Fig. 1. The cellular synthesis of an aminoacyl-tRNA and its role in protein synthesis. An uncharged tRNA and the corresponding amino acid are first selected from the cellular pools of similar molecules by the appropriate AARS (see text for details). After synthesis and release from the AARS the aminoacyl-tRNA is delivered to the ribosome, where its anticodon can then interact with the corresponding codon in mRNA (reviewed in Al-Karadaghi et al., 2000). The example shown illustrates how this leads to the translation of the codon UGG as tryptophan during the elongation phase of protein synthesis.

Fig. 2. Examples of phylogenetically diverse bacteria and archaea with non-canonical aminoacyl-tRNA synthesis pathways as inferred by genome sequence analysis [see text for details; data derived from DDBJ/EMBL/GenBank and IGwit, Bifidus data (F. Arigoni, Nestle Research Center, personal communication)]. Organisms within the blue oval do not encode CysRS; within the red oval encode a class I LysRS; within the green oval lack GlnRS; enclosed within the yellow oval lack AsnRS. Thus, for example, Rhodobacter lacks AsnRS and GlnRS and contains a class I LysRS, whereas Chlamydia also lacks AsnRS and GlnRS but does not contain a class I LysRS. The position of Rhodopseudomonas indicates that it contains GlnRS and a class I LysRS but lacks AsnRS, while Pseudomonas only lacks AsnRS.

Michael Ibba and Dieter Söll