Amino acid acceptor identity switch of Escherichia coli tmRNA from alanine to histidine in vitro.

According to a trans -translation model, tmRNA facilitates the resumption of translation that has been stalled on the ribosome with the 3' end of a terminator-less mRNA, to produce a chimera polypeptide of the nascent peptide and the tmRNA-encoding 11 amino acid-tag. The first alanine residue of the tag-sequence is encoded neither by mRNA nor by tmRNA. This alanine is a key molecule for this model, in which it is derived from the alanine moiety aminoacylated to tmRNA. This is supported only by the observation that a point mutation at the third base-pair position of the acceptor stem of Escherichia coli tmRNA that deprives it of its aminoacylation ability causes abolishment of tag-peptide synthesis in vitro. Here, we made an E. coli tmRNA mutant with a completely switched amino acid acceptor identity from alanine to histidine by transplanting the upper half of the acceptor stem of tRNAHis. This histidine acceptor tmRNA mutant still retained an ability of tag-specific amino acid incorporation into the polypeptide in an in vitro poly(U)-dependent tag-peptide synthesis system, with an altered amino acid composition. Histidine, which is not a constituent of the original tag-peptide, was incorporated into the mutant-directed tag. The molar ratio of amino acids incorporated is consistent with that in the tag-sequence with the only expected change being the first amino acid from alanine to histidine. These results indicate that the first alanine residue of the tag-peptide is actually derived from that aminoacylated to tmRNA and is substitutable by other amino acids during the trans -translation processes. Copyright 1999 Academic Press.