spacer to either of the LRCs. Experiments in vitro confirmed that the LRCs are each important for RNA binding. Thus, this nonspecific RNA binding domain has two dissimilar lysine-rich sequence elements that are functionally redundant. Further experiments suggest that this redundancy is not used to dock two molecules of RNA but rather to enhance the overall affinity for a single RNA molecule.