Evolutionary relationships between "Q-type" photosynthetic reaction centres: hypothesis-testing using parsimony.

Area of Science:

• Biochemistry
• Molecular Biology
• Evolutionary Biology

Background:

• Q-type photosynthetic reaction centers are crucial for energy conversion in various organisms.
• Understanding their evolutionary history is key to deciphering early life and photosynthesis.
• Previous models often extrapolated findings from purple bacterial reaction centers to Photosystem-II.

Purpose of the Study:

• To investigate the evolutionary relationships between different Q-type photosynthetic reaction centers.
• To test hypotheses regarding gene duplication events in reaction center evolution.
• To clarify the ancestral state of bacterial photosynthetic reaction centers.

Main Methods:

• Amino acid parsimony analysis of subunit sequences.
• Sequence alignment using dot matrix comparisons.
• Phylogenetic reconstruction to infer evolutionary relationships.

Main Results:

• Strong evidence for independent gene duplications producing L/M subunits (purple bacteria) and D1/D2 (Photosystem-II).
• Support for L/M subunits of Chloroflexus aurantiacus originating from the same duplication event as purple bacterial subunits.
• Suggests an ancestral heterodimeric bacterial reaction center.

Conclusions:

• The evolutionary paths of purple bacterial reaction centers and Photosystem-II likely involved independent gene duplications.
• An ancestral heterodimeric reaction center is proposed, predating divergence into different types.
• Photosystem-II may be more ancient than previously assumed, cautioning against direct extrapolation from purple bacterial systems.