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The recognition process of tRNASerand tRNASecby human seryl‐tRNA synthetase (SerRS) was studied using T7 transcripts representing defined regions of human tRNASeror tRNASecand the influence of the tRNA elements on serylation and tertiary structure was elucidated. The anticodon arms of both tRNAs showed no contribution to serylation in contrast to the acceptor stems and the long extra arms. D and T arms were only involved in formation of the L‐shaped tRNA structure, not in the recognition process between tRNAs and SerRS. This is the first report of microhelices adapted from human tRNAs being aminoacylated by their homologous synthetase.

We have previously shown that the exchange of the discriminator base A73 of human tRNALeu for G is alone sufficient to achieve complete loss of leucine acceptance and to create an efficient serine acceptor. The reverse identity switch, however, which was studied using T7 RNA polymerase transcripts of in vitro mutagenized tRNA genes, reveals a far more complex pattern of identity elements for tRNALeu Introduction of the following tRNALeu-specific structures is necessary to transform human tRNASer into an efficient leucine acceptor: the discriminator base A73, the base pairs C3:G70, A4:U69 and G5:C68 of the acceptor stem, C20a of the DHU loop and the long extra arm. In contrast to tRNASer human tRNALeu identity requires both the sequence and the correct orientation of the long extra arm, whereas only its orientation is essential for serine identity.