Spectral properties of phytochrome Agp2 from Agrobacterium tumefaciens are specifically modified by a compound of the cell extract

Phytochromes are widely distributed photoreceptors that are converted by light between the red absorbing Pr and the far-red absorbing Pfr form. The soil bacterium Agrobacterium tumefaciens contains two phytochromes, Agp1 and Agp2, which act as light-regulated histidine kinases. Whereas most phytochromes are stable in the Pr form, Agp2 and few other phytochromes convert into Pfr in darkness. We have shown in a previous publication that the spectral properties of recombinant Agp2 are modified by compounds of the cell extract from an Agrobacterium agp1−/agp2− double knockout mutant. In the present work we performed concentration series which show that the interaction is specific and that the modifying factor has a concentration of ca. 0.2 μM. We have also performed a series of mixing experiments with the truncated protein Agp2–M2, which consists of the N-terminal chromophore module (501 amino acids). The cell extract inhibited the photoconversion of Agp2–M2 in an unspecific way. In concentration series, this negative effect was less pronounced when lower concentrations of Agp2–M2 were used. In the presence of excess Agp2–M2 apoprotein, the cell extract did no longer modify the spectral properties of Agp2. The data suggest that the factor of the cell extract interacts specifically with the N-terminal moiety of Agp2.