Origin of Type II tRNA Variable Loops, Aminoacyl-tRNA Synthetase Allostery from Distal Determinants, and Diversification of Life

The three 31 nucleotide minihelix tRNA evolution theorem describes the evolution of type I and type II tRNAs to the last nucleotide. In databases, type I and type II tRNA V loops (V for variable) were improperly aligned, but alignment based on the theorem is accurate. Type II tRNA V arms were a 3′-acceptor stem (initially CCGCCGC) ligated to a 5′-acceptor stem (initially GCGGCGG). The type II V arm evolved to form a stem–loop–stem. In Archaea, tRNALeu and tRNASer are type II. In Bacteria, tRNALeu, tRNASer, and tRNATyr are type II. The trajectory of the type II V arm is determined by the number of unpaired bases just 5′ of the Levitt base (Vmax). For Archaea, tRNALeu has two unpaired bases, and tRNASer has one unpaired base. For Bacteria, tRNATyr has two unpaired bases, tRNALeu has one unpaired base, and tRNASer has zero unpaired bases. Thus, the number of synonymous type II tRNA sets is limited by the possible trajectory set points of the arm. From the analysis of aminoacyl-tRNA synthetase structures, contacts to type II V arms appear to adjust allosteric tension communicated primarily via tRNA to aminoacylating and editing active sites. To enhance allostery, it appears that type II V arm end loop contacts may tend to evolve to V arm stem contacts.