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Cyclic  N   6 -threonylcarbamoyladenosine (‘cyclic t 6 A’, ct 6 A) is a non-thiolated hypermodification found in transfer RNAs (tRNAs) in bacteria, protists, fungi and plants. In bacteria and yeast cells ct 6 A has been shown to enhance translation fidelity and efficiency of ANN codons by improving the faithful discrimination of aminoacylated tRNAs by the ribosome. To further the understanding of ct 6 A biology we have determined the high-resolution crystal structures of CsdL/TcdA in complex with AMP and ATP, an E1-like activating enzyme from  Escherichia coli , which catalyzes the ATP-dependent dehydration of t 6 A to form ct 6 A. CsdL/TcdA is a dimer whose structural integrity and dimer interface depend critically on strongly bound K +  and Na +  cations. By using biochemical assays and small-angle X-ray scattering we show that CsdL/TcdA can associate with tRNA with a 1:1 stoichiometry and with the proper position and orientation for the cyclization of t 6 A. Furthermore, we show by nuclear magnetic resonance that CsdL/TcdA engages in transient interactions with CsdA and CsdE, which, in the latter case, involve catalytically important residues. These short-lived interactions may underpin the precise channeling of sulfur atoms from cysteine to CsdL/TcdA as previously characterized. In summary, the combination of structural, biophysical and biochemical methods applied to CsdL/TcdA has afforded a more thorough understanding of how the structure of this E1-like enzyme has been fine tuned to accomplish ct 6 A synthesis on tRNAs while providing support for the notion that CsdA and CsdE are able to functionally interact with CsdL/TcdA.

The Crystal Structure and Small-Angle X-Ray Analysis of CsdL/TcdA Reveal a New tRNA Binding Motif in the MoeB/E1 Superfamily