To study the relationship between transcription and strand-specific repair of UV-induced cyclobutane pyrimidine dimers, dimer removal was analyzed in a cell line containing two alleles of an inactivated adenosine deaminase (ADA) gene. The cell line was derived from a patient suffering from severe combined immunodeficiency. The disease was caused by a deletion of the complete promoter of the gene as well as the first exon of the ADA gene. This resulted in a true null allele without any

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... transcription (Berkvens, T.M., Gerritsen, E. J. A., Oldenburg, M., Breukel, C., Wijnen, J. T. H., Van Ormondt, H., Vossen, J. M., Van der Eb, A. J., and Meera Khan, P. (1987) Nucleic Acids Res. 15, 9365-9378). Despite this lack of transcription, repair of the ADA gene in this cell line was found to be very efficient with 80% of the dimers being removed within 24 h after UV irradiation. However, the initial rapid repair which is associated with the transcribed strand in normal cells is absent. Dimer removal from two inactive loci, 754 and coagulation factor IX, was much less efficient with only 40% dimers removed after 24 h. From this data, we conclude that transcription is not required for efficient repair of a gene, but forms an additional signal for accelerated repair of the transcribed strand. Furthermore, we suggest that different levels of repair exist between non-transcribed sequences in active genes and those in repressed loci. The results are discussed in terms of the current ideas about the mechanism of preferential DNA repair in human cells.