Lysine 207 as the site of cross-linking between the 3'-end of Escherichia coli initiator tRNA and methionyl-tRNA formyltransferase

The specific formylation of initiator methionyl-tRNA by methionyl-tRNA formyltransferase (MTF) is important for initiation of protein synthesis in Escherichia coli. In attempts to identify regions of MTF that come close to the 3'-end of the tRNA, we oxidized P-32-3'-end-labeled E. coli initiator methionine tRNA with sodium metaperiodate and cross-linked it to MTF. The cross-linked MTF was separated from uncross-linked MTF by DEAE-celulose chromatography, and the tRNA in the cross-linked MTF was hydrolyzed with nuclease P1 and RNase T1, leaving behind an oxidized fragment of [P-32]AMP attached to MTF. Trypsin digestion of the cross-linked MTF followed by high pressure liquid chromatography of the digest yielded two peaks of radioactive peptides, I* and II*. These peptides were characterized by N- and/or C-terminal sequencing and by matrix-assisted laser desorption ionization mass spectroscopy. Peptide I* contained amino acids Gln(186)-Lys(210) with Lys(207) as the site of the cross-link. Peptide II*, a partial digestion product, contained amino acids Gln(186)-Arg(214) also with Lys(207) as the site of the cross-link, The molecular masses of peptides I* and II* indicate that the final product of the cross-linking reaction between the periodate-oxidized AMP moiety of the tRNA and Lys(207) is most likely a morpholino derivative rather than a reduced Schiffs base.