A single-site mutation in the XPAC gene alters photoproduct recognition

The XPAC (xeroderma pigmentosum group A complementing) gene, which is located on chromosome 9, carries a variety of point mutations in XP group A patients. We investigated the role of the XPAC gene product in excision repair by generating revertants of an XP group A cell line (XP12RO) that have increased resistance to ultraviolet light. One of these cell lines, XP129, can repair (6-4) pyrimidine-pyrimidone photoproducts normally but has reduced repair of cyclobutane dimers, as in XP12RO. Sequence analysis of cDNA from the XPAC gene indicated that XP12RO contains a termination codon at amino acid position 207, resulting in a reduced amount of mRNA and no detectable protein. In the revertant XP129 line, this termination codon has been mutated further and now encodes glycine in one allele instead of the wild-type arginine. The mRNA level detected by allele-specific polymerase chain reaction amplification was greater for the reverted sequence than for the chain-terminating sequence. These observations indicated that a point mutation resulting in a mis-sense mutation in the XPAC gene and altered expression of the XPAC protein can alter the substrate specificity of the excision repair system, and imply that the XPAC gene product plays an important role in photoproduct recognition.