This thesis presents structural and biochemical studies of a phytochrome, Agp1, a bilin-binding red-light receptor protein. Crystallographic studies were undertaken in order to obtain structural insight into the mechanism of photoconversion and signal transduction from the sensor domain to the signalling domain, the latter hypothesised to be universally conserved among two-component histidine kinases. Using Agp1 from a non-photosynthetic plant pathogen, Agrobacterium tumefaciens as a model phytochrome, structural determination of both Pr and Pfr-forms was attempted, in order to permit unbiased structural comparison of the two. Application of the surface entropy reduction strategy led to determination of the structure of Agp1 in its Pr-form and crystals of Agp1 in the Pfr-like form were obtained for the first time. Biochemical studies were undertaken to probe the conformational differences between Agp1 as apoprotein, Pr-form, and Pfr-like form. Limited differences in secondary structure exist between the forms of Agp1. Conformational differences between the Pr and the Pfr-like form seem to underlie the fact that the space group of Agp1 crystals in the Pfr-like form is different from that of Agp1 in the Pr-form. The ability of the Agp1 apoprotein to form a dimer via a disulphide bond at the N-terminal chromophore-binding cysteine residue implies flexibility of the N-terminal region which allows for the initial bilin incorporation during holoprotein formation.