Park E, Guzder S N, Koken M H, Jaspers-Dekker I, Weeda G, Hoeijmakers J H, Prakash S, Prakash L
Department of Biophysics, University of Rochester School of Medicine, NY 14642.
Proc Natl Acad Sci U S A. 1992 Dec 1;89(23):11416-20. doi: 10.1073/pnas.89.23.11416.
Xeroderma pigmentosum (XP) patients are extremely sensitive to ultraviolet (UV) light and suffer from a high incidence of skin cancers, due to a defect in nucleotide excision repair. The disease is genetically heterogeneous, and seven complementation groups, A-G, have been identified. Homologs of human excision repair genes ERCC1, XPDC/ERCC2, and XPAC have been identified in the yeast Saccharomyces cerevisiae. Since no homolog of human XPBC/ERCC3 existed among the known yeast genes, we cloned the yeast homolog by using XPBC cDNA as a hybridization probe. The yeast homolog, RAD25 (SSL2), encodes a protein of 843 amino acids (M(r) 95,356). The RAD25 (SSL2)- and XPBC-encoded proteins share 55% identical and 72% conserved amino acid residues, and the two proteins resemble one another in containing the conserved DNA helicase sequence motifs. A nonsense mutation at codon 799 that deletes the 45 C-terminal amino acid residues in RAD25 (SSL2) confers UV sensitivity. This mutation shows epistasis with genes in the excision repair group, whereas a synergistic increase in UV sensitivity occurs when it is combined with mutations in genes in other DNA repair pathways, indicating that RAD25 (SSL2) functions in excision repair but not in other repair pathways. We also show that RAD25 (SSL2) is an essential gene. A mutation of the Lys392 residue to arginine in the conserved Walker type A nucleotide-binding motif is lethal, suggesting an essential role of the putative RAD25 (SSL2) ATPase/DNA helicase activity in viability.
着色性干皮病（XP）患者对紫外线（UV）极度敏感，由于核苷酸切除修复缺陷，患皮肤癌的几率很高。该疾病在遗传上具有异质性，已鉴定出七个互补组，A - G。在酿酒酵母中已鉴定出人类切除修复基因ERCC1、XPDC/ERCC2和XPAC的同源物。由于在已知的酵母基因中不存在人类XPBC/ERCC3的同源物，我们使用XPBC cDNA作为杂交探针克隆了酵母同源物。酵母同源物RAD25（SSL2）编码一个843个氨基酸的蛋白质（分子量95,356）。RAD25（SSL2）和XPBC编码的蛋白质有55%的相同氨基酸残基和72%的保守氨基酸残基，并且这两种蛋白质在包含保守的DNA解旋酶序列基序方面彼此相似。RAD25（SSL2）中第799位密码子的无义突变删除了45个C末端氨基酸残基，导致对紫外线敏感。该突变与切除修复组中的基因表现出上位性，而当它与其他DNA修复途径中的基因突变结合时，紫外线敏感性会协同增加，这表明RAD25（SSL2）在切除修复中起作用，而在其他修复途径中不起作用。我们还表明RAD25（SSL2）是一个必需基因。保守的沃克A型核苷酸结合基序中赖氨酸392残基突变为精氨酸是致命的，这表明推定的RAD25（SSL2）ATP酶/DNA解旋酶活性在生存能力中起重要作用。
