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

The 3 31 nucleotide minihelix tRNA theorem describes 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 positioning 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 2 unpaired bases, and tRNASer has 1 unpaired base. For Bacteria, tRNATyr has 2 unpaired bases, tRNALeu has 1 unpaired base and tRNASer has 0 unpaired bases. Thus, the number of type II tRNAs is limited by the possible set points of the arm. From 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.