REDUCED MISREADING OF ASPARAGINE CODONS BY ESCHERICHIA-COLI TRNA(LYS)WITH HYPOMODIFIED DERIVATIVES OF 5-METHYLAMINOMETHYL-2-THIOURIDINE INTHE WOBBLE POSITION

It has been suggested that modified nucleosides of the xm(5)(s(2))U(m) 34-type restrict the wobble capacity of the base, and that their funct ion is to prevent misreading in the third position of the codon in mix ed codon family boxes that encode two different amino acids. In this s tudy in Escherichia coli, the misreading in vivo of asparagine codons in bacteriophage MS2 mRNA by different hypomodified derivatives of tRN A(Lys), normally containing 5-methylaminomethyl-2-thiouridine (mnm(5)s (2)U34) in the wobble position, has been analysed. Contrary to what wo uld be predicted from the general hypothesis for the function of mnm(5 )s(2)U, it was found that the misreading of asparagine codons by tRNA( Lys) was greatly reduced in the mnmA (formerly asuE or trmU) and mnmE (formerly trmE) mutants which contain the hypomodified mnm(5)U34 and s (2)U34, respectively, instead of the fully modified mnm5s2U34. In addi tion, it was found that these hypomodified tRNAs were efficiently char ged with lysine in vivo, under the growth conditions employed. The lat ter result is at variance with results obtained in vitro. The results are discussed in relation to the postulated function for modified nucl eosides of the xm(5)s(2)U type. (C) 1998 Academic Press.