Figure 2.

Cartoon schematic of early aaRS evolution. (A) A schematic summary of early aaRS evolution. Very early aaRS precursors are predicted to have aminoacylated tRNA micro- or mini- helices with substrate promiscuity. One minihelix would be recognized simultaneously by class I and class II precursors. Gene duplication (i) would have allowed these early aaRS precursors to achieve greater substrate specificity as they (ii) acquired additional domains like the anticodon-binding domains shown here to become the ancestors of modern aaRSs. Figure adapted from [8,16]. (B) List of aaRSs that existed at LUCA versus those that emerged post-LUCA. Enzymes discussed in this article are highlighted in red. CysRS might have existed at LUCA or emerged post-LUCA; this possibility is noted with a question mark.