A G-wire composing of multiple G-rich sequences in parallel orientations via G-quartet formations is promising as a structurally well-defined nanowire. Here, in order to control the G-wire by the metal ion, we incorporated 2,2'-bipyridine unit, which has two aromatic rings rotating arbitrarily by coordination with metal ions, into the main chain of d(G4T4G4) instead of thymine (G1). Circular dichroism (CD) spectra showed that G1 formed an antiparallel G-quadruplex. Interestingly, Ni(2+) induces G1 to switch from an antiparallel G-quadruplex to a parallel. On the other hand, the parallel G-quadruplex reversed the structural transition to an antiparallel G-quadruplex by the addition of Na2EDTA. Moreover, the study of the atomic force microscopy (AFM) showed that G1 formed a long nanowire ( approximately 200 nm) in the presence of Ni(2+), whereas no significant image was observed in the absence of Ni(2+) or in the presence of both Ni(2+) and Na2EDTA. From these results, it can be concluded that Ni(2+) induces a structural transition of G1 from an antiparallel G-quadruplex to a G-wire and that metal ion chelator induces the reverse structural transition from the G-wire to the antiparallel G-quadruplex. The G-wire responsive to metal ions would be useful for development of functional nanomaterials.