Figure 2

The emergence of adaptors. (a) The replication of a DRT would produce its complementary strand, "C-DRT". The drawing style of the DRT is the same as that in Figure 1b, except that the aa-binding sites and spacer sequences are drawn in a linear way. In the C-DRT, the complements of the aa-binding sites are correspondingly drawn in dark gray, black and light gray, but as "empty lines", while the complements of the spacer sequences are still drawn as dashed lines. (b) Degradation or partial replication of DRTs might result in segments with a core aa-aptamer domain flanked by some spacer sequences. If the aa-aptamer domain could transform into a conformation with a large "recognition loop", the segments might become adaptors using the C-DRT as a template. (c) To become a qualified adaptor, the segment's 3'-arm (the dotted line) should fold towards its aa-binding site to accept the amino acid (Left), and then be "grasped" back by its 5'-arm, a simple situation for which could be that the 5'-arm (the dashed-dotted line) is complementary to the 3'-arm (Middle and Right). It should be noted that the adaptors derived from the aa-aptamers in such a way would act on the C-DRT (as shown in b) to direct the synthesis of the same peptide synthesized by the DRT. (d) The L-shape tRNA-like adaptor --- proto-tRNA. Its 3'-arm (the dotted line), containing the "T-loop", would fold towards its aa-binding site (Left), while its 5'-arm (the dashed-dotted line), containing the "D-loop", could grasp the 3'-arm back via spatial folding (Right). (e) The L-shape proto-tRNAs, with a "leg" vertically binding on the C-DRT and an "arm" horizontally delivering amino acids, might turn out to be very suitable for the successive peptide synthesis. Note: the proto-tRNA in d is drawn in reference to the real structure of a tRNA, while the proto-tRNAs in e are drawn in a simplified form.