FeSBCP analogue from cyanobacteria: insights from in vitro and in silico studies

The blue light responsive Cryptochrome/Photolyase Family harbours two important photoactivatable proteins - cryptochromes and photolyases. While cryptochromes are essentially photoreceptors with diverse biological activities including circadian rhythm, photolyases repair UV damaged cyclobutane pyrimidine dimers (CPD) or (6-4) pyrimidine-pyrimidone photoadducts. Both cryptochromes and photolyases share a common Photolyase Homology Region (PHR) where they harbour a light harvesting antenna chromophore and redox-active flavin di-nucleotide (FAD). Even though photolyases do not possess an extension in the C-terminal region unlike cryptochromes, a recent discovery of FeS cluster containing bacterial cryptochrome-photolyase proteins (FeSBCPs) present an interesting retreat from the conventional photolyase architecture. They possess an additional modular redox center, [4Fe4S] iron-sulfur cluster connected by conserved cysteine residues. Herein, we characterize a cyanobacterial FeSBCP from Cyanobium sp. (CyPhrB) using tools from bioinformatics, biochemistry and biophysics. Sequence analysis reveals substitution in the well-conserved aromatic residues, meant for electron transfer. In silico modeling and docking supported by electrophoretic mobility shift assay as well as spectroscopic measurements do suggest efficient binding and repair of CyPhrB to damaged single/double-stranded substrates containing 6 -4  photolesion. Considering FeS clusters' established roles in DNA binding/repair activities, we speculate the role of FeS clusters in FeSBCPs and existence of FeS-FAD-DNA triangle towards efficient electron transfer.