Chiral proofreading during protein biosynthesis and its evolutionary implications.

Homochirality of biomacromolecules is a prerequisite for their proper functioning and hence essential for all life forms. This underscores the role of cellular chiral checkpoints in enforcing homochirality during protein biosynthesis. d-Aminoacyl-tRNA deacylase (DTD) is an enzyme that performs 'chirality-based proofreading' to remove d-amino acids mistakenly attached to tRNAs, thus recycling them for further rounds of translation. Paradoxically, owing to its l-chiral rejection mode of action, DTD can remove glycine as well, which is an achiral amino acid. However, this activity is modulated by discriminator base (N73) in tRNA, a unique element that protects the cognate Gly-tRNA . Here, we review our recent work showing various aspects of DTD and tRNA coevolution and its key role in maintaining proper translation surveillance in both bacteria and eukaryotes. Moreover, we also discuss two major optimization events on DTD and tRNA that resolved compatibility issues among the archaeal and the bacterial translation apparatuses. Importantly, such optimizations are necessary for the emergence of mitochondria and successful eukaryogenesis.