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 hyper modified tRNA nucleoside queuosine. The enzyme catalyzes the synthesis of an epoxycyclopentane moiety and transfers this compound to specifi c tRNAs containing the queuosine precursor 7-(aminomethyl)-7-deazaguan ine (preQ1). S-adenosylmethionine (AdoMet) is the sole cofactor that i s 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 diffe rent 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, containi ng 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-ad enosylmethionine:tRNA ribosyltransferase-isomerase QueA is proposed.