Demonstrating evolutionary relationships between macromolecular sequences through mutual relationships with a third sequence.

Corresponding sites of the Euglena chloroplast and yeast small subunit ribosomal RNAs (rRNAs) show only an insignificant match with each other but show extensive matches with Euglena chloroplast tRNA(arg). The match with the tRNA extends farther toward the 5' end of the Euglena rRNA and toward the 3' end of the yeast rRNA. The expected number of such configurations given the number of RNAs searched is about 1 in 100,000. Comparison of two sequences with a third sequence frequently reveals relationships where pairwise comparisons fail to do so.