Synergistic mutations produce blue-shifted bioluminescence in firefly luciferase

Light emission from the North American firefly Photinus pyralis, which emits yellow-green (557 nm) light, is widely believed to be the most efficient bioluminescence system known, making this luciferase an excellent tool for monitoring gene expression. In a previous study designed to produce luciferases for simultaneously monitoring two gene expression events, we identified a very promising blue-shifted emitter (548 nm) that contained the mutations Val241Ile, Gly246Ala, and Phe250Ser [Branchini, B. R., Southworth, T. L., Khattak, N. F., Michelini, E., and Roda, A. (2005) Red- and green-emitting firefly luciferase mutants for bioluminescent reporter applications, Anal. Biochem. 345, 140-148]. To establish the basis of the unusual blue-shifted emission, we determined that a simple additive effect of the three individual mutations did not account for the spectral properties of the triple mutant. Instead, the bioluminescence emission spectra of two double mutants containing Phe250Ser and either Val241Ile or Gly246Ala very closely resembled that of the triple mutant. Additional mutagenesis results confirmed that the blue-shifted emission of the double mutants was determined by the synergistic behavior of active site residues. Molecular modeling studies of the Gly246Ala and Phe250Ser double mutant supported the notion that the blue-shifted emission was due to localized changes that increased the hydrophobicity at the emitter site as a result of the addition of a single methyl group at position 246. Moreover, the modeling data suggested that the Ala246 side chain remained close to the emitter through an additional H-bond between Ala246 and the hydroxyl group of Phe250, providing a possible structural basis for the synergistic behavior.