Abstract
Agrobacterium tumefaciens is a model organism which delivers genes into plants and induces plant tumors. We study the photobiology of A .tumefaciens, which contains two phytochromes, Agp1 and Agp2, and two photolyases, PhrA and PhrB. Photolyases are DNA repair enzymes that revert the UV-induced formation of cyclobutan pyrimidine dimers (CPD) or (6-4) photoproducts in a light dependent manner. PhrA is a CPD photolyase, whereas PhrB serves as (6-4) photolyase, as shown by HPLC based repair assays. We have obtained the crystal structures of PhrA and PhrB . Both structures revealed interesting insight into function and evolution of photolyases. PhrA has two Trp triades that serve as electron pathways for photoreduction of the chromophore, FAD. An antenna chromophore, methenyl tetrahydrofolate (MTHF) is bound to a site of the protein that is different from other photolyases. This finding could be a clue for the antenna chromophore of plant cryptochromes, which are closely related with PhrA. PhrB, which is so far the only prokaryotic protein with documented (6-4) photorepair activity, has a 6-7 dimethyl 8 ribityllumazin antenna chromophore and an FeS cluster. Phylogenetic studies reveal that PhrB is a representative of an ancient type of photolyases.
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