Nature Struct. Biol. 6, 557– 560 (1999).

A printer's error resulted in deletion of the last half of a sentence, which should have appeared as the last line on page 557. The full-text and PDF versions on the web site are correct. The final sentence on page 557 of the print version should have read:

For λexc ≥ 435 nm, only the B form is excited and emits light, but no B* →* I* conversion takes place.

In addition, in this paper, the legend of Fig. 4b should have read:

b, Emission spectrum of I upon direct excitation into I. The photoinduced reaction between A and I is reversible.

We regret any confusion these errors may have caused.