A molecular movie at 1.8 angstrom resolution displays the photocycle of photoactive yellow protein, a eubacterial blue-light receptor, from nanoseconds to seconds

The photocycle of the bacterial blue-light photoreceptor, photoactive yello w protein, was stimulated by illumination of single crystals by a 7 ns lase r pulse. The molecular events were recorded at high resolution by time-reso lved X-ray Laue diffraction as they evolved in real time, from I ns to seco nds after the laser pulse. The complex structural changes during the photoc ycle at ambient temperature are displayed in a movie of difference electron density maps relative to the dark state. The step critical to entry into t he photocycle is identified as flipping of the carbonyl group of the 4-hydr oxycinnamic acid chromophore into an adjacent, hydrophobic environment rath er than the concomitant isomerization about the double bond of the chromoph ore tail. The structural perturbation generated at the chromophore propagat es throughout the entire protein as a light-induced "protein quake" with it s "epicenter" at the carbonyl moiety of the chromophore.