Figure 7
The dual complemenarity might provide indirect evidence in support of the existence of small local "bulges" in the earliest precursors of the acceptor stem. (A) Shown is a pair of proto-tRNAs with complementary anticodons that has, in parallel, complementary 2nd bases in their acceptor stems. When aligned head-to-tail, these sequences reveal mismatches at the 2nd position (at least), meaning that the two could not have originated concertedly (as a pair) with one proto-tRNA being a complementary replica of another [26, 28, 29]. (B) The 2nd base pairs are identical in these two tRNAS with complementary anticodons meaning that, in contrast to "A", their concerted origin is possible, but the dual complementarity is not. We have noted this difference between A and B before, but interpreted it in a different way 2[6]. (C) In this case, proto-tRNAs with complementary anticodons could originate concertedly (as a pair of + and - sequences) but, in contrast to "B", their 2nd bases in the acceptors are also complementary. However, simultaneous maintenance of these two properties is possible if and only if there are local distortions of acceptor helices ("bulges") just at the 2nd position. Remarkably, such local mismatches could facilitate recognition of anticodon precursors adjacent to the universal NCCA 3' terminus by putative ribozymes with aminoacylating activity. The fact that the dual complementarity does exist (Table 1) favors this ("C") case.