Photolyase is a light‐dependent enzyme that repairs pyrimidine dimers in DNA. Two types of photolyases have been found in frog Xenopus laevis, one for repairing cyclobutane pyrimidine dimers (CPD photolyase) and the other for pyrimidine–pyrimidone (6–4)photoproduct [(6–4)photolyase]. However, little is known about the former type of the Xenopus photolyases. To characterize this enzyme and its expression profiles, we isolated the entire coding region of a putative CPD photolyase cDNA by extending an EST (expressed sequence tag) sequence obtained from the Xenopus database. Nucleotide sequence analysis of the cDNA revealed a protein of 557 amino acids with close similarity to CPD photolyase of rat kangaroo. The identity of this cDNA was further established by the molecular mass (65 kDa) and the partial amino acid sequences of the major CPD photolyase that we purified from Xenopus ovaries. The gene of this enzyme is expressed in various tissues of Xenopus. Even internal organs like heart express relatively high levels of mRNA. A much smaller amount was found in skin, although UV damage is thought to occur most frequently in this tissue. Such expression profiles suggest that CPD photolyase may have roles in addition to the photorepair function.
A fixed-point radiation monitoring system based on the gated counting method, which was previously reported by us, was constructed for a 45-MeV electron LINAC facility. It was tested under different beam intensities of the accelerator both at the current and repetition rates. It showed excellent dynamic response (from 2.5 X 10(-11) to 1.0 X 10(-8) Sv h-1) to the intensity variation of the radiation source. It was demonstrated that the system was very useful in monitoring an extremely low-level radiation from a pulsed source.
By the gated counting method that had been developed by the authors the spatial dose distributions at the boundary and inside of a 45-MeV electron linear accelerator (linac) facility were obtained using a NaI(Tl) scintillation probe and a counting system. Both distributions of pulse height and radiation propagation time were also measured. At the facility boundary, dose rates measured ranged from 0.16 microR hr-1 to 0.86 microR hr-1, which were less than one-tenth the natural background. At measurement in the linac control room, two different leakage sources, one originating at and around the accelerator and the other at a microwave power system, were discriminated by measuring pulse height and time distributions simultaneously.
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