Large tumor antigen (T antigen) was extracted from SV40‐infected African Green Monkey cells and purified to homogeneity by immunoaffinity chromatography. The purified T antigen preparations unwind DNA duplices of greater than 120 bp in a reaction which is dependent on magnesium ions and ATP hydrolysis. Based on these and other properties of the reaction we classify this newly discovered enzymatic activity as a eukaryotic DNA helicase. The helicase and the known ATPase function of T antigen cosediment with the mono‐ or dimeric 4‐6 S form of T antigen, but not with higher T antigen aggregates. The helicase activity seems to be an intrinsic function of SV40 T antigen. First, several different T antigen‐specific monoclonal antibodies interfere with the DNA unwinding activity; monoclonals which are known to reduce the T antigen‐specific ATPase most strongly inhibited the helicase reaction. Second, mutant T antigens with impaired ATPase function also showed a reduced DNA unwinding activity.

Ruthenium compounds have become promising alternatives to platinum drugs by displaying specific activities against different cancers and favourable toxicity and clearance properties. Nonetheless, their molecular targeting and mechanism of action are poorly understood. Here we study two prototypical ruthenium-arene agents—the cytotoxic antiprimary tumour compound [(η6-p-cymene)Ru(ethylene-diamine)Cl]PF6 and the relatively non-cytotoxic antimetastasis compound [(η6-p-cymene)Ru(1,3,5-triaza-7-phosphaadamantane)Cl2]—and discover that the former targets the DNA of chromatin, while the latter preferentially forms adducts on the histone proteins. Using a novel ‘atom-to-cell’ approach, we establish the basis for the surprisingly site-selective adduct formation behaviour and distinct cellular impact of these two chemically similar anticancer agents, which suggests that the cytotoxic effects arise largely from DNA lesions, whereas the protein adducts may be linked to the other therapeutic activities. Our study shows promise for developing new ruthenium drugs, via ligand-based modulation of DNA versus protein binding and thus cytotoxic potential, to target distinguishing epigenetic features of cancer cells.

The nitrogen regulatory protein NtrC of enteric bacteria activates transcription of the glnA gene by catalyzing isomerization of closed complexes between RNA polymerase and the glnA promoter to open complexes. NtrC binds to sites upstream of glnA that have properties of eukaryotic transcriptional enhancers. NtrC-binding sites were found to facilitate open complex formation when these sites and the glnA promoter were located on different rings of a singly linked catenane, but not when the two rings were decatenated. The results provide evidence that NtrC contacts RNA polymerase-promoter complexes in a process mediated by formation of a DNA loop. NtrC-binding sites serve to tether NtrC near the glnA promoter, thereby increasing the frequency of collisions between NtrC and polymerase-promoter complexes.

The small, chromatin-associated HMGA proteins contain three separate DNA binding domains, so-called AT hooks, which bind preferentially to short AT-rich sequences. These proteins are abundant in pluripotent embryonic stem (ES) cells and most malignant human tumors, but are not detectable in normal somatic cells. They act both as activator and repressor of gene expression, and most likely facilitate DNA architectural changes during formation of specialized nucleoprotein structures at selected promoter regions. For example, HMGA2 is involved in transcriptional activation of certain cell proliferation genes, which likely contributes to its well-established oncogenic potential during tumor formation. However, surprisingly little is known about how HMGA proteins bind DNA packaged in chromatin and how this affects the chromatin structure at a larger scale. Experimental evidence suggests that HMGA2 competes with binding of histone H1 in the chromatin fiber. This could substantially alter chromatin domain structures in ES cells and contribute to the activation of certain transcription networks. HMGA2 also seems capable of recruiting enzymes directly involved in histone modifications to trigger gene expression. Furthermore, it was shown that multiple HMGA2 molecules bind stably to a single nucleosome core particle whose structure is known. How these features of HMGA2 impinge on chromatin organization inside a living cell is unknown. In this commentary, we propose that HMGA2, through the action of three independent DNA binding domains, substantially contributes to the plasticity of ES cell chromatin and is involved in the maintenance of a un-differentiated cell state.

Maintaining genome integrity requires the accurate and complete replication of chromosomal DNA. This is of the utmost importance for embryonic stem cells (ESCs), which differentiate into cells of all lineages, including germ cells. However, endogenous and exogenous factors frequently induce stalling of replication forks in every cell cycle, which can trigger mutations and chromosomal instabilities. We show here that the oncofetal, nonhistone chromatin factor HMGA2 equips cells with a highly effective first-line defense mechanism against endonucleolytic collapse of stalled forks. This fork-stabilizing function most likely employs scaffold formation at branched DNA via multiple DNA-binding domains. Moreover, HMGA2 works independently of other human factors in two heterologous cell systems to prevent DNA strand breaks. This fork chaperone function seemingly evolved to preserve ESC genome integrity. It is hijacked by tumor (stem) cells to also guard their genomes against DNA-damaging agents widely used to treat cancer patients.