Antonia M. Pedrini

Mutations in the XPD helicase gene result in XP and TTD phenotypes, preventing interaction between XPD and the p44 subunit of TFIIH

In most cases, xeroderma pigmentosum group D (XP-D) and trichothiodystrophy (TTD) patients carry mutations in the carboxy-terminal domain of the evolutionarily conserved helicase XPD, which is one of the subunits of the transcription/repair factor TFIIH (Refs 1,2). In this study, we demonstrate that XPD interacts specifically with p44, another subunit of TFIIH, and that this interaction results in the stimulation of 5´→3´ helicase activity. Mutations in the XPD C-terminal domain, as found in most patients, prevent the interaction with p44, thus explaining the decrease in XPD helicase activity and the nucleotide excision repair (NER) defect.

New functions of XPC in the protection of human skin cells from oxidative damage

Xeroderma pigmentosum (XP) C is involved in the recognition of a variety of bulky DNA‐distorting lesions in nucleotide excision repair. Here, we show that XPC plays an unexpected and multifaceted role in cell protection from oxidative DNA damage. XP‐C primary keratinocytes and fibroblasts are hypersensitive to the killing effects of DNA‐oxidizing agents and this effect is reverted by expression of wild‐type XPC. Upon oxidant exposure, XP‐C primary keratinocytes and fibroblasts accumulate 8,5′‐cyclopurine 2′‐deoxynucleosides in their DNA, indicating that XPC is involved in their removal. In the absence of XPC, a decrease in the repair rate of 8‐hydroxyguanine (8‐OH‐Gua) is also observed. We demonstrate that XPC–HR23B complex acts as cofactor in base excision repair of 8‐OH‐Gua, by stimulating the activity of its specific DNA glycosylase OGG1. In vitro experiments suggest that the mechanism involved is a combination of increased loading and turnover of OGG1 by XPC‐HR23B complex. The accumulation of endogenous oxidative DNA damage might contribute to increased skin cancer risk and account for internal cancers reported for XP‐C patients.

Determination of pyrimidine dimer unwinding angle by measurement of DNA electrophoretic mobility

Abstract The unwinding angle produced by the formation of one pyrimidine dimer has been estimated to be −14.3 ° ±0.2 °. This value has been obtained by titrating the number of pyrimidine dimers necessary to reduce the number of superhelical turns by one in each topoisomer obtained by treatment of a supercoiled DNA with DNA topoisomerase I.

Induction of polynucleotide ligase in human lymphocytes stimulated by phytohemoagglutinin

Abstract The treatment of human lymphocytes with phytohemoagglutinin causes the appearance of the activity of polynucleotide ligase, rising at least 50 fold from levels below the background. This increase takes place in the fourth or fifth day after treatment, and is delayed by one day approximately with respect to the rise of DNA synthesis rate; the activity of two other enzymes of DNA metabolism, DNA polymerase and a DNase acting on single-stranded DNA, increases in parallel with the DNA synthesis rate.

Nalidixic acid does not inhibit bacterial transformation

SummaryNalidixic acid at concentrations that block completely the DNA replication of Bacillus subtilis does not inhibit its genetic transformation and does not appreciably affect the growth of the DNA phage SPP1 on the same organism.

DNA Repair in Human Cells: Enzymes and Mutants

A variety of chemical and physical agents may cause damage to DNA, i.e., they may lead to the presence of chemical structures different from those of the standard base pairs, or to chain breaks. The cells of all organisms have a variety of repair mechanisms available, still only partially understood even in prokaryote systems [see the recent reviews of Hanawalt (1) and Grossman (2)]. Most of these mechanisms involve a certain amount of DNA synthesis, which is operationally distinguishable from the synthesis occurring at the growing point and is demonstrably due, in part at least, to distinct molecules. Still, in bacteria, a certain overlapping can be shown between the replication and repair systems, and also with the related recombination process. Thus a given molecule mainly involved in one process may, in particular situations, act in another one. In mammalian systems the DNA repair mechanisms are much less known than in microorganisms and many conflicting observations limit the applicability of bacterial models; furthermore, important differences exist between the repair responses, e.g., of rodent and human cells, to W, forbidding thus the simple utilization of bacterial data for interpretations.