A helix with a boronate ester linkage was constructed by DNA- or RNA-templated assembly through the plugging together of appropriately designed half-sequences, one with a boronic acid at its 5' end, the other with a ribonucleotide at its 3' end. The two hybridization steps in the three-component assembly (see picture) could be controlled reversibly by external stimuli, including an acid or base, cyanide ions, or fructose.