Structure of crystalline Escherichia coli methionyl-tRNA(f)(Met) formyltransferase: Comparison with glycinamide ribonucleotide formyltransferase

Formylation of the methionyl moiety esterified to the 3' end of tRNA(f)(Met) is a key step in the targeting of initiator tRNA towards the translation start machinery in prokaryotes. Accordingly, the presence of methionyl-tRNA(f)(Met) formyltransferase (FMT), the enzyme responsible for this formylation, is necessary for the normal growth of Escherchia coli. The present work describes the structure of crystalline E.coli FMT at 2.0 Å resolution. The protein has an N-terminal domain containing a Rossmann fold. This domain closely resembles that of the glycinamide ribonucleotide formyltransferase (GARF), an enzyme which, like FMT, uses N-10 formyltetrahydrofolate as formyl donor. However, FMT can be distinguished from GARF by a flexible loop inserted within its Rossmann fold. In addition, FMT possesses a C-terminal domain with a β-barrel reminiscent of an OB fold. This latter domain provides a positively charged side oriented towards the active site. Biochemical evidence is presented for the involvement of these two idiosyncratic regions (the flexible loop in the N-terminal domain, and the C-terminal domain) in the binding of the tRNA substrate.