ZINC-DEPENDENT TRANSFER-RNA BINDING BY A PEPTIDE ELEMENT WITHIN A TRANSFER-RNA SYNTHETASE

The class I aminoacyl-tRNA synthetases are defined by an N-terminal nu cleotide binding fold that contains the active site for adenylate synt hesis, Insertions and additions of idiosyncratic RNA binding elements that facilitate docking of the L-shaped tRNA structure are superimpose d onto this basic fold. These RNA binding elements are imagined to hav e been acquired during the evolution and development of the modern gen etic code. The monomeric Escherichia coli isoleucyl-tRNA synthetase ha s a zinc-containing peptide at its C terminus, Removal of the zinc-con taining peptide was shown previously to create a shortened enzyme with activity for adenylate synthesis but no detectable binding to tRNA(Il e) We show here that the isolated zinc-containing peptide binds to tRN A with relatively low affinity, This binding is not tRNA-specific but shows a strict requirement for zinc. In contrast, the zinc-containing peptide conferred specific and high-affinity binding when combined wit h the shortened enzyme. Thus, when combined with another protein, a no nspecific tRNA binding peptide is essential for formation of a high-af finity and specific tRNA binding site. These results demonstrate the f easibility of the idea that noncovalent complexes of general RNA-bindi ng peptides with a domain for adenylate synthesis were precursors to m odern tRNA synthetases, In addition, the results offer the first direc t evidence of a role for zinc in the tRNA-binding activity of one of t hese peptide elements.