Since XAB harbors tandem arrays of TPR, a possible scaffolding function for XAB within cellular processes including NER and transcription is in line with its deduced amino acid sequence.XAB may function as a bridging protein, by simultaneously interacting with several other proteins or protein complexes.In addition, it would be of interest to find out whether defects in XAB also give rise to a human condition, since both CSA and CSB are associated with the severe neurodevelopmental, UVsensitive TCR disorder Cockayne syndrome.JeanMarc Egly for kindly providing antiRNA polymerase II monoclonal antibody and Dr.jbc.M When a correction for this article is posted Click here to choose from all of JBCs email alerts This article cites references, of which can be accessed free at http:www.jbc.orgcontent.full.htmlreflist Downloadedfromhttp: www.jbc.orgbyguestonSeptember, To define the mechanisms of action of MED, we implemented a sensitive glycosylase assay amenable to kinetic analysis.MED lacks uracil glycosylase activity on singlestrand DNA and abasic site lyase activity.A kinetic analysis revealed that MED displays a fast first cleavage reaction followed by slower subsequent reactions, resulting in biphasic time course; this is due to the tight binding of MED to the abasic site reaction product rather than a consequence of enzyme inactivation.The integrity of genetic information is constantly challenged by a variety of endogenous and exogenous DNA damaging agents. Cellular DNA transactions occur in aqueous solution containing reactive oxygen species, and, as such, DNA is prone to both hydrolytic and oxidative damage.The costs of publication of this article were defrayed in part by the payment of page charges.This article must therefore be hereby marked advertisement in accordance with U.Oxidative lesions include oxoguanine, thymine glycol, and formamidopyrimidine derivatives of reasch Azathioprine adenine and guanine. In addition to endogenous damaging processes, DNA is exposed to the attack of exogenous reactive species, including alkylating agents and the carcinogens vinyl chloride and ethyl carbamate.Efficient correction of these DNA lesions relies on the action of several enzymes belonging to the base excision repair system, base excision repair enzymes usually act in a lesionspecific fashion on a single damaged or mismatched nucleotide.Given the mutagenic potential of DNA lesions, continuing elucidation of the biochemical activities, damage spectrum and specificity of base excision repair enzymes has direct implications on cancer and aging. We previously showed that MED coimmunoprecipitates with MLH in human cells, binds to oligonucleotides containing methylcytosine on one or both strands, and has endonuclease activity on a supercoiled plasmid DNA substrate.We and others have previously shown that the MED gene is mutated in human colorectal, endometrial, and pancreatic carcinomas exhibiting a defect in the MMR genes MLH and MSH. Thus, the present findings suggest that a fraction of human carcinomas are defective not only in longpatch MMR but also in MED thymine, uracil, and FU glycosylase activity.The defect in MED activity may facilitate accumulation of DNA damage, thereby contributing to tumor development, and, in addition, may have implications in cancer treatments based on chemotherapy with FU.The AP site reaction product was purified by ion exchange chromatography at pH. Unincorporated label was removed with G spin columns.