ABSTRACT
The hexactinellid sponges (phylum Porifera) represent the phylogenetically oldest metazoans that evolved 570-750 million years ago. At this period exposure to ultraviolet (UV) light exceeded that of today and it may be assumed that this old taxon has developed a specific protection system against UV-caused DNA damage. A cDNA was isolated from the hexactinellid Aphrocallistes vastus which comprises high sequence similarity to genes encoding the protostomian and deuterostomian (6-4) photolyases. Subsequently functional studies were performed. It could be shown that the sponge gene, after transfection into mutated Escherichia coli, causes resistance of the bacteria against UV light. Recombinant sponge photolyase was prepared to demonstrate that this protein binds to DNA treated with UV light (causing the formation of thymine dimers). Finally, it is shown that the photolyase gene is strongly expressed in the upper part of the animals and not in their middle part or their base. It is concluded that sponges not only have an excision DNA repair system, as has been described earlier by us, but also a photolyase-based photo-reactivating system.
