Photochemical sterilization of 3SR reactions.

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Abstract
The self-sustained sequence replication (3SR) reaction is an extremely efficient method for amplifying target DNA and RNA sequences that may be present in minute quantities. A serious problem often encountered in its practice is carryover contamination from products of previous 3SR reactions. A postamplification treatment of 3SR reaction products with the photoactive agent 4'-aminomethyl-4,5-dimethylisopsoralen (IP-10) was investigated as an approach for preventing carryover contamination by 3SR amplicons. Initially, inhibition of the amplification reaction by high concentrations of the reagent was observed. This problem was circumvented by developing a gel-based delivery of IP-10, and the method was found to provide highly efficient sterilization (approximately 10(6)-fold) of 3SR amplicons. Evaluation of this strategy on a number of 3SR targets has indicated that the degree of sterilization is dependent on the length of the amplified region and on the concentration of IP-10. It appears that the sterilization effect is caused by covalent modification of the pyrimidine bases of RNA and DNA, which renders them unusable as templates for the 3SR reaction. Modification of a purified RNA transcript with IP-10 was shown to prevent effectively reverse transcription by avian myeloblastosis virus reverse transcriptase (AMV RT). Similarly, treatment of a T7 RNA polymerase promoter-containing DNA template with IP-10 eliminated full-length transcription by T7 RNA polymerase. This isopsoralen method may be used to sterilize multiple 3SR reactions in a clinical assay with a convenient UV irradiation step.

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