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We found that a group of rubromycins and their analogues, a class of quinone antibiotics that possesses benzofuran and benzodipyran rings to form a spiroketal system, strongly inhibited human telomerase as assessed with a modified telomeric repeat amplification protocol. beta- and gamma-Rubromycins and purpuromycin appeared to be the most potent telomerase inhibitors, with 50% inhibitory concentrations (IC(50)) of about 3 microM, and griseorhodins A and C also showed comparable potencies for the inhibition (IC(50) = 6-12 microM). In contrast, opening of the spiroketal system of beta-rubromycin, giving rise to alpha-rubromycin, substantially decreased its inhibitory potency toward telomerase (IC(50) > 200 microM), indicating the essential role of the spiroketal system in telomerase inhibition. A kinetic study of the inhibition by beta-rubromycin revealed a competitive interaction with respect to the telomerase substrate primer, with a K(i) of 0.74 microM, whereas a mixed type inhibition was observed with respect to the nucleotide substrate. beta-Rubromycin was also potent in inhibiting retroviral reverse transcriptases but had virtually no effect on other DNA/RNA-modifying enzymes including DNA and RNA polymerases, deoxyribonuclease, and topoisomerase. Although beta-rubromycin showed nonspecific cytotoxicities, reducing proliferation of cancer cells (IC(50) approximately 20 microM), we conclude that beta-rubromycin appears to be a lead structure for the development of more potent and selective inhibitors of human telomerase.

The genome of an organism is under constant attack from endogenous and exogenous DNA damaging factors, such as reactive radicals, radiation, and genotoxins. Therefore, DNA damage response systems to sense DNA damage, arrest cell cycle, repair DNA lesions, and/or induce programmed cell death are crucial for maintenance of genomic integrity and survival of the organism. Genome sequences revealed that, although plants possess many of the DNA damage response factors that are present in the animal systems, they are missing some of the important regulators, such as the p53 tumor suppressor. These observations suggest differences in the DNA damage response mechanisms between plants and animals. In this review the DNA damage responses in plants and animals are compared and contrasted. In addition, the function of SUPPRESSOR OF GAMMA RESPONSE 1 (SOG1), a plant-specific transcription factor that governs the robust response to DNA damage, is discussed.

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We reported previously that long-chain fatty acids are potent inhibitors of mammalian DNA polymerase ␤. At present, based on information available from the NMR structure of the N-terminal 8-kDa domain, we examined the structural interaction with the 8-kDa domain using two species, C 18 -linoleic acid (LA) or C 24 -nervonic acid (NA). In the 8-kDa domain with LA or NA, the structure that forms the interaction interface included helix-1, helix-2, helix-4, the three turns (residues 1-13, 48 -51, and 79 -87) and residues adjacent to an ⍀-type loop connecting helix-1 and helix-2 of the same face. No significant shifts were observed for any of the residues on the opposite side of the 8-kDa domain. The NA interaction interface on the amino acid residues of the 8-kDa domain fragment was mostly the same as that of LA, except that the shifted cross-peaks of Leu-11 and Thr-79 were significantly changed between LA and NA. The 8-kDa domain bound to LA or NA as a 1:1 complex with a dissociation constant (K D ) of 1.02 or 2.64 mM, respectively.We reported previously that long-chain fatty acids strongly inhibited the activities of mammalian DNA polymerase ␣ (pol ␣) 1 and DNA polymerase ␤ (pol ␤) in vitro and plant DNA polymerases, albeit less potently, but that at the concentrations used, the fatty acids hardly influenced the activities of prokaryotic DNA polymerases or other DNA metabolic enzymes such as DNase I (1, 2). The most potent inhibitors were fatty acids, which have the following characteristics: hydrocarbon chain containing 18 or more carbons, a free carboxyl end, and the cis-configuration is preferred to the trans-configuration. Fatty acids in the trans-configuration have a much weaker inhibitory effect on pol ␤, and those in which the carboxyl end is chemically modified can lose the inhibitory effect on both pol ␣ and pol ␤. The mode of inhibition by longer chain fatty acids showed the same characteristics, except that the minimum inhibitory doses of these longer chain fatty acids were much lower (2, 3). Lineweaver-Burk plots of the fatty acids indicated that both the substrate (i.e. deoxynucleotide)-binding and the template DNA-binding sites of pol ␣ were nonantagonistically inhibited by the fatty acids, but they were effective as antagonists against the sites of pol ␤. For pol ␤, fatty acids acted by competing with not only the substrate but also the template-primer DNA. In screening inhibitors of eukaryotic DNA polymerase, we also found several natural compounds which inhibited pol ␤ in the same manner as fatty acids (4 -10).Pol ␤ is the smallest known DNA polymerase in animal cells with a molecular mass of 39 kDa, and its structure is highly conserved among mammals (11). This protein has a modular two-domain structure, with apparent flexibility within a protease-sensitive region between residues 82 and 86, which separates the two domains. Treatment with trypsin yields an N-terminal domain fragment (8 kDa), which retains binding affinity for single-stranded DNA (ssDNA), and a C-terminal domain fragment (31 kDa) ...

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