We have investigated mutants of phytochrome Cph1 from the cyanobacter-ium SynechocystisPCC6803 in order to study chromophore–protein inter-actions. Cph1D2, the 514-residue N-terminal sensor module produced as a recombinant His6-tagged apoprotein in Escherichia coli, autoassembles in vitro to form a holoprotein photochemically indistinguishable from the full-length product.