TRANSFER AND ISOMERIZATION OF THE RIBOSE MOIETY OF ADOMET DURING THE BIOSYNTHESIS OF QUEUOSINE TRANSFER-RNAS, A NEW UNIQUE REACTION CATALYZED BY THE QUEA PROTEIN FROM ESCHERICHIA-COLI

The enzyme QueA of E coli is involved in the biosynthesis of the hypermodified tRNA nucleoside queuosine. The enzyme catalyzes the synthesis of an epoxycyclopentane moiety and transfers this compound to specific tRNAs containing the queuosine precursor 7-(aminomethyl)-7-deazaguanine (preQ1). S-adenosylmethionine (AdoMet) is the sole cofactor that is required for this reaction (Slany et al, 1993, Biochemistry 32, 7811-7817). To proof that the ribose moiety of AdoMet is the precursor of the epoxycyclopentane moiety, labeled AdoMet, was generated from different types of H-3 ATP and methionine by the AdoMet synthetase enzyme (MetK) from E coli. The resulting H-3 labeled AdoMet was directly used as the cofactor for the QueA reaction. Using [2,5',8-H-3]ATP, containing tritium at C5' of the ribose ring, resulted in an incorporation of radioactivity into preQ(1) tRNA, whereas this was not the case when [2,8-H-3]ATP was applied. A model for the reaction catalyzed by the S-adenosylmethionine:tRNA ribosyltransferase-isomerase QueA is proposed.