Book of Abstracts: Albany 2007
June 19-23 2007
A G-Quadruplex as a Light-Harvesting Antenna Within a Deoxyribozyme With Photolyase Activity
We usedin vitro selection to investigate whether enzymes made of nucleic acids (ribozymes or deoxyribozymes) are capable of catalyzing photochemical reactions. The reaction chosen was photo-reactivation of thymine cyclobutane dimers in DNA, using serotonin as cofactor and light of wavelengths longer than the typical absorption spectrum of DNA. Curiously, two distinct classes of deoxyribozymes were selected -- one, Sero1A, has an absolute requirement for serotonin, whereas the other, UV1C, does not require any extraneous cofactor for its catalysis. The two deoxyribozymes show some important differences -- Sero1A is optimally catalytic with ∼315 nm irradiation (although activity extends upto 350 nm), whereas UV1C shows a narrower action spectrum, with optimal catalysis at 305 nm. Although the catalytic enhancement parameters of the two enzymes are comparable, UV1C is limited in its substrate usage, efficiently repairing only thymine dimers (and not uracil dimers). Sero1A, by contrast, repairs both kinds of pyrimidine dimers efficiently. Whereas Sero1A undoubtedly uses its bound indole moiety for a reductive repair of its substrates, mechanistic and crosslinking studies of UV1C suggest that a guanine quadruplex within the latter?s higher order structure likely serves as a light-harvesting antenna, with photo-reactivation of the thymine dimer proceeding via electron donation from an excited guanine base. In a primordial RNA World, nucleic acid populations may have been vulnerable to deactivation via ultraviolet light-induced pyrimidine dimer formation. Photolyase nucleic acid enzymes such as the ones reported here could have played a role in preserving the integrity of such an RNA World.
References and Footnotes
Simon Fraser University,