In addition to the geometry confinement exerted mainly by steric effects, the electrostatic interactions with the protein environment may play a key role in the modulation of photophysical and photochemical properties of chromophores. Indeed, an increasing number of recent studies have shown that proteins not only provide cavities for their substrates but also modulate reactivity and reaction selectivity through electrostatic interactions with substrates. In green fluorescent protein (GFP) and its mutants, the electrostatic environments imposed by different proteins were found to play key roles in spectral modulation. Similarly, among the species of fireflies and their analogous beetles, it has been well-established that the effect of the electrostatic environments is the main factor in the emission colortuning mechanism. Mutant firefly luciferases can yield different colors by inducing different local electric fields around the luciferin. Therefore, it is important to gain an in-depth understanding of the electrostatic effect in bacterial luciferase system. The elucidation of this question will contribute to the rationalization of the color modulation mechanism in luciferase and fluorescent proteins caused by amino acid mutations or pH changes, and to their rational design to emit light at specific wavelengths.