Chromatid exchanges may be induced by damage in sites of transcriptional activity.

A conditional expression system has allowed us to vary the expression level of the xeroderma pigmentosum group A (XPA) photoproduct-specific DNA-binding protein in human cells and so control the response of cells to damage by UV light. Using a form of XPA that contains a single missense mutation (R207G) enabled us to study a lower range of function than that obtained with the wild-type sequence. This form of XPA has been previously shown to stimulate pyrimidine dimer excision preferentially in actively transcribed genes. We found that UV resistance increased as a linear function of XPA expression levels. Excision of (6-4) pyrimidine-pyrimidone photoproducts in the whole genome increased to a maximum at about the haploid level of XPA expression, but there was little pyrimidine dimer excision from the whole genome. SCE frequency induced by UV light was high in cells with no SPA expression and fell rapidly with increasing levels of SPA expression within 0-50% of the haploid level of expression. No further reduction in SCE frequency was produced at the highest levels of XPA expression, when repair replication extended to the overall genome. We speculate that a low level of repair, especially that occurring in actively transcribed genes, may selectively eliminate photoproducts that are particularly important in causing cell killing and SCEs.