Evidence for weak interaction between phytochromes Agp1 and Agp2 from Agrobacterium fabrum

During bacterial conjugation, plasmid DNA is transferred from cell to cell. In Agrobacterium fabrum, conjugation is regulated by the phytochrome photoreceptors Agp1 and Agp2. Both contribute equally to this regulation. Agp1 and Agp2 are histidine kinases, but, for Agp2, we found no autophosphorylation activity. A clear autophosphorylation signal, however, was obtained with mutants in which the phosphoaccepting Asp of the C-terminal response regulator domain is replaced. Thus, the Agp2 histidine kinase differs from the classical transphosphorylation pattern. We performed size exclusion, photoconversion, dark reversion, autophosphorylation, chromophore assembly kinetics and fluorescence resonance energy transfer measurements on mixed Agp1/Agp2 samples. These assays pointed to an interaction between both proteins. This could partially explain the coaction of both phytochromes in the cell.