Evidence for evolutionary relationship between archaeplastidal and cyanobacterial phytochromes based on their chromophore pockets.

Phytochromes are photoreceptor proteins with a bilin chromophore that undergo photoconversion between two spectrally different forms, Pr and Pfr. In plants, phytochromes play a central role in growth and differentiation during the entire life cycle. Phytochromes of plants and other groups of archaeplastida have a common evolutionary origin in prokaryotes, but the exact prokaryotic origin is as yet uncertain. Two possibilities are presently discussed: either, archaeplastidal phytochromes arose from the last eukaryotic common ancestor (LECA) or they arose from the cyanobacterial endosymbiont that gave rise to plastids. We first constructed standard phylogenetic trees based on N-terminal protein sequences of the chromophore module. As usual, variation of algorithms and parameters led to different trees. A relationship between cyanobacteria and archaeplastida was observed in 7 out of 36 trees. The lack of consistency between results obtained from variation of parameters of tree constructions reflects the uncertainty of archaeplastidal origin. To gain more information about a possible cyanobacterial and archaeplastidal relationship, we performed phylogenetic studies based on the amino acids that line the chromophore pockets. These amino acids are highly conserved and could provide more accurate information about long evolutionary time scales, but the reduction of traits could also lead to insignificant results. From 30 selected chromophore-binding amino acids, 6 were invariant. The subsequent studies were thus based on the information dependent on 24 or fewer amino acid positions. Again, multiple trees were constructed to get information about the robustness of relationships. The very low number of information-containing traits resulted in low bootstrap values and many indistinguishable leaves. However, the major groups fungi, bacteria, cyanobacteria, and plants remained united. Without exception, cyanobacteria and archaeplastida were always closely linked. In this respect, the results were more robust than those of the classic approach, based on long contiguous sequences. We therefore consider cyanobacteria as the most likely origin of archaeplastidal phytochromes.