Phytochromes are photochromic photoreceptors with a bilin chromophore that have been found in plants and bacteria. Typical bacterial phytochromes are composed of an N-terminal photosensory chromophore module and a C-terminal protein kinase. The former contains the chromophore, which allows phytochromes to adopt the two interconvertible spectral forms, Pr and Pfr. The N-terminal photosensory module of Agrobacterium phytochrome Agp1, Agp1-M15, was used for crystallization studies. The protein was either assembled with the natural chromophore biliverdin or a sterically locked synthetic biliverdin-derivative, termed 15Za. The last-named adduct does not undergo photoisomerization due to an additional carbon chain between the rings C and D of the chromophore. Both adducts could be crystallized, but the resolution was largely improved by the use of 15Za. Crystals of biliverdin-Agp1-M15 diffract to 6 angstrom resolution and belong to the tetragonal space group I422 with unit cell dimensions a = b = 171 angstrom, c = 81 angstrom, crystals of 15Za-Agp1-M15 belong to the same space group with similar unit cell dimensions a = b = 174 angstrom, c = 80 angstrom, but diffract to 3.4 angstrom resolution. Assuming the asymmetric unit to be occupied by one monomer of 55 kDa, the unit cell contains 54-55% solvent with a crystal volume per protein mass, V-m, of 2.7 angstrom(3) Da(-1). (C) 2005 Elsevier Inc. All rights reserved.