We investigated the earliest steps of the photocycle of photoactive yellow protein (PYP) and its R52Q mutant in aqueous solution, by subpicosecond transient absorption spectroscopy. Our aim was to address the role of the positively charged Arg52 residue. We found that the relaxation mechanism of R52Q is similar to that of wt-PYP and discarded that Arg52 plays a key role in the chromophore photoreactivity. The excited-state decay of R52Q is however slower, confirming previous fluorescence up-conversion data. The experiments also reveal a slower stabilization of the cis isomer product. The loosening of the protein pocket and structural heterogeneities are discussed. (c) 2006 Elsevier B.V. All rights reserved.