Serum response factor and protein-mediated DNA bending contribute to transcription of the dystrophin muscle-specific promoter.

The minimal muscle-specific dystrophin promoter contains the consensus sequence CC(A/T)6GG, or the CArG element, which can be found in serum-inducible or muscle-specific promoters. The serum response factor (SRF), which mediates the transcriptional activation of the c-fos gene in response to serum stimulation, can bind to different CArG box elements, suggesting that it could be involved in muscle-constitutive transcription. Here we show that SRF binds to the dystrophin promoter and regulates its muscle-specific transcription. In transient transfections, an altered-binding-specificity SRF mutant restores the muscle-constitutive transcription of a dystrophin promoter with a mutation in its CArG box element. The muscle-constitutive transcription of the dystrophin promoter also requires the sequence GAAACC immediately downstream of the CArG box. This sequence is recognized by a novel DNA bending factor which was named dystrophin promoter-bending factor (DPBF). Mutations of the CArG flanking sequence abolish both DPBF binding and the promoter activity in muscle cells. Its replacement with a p62/ternary complex factor binding site changes the promoter specificity from muscle constitutive to serum responsive. These results show that, on the dystrophin promoter, the transcriptional activation induced by SRF requires the DNA bending induced by DPBF. The bending, next to the CArG box, could promote interactions between SRF and other proteins in the transcriptional complex.

Lack of discrimination between DNA ligases I and III by two classes of inhibitors, anthracyclines and distamycins.

We have measured the effects of eight distamycin and two anthracycline derivatives on polynucleotide joining and self-adenylating activities of human DNA ligase I and rat DNA ligases I and III. All test drugs show good inhibitory activity against the three enzymes in the poly[d(A-T)] joining assay. Several distamycins also inhibit the DNA-independent self-adenylation reaction catalysed by the human enzyme and, to a lesser extent, by rat DNA ligases. These results confirm that anthracyclines and distamycins express their inhibitory action against DNA joining activities mainly via specific interactions with the substrate, and suggest that the three test DNA ligases utilize similar, if not identical, mechanisms of recognition and interaction with DNA-drug complexes. Our findings also indicate that distamycins have a greater affinity for human DNA ligase I than for rat enzymes, suggesting that, in this respect, rat DNA ligase I is more similar to rat DNA ligase III than to human DNA ligase I.

Bacteriophage T4 and human type I DNA ligases relax DNA under joining conditions.

Both bacteriophage T4 and human type I DNA ligases in the presence of a mixture of ATP, AMP and PPi altered the topological properties of a supercoiled substrate by a step-wise reaction eventually leading to a population of fully relaxed, covalently closed products. In the presence of only AMP and PPi DNA products containing nicks with 3'OH/5'P termini accumulated in the presence of bacteriophage T4 DNA ligase, suggesting reversal of the entire joining reaction, but not in the presence of human DNA ligase I. Both DNA ligases became deoxyadenylylated in the presence of dATP, but the joining reaction did not proceed to completion. However, with both enzymes the full relaxing reaction took place in the presence of dAMP alone and in the presence of a mixture of dATP, dAMP and PPi. In no case could the joining reaction be reversed by dAMP and PPi. Related experiments with modified derivatives of deoxyribonucleoside 5'-triphosphates and PPi gave results in accord with these observations. The AMP dependent DNA relaxation catalysed by DNA ligases was insensitive to the presence of exonuclease III. These results indicate that controlled relaxation of the substrate by both DNA ligases occurs as a separate reaction rather than by simple reversal of the joining reaction. These findings support the hypothesis that in vivo the DNA topoisomerising ligases relax their substrate at the replication fork both during and separately from ligation of a pre-existing nick.

Specific inhibition of human DNA ligase adenylation by a distamycin derivative possessing antitumor activity.

The antiviral distamycin A and its phenyl mustard derivative FCE24517 possessing antitumor activity were tested for their ability to inhibit macromolecular synthesis in three human and one murine cell line. While distamycin A was poorly active in these systems, FCE24517 inhibited DNA synthesis efficiently, RNA synthesis to a lower extent and had little effect on protein synthesis. These findings suggest that the in vivo activity of FCE24517 derives from the specific inhibition of DNA synthesis. When the two drugs were tested on several enzymes involved in human DNA metabolism a strikingly similar pattern of inhibition appeared, with distamycin A being the more potent. Both drugs showed: A), no inhibitory activity against thymidine kinase and DNA primase; B), low activity against DNA topoisomerases I and II and the 3'-5' exonuclease associated with the DNA polymerase epsilon; C), high activity against DNA polymerases alpha and epsilon, uracil-DNA glycosylase and the joining activity of the replicative DNA ligase; D), the highest inhibitory activity against the AMP-dependent DNA relaxing activity of DNA ligase. The strong in vitro inhibition of several DNA enzymatic activities, including DNA ligase, do not match with the in vivo activities of the two drugs. However a unique difference was observed: only FCE24517 inhibited the DNA-independent reaction of adenylation of human DNA ligase while the adenylation reaction of T4 and E. coli DNA ligase was unaffected by either drug. It is still unclear whether these properties are relevant for modulating the killing activity of FCE24517 against proliferating cells both in culture and in vivo. Nevertheless FCE24517 is the first known molecule capable of interacting directly and specifically with human DNA ligase.

Effects of DNA-binding drugs on T4 DNA ligase.

A number of DNA intercalating and externally binding drugs have been found to inhibit nick sealing, cohesive and blunt end ligation, AMP-dependent DNA topoisomerization and EDTA-induced DNA nicking mediated by bacteriophage T4 DNA ligase. The inhibition seems to arise from drug-substrate interaction so that formation of active DNA-Mg2(+)-AMP-enzyme complex is impaired while assembled and active complexes are not disturbed by drug binding to the substrate.

Use of ATP, dATP and their alpha-thio derivatives to study DNA ligase adenylation.

Bacteriophage-T4 and human type I DNA ligases were found capable of self-adenylating upon exposure to both ribo- and deoxyribo-[alpha-35S]thio-ATP. However, the joining reaction does not take place in the presence of the deoxyribotriphosphates. Enzyme adenylation is reversed in all cases by an excess of PPi, but the rate of reversion is lower with thio derivatives. Therefore thio derivatives can be used to study the adenylation of DNA ligases and to search for specific inhibitors of the first step of the ligation reaction. In addition we show that thio derivatives can be used to detect DNA ligase adenylation activity covalently bound to a solid matrix.

DNA binding properties of FCE24517, an electrophilic distamycin analogue.

The distamycin derivative FCE24517 binds both reversibly and irreversibly to DNA. At 37 degrees C, the drug originates reversible complexes that are strong enough to survive to the electrophoretic separation of the substrate. These complexes slowly evolve to covalent adducts (10(-4) adducts/bp/h) that eventually degenerate to single-strand breaks (1.5 x 10(-5) nicks/bp/h). The site of attack by the drug can be any base in the vicinity of AT-rich regions of the double helix. Rapidly reassociating duplex DNA molecules, indicative of the presence of cross-links, are observed only upon boiling of DNA with FCE24517. While the low rates of formation of covalent adducts and DNA breaks could be relevant for the long-term biological effects of FCE24517, the specific formation of strong but still reversible complexes with DNA could be matched to the drastic and sudden reduction of thymidine incorporation induced by this electrophilic distamycin.

Correlation between anthracycline structure and human DNA ligase inhibition.

A total of 19 anthracycline derivatives were tested for their ability to interfere in vitro with the action of the human replicative DNA ligase. Only those with the sugar devoid of unmodified amino groups or with large configurational modifications were found to be inactive. Maximal inhibition of DNA-joining activity was found to require a 4'-deoxy-3'-amino sugar. Self-adenylation of DNA ligase was largely insensitive to these drugs. An important general finding is that slight modifications of the anthracycline structure have pronounced effects on DNA-ligase-inhibitory activity and might be related to the specificity of anthracycline anti-tumour activity.

Human FIGF: cloning, gene structure, and mapping to chromosome Xp22.1 between the PIGA and the GRPR genes.

We report the identification, structural characterization, and mapping of the human FIGF gene. FIGF is the human homologue of mouse figf (c-fos-induced growth factor), a new member of the platelet-derived growth factor/vascular endothelial growth factor (PDGF/VEGF) family. It codes for a secreted factor with mitogenic and morphogenic activity on fibroblast cells. The predicted amino acid sequence of FIGF is 84% identical to that of the mouse protein, and it is highly conserved (up to 40%) in the dimerization domain with respect to the VEGF members of the family. The 2.5-kb mRNA of FIGF was detected in adult lung and heart tissues. The gene spans about 50 kb and is organized into seven exons and six introns. The FIGF promoter contains an optimal AP-1-binding site and lacks a canonical TATA box. Fluorescence in situ hybridization mapped FIGF to chromosomal region Xp22.1. The subsequent identification of YAC positive clones from this region allowed us to refine the map and localize FIGF centromeric to the phosphatidylinositol glycan complementation class A (PIGA) gene and telomeric to the gastrin-releasing peptide receptor (GRPR) gene. FIGF and PIGA genes lie next to each other in a head-to-tail orientation, with the FIGF polyadenylation signal about 12 kb from the PIGA transcriptional start site.

Formación de ayudantes en Alimentación del niño sano / Assistant professors' training in Feeding Healthy Children

Los procesos de enseñanza–aprendizaje son fundamentales para la formación de cualquier profesional. Quienes integran el equipo docente de “Alimentación del niño sano” de la Universidad de Buenos Aires buscan constantemente reformular las estrategias de enseñanza–aprendizaje. Es por eso que se organizó dentro de ésta materia, la “Escuela de ayudantes”. La misma ha tenido sus orígenes en el año 1998. La finalidad, por aquel entonces, fue crear un ámbito de trabajo en equipo, sobre los temas relacionados con la materia y permitió en el 2004 publicar el libro “Nutrición Pediátrica” y su actualización “Nutrición del Niño Sano” en el 2007. Hoy, gracias al esfuerzo de todos los que han formado parte de ella, se ha logrado sistematizar la formación de los ayudantes para lograr mantener al grupo actualizado y preparado para afrontar la tarea de enseñanza. El equipo docente esta integrado por un pediatra y seis licenciados en Nutrición. En el año 2007 se realizó una evaluación de los ayudantes y del funcionamiento del sistema de formación docente. Al finalizar la evaluación el equipo se encuentra conforme con los resultados y con la modalidad de trabajo. La mayoría de los ayudantes considera fundamental la formación docente y cree que la escuela de ayudantes le ha servido para alcanzar ese objetivo. Desde el punto de vista de los contenidos abordados, los conocimientos, las actitudes y vínculos, las experiencias transmitidas, las horas dedicadas y los tópicos discutidos, tanto ayudantes como docentes a cargo están conformes y lo consideran un espacio de enriquecimiento mutuo. Los autores de esta comunicación consideran de vital importancia la “formación de ayudantes” entre los profesionales de la salud que integran equipos docentes universitarios, así como el trabajo intra e ínter – materias