BackgroundHistone H1 is involved in the formation and maintenance of chromatin higher order structure. H1 has multiple isoforms; the subtypes differ in timing of expression, extent of phosphorylation and turnover rate. In vertebrates, the amino acid substitution rates differ among subtypes by almost one order of magnitude, suggesting that each subtype might have acquired a unique function. We have devised a competitive assay to estimate the relative binding affinities of histone H1 mammalian somatic subtypes H1a-e and H1° for long chromatin fragments (30–35 nucleosomes) in physiological salt (0.14 M NaCl) at constant stoichiometry.ResultsThe H1 complement of native chromatin was perturbed by adding an additional amount of one of the subtypes. A certain amount of SAR (scaffold-associated region) DNA was present in the mixture to avoid precipitation of chromatin by excess H1. SAR DNA also provided a set of reference relative affinities, which were needed to estimate the relative affinities of the subtypes for chromatin from the distribution of the subtypes between the SAR and the chromatin. The amounts of chromatin, SAR and additional H1 were adjusted so as to keep the stoichiometry of perturbed chromatin similar to that of native chromatin. H1 molecules freely exchanged between the chromatin and SAR binding sites. In conditions of free exchange, H1a was the subtype of lowest affinity, H1b and H1c had intermediate affinities and H1d, H1e and H1° the highest affinities. Subtype affinities for chromatin differed by up to 19-fold. The relative affinities of the subtypes for chromatin were equivalent to those estimated for a SAR DNA fragment and a pUC19 fragment of similar length. Avian H5 had an affinity ~12-fold higher than H1e for both DNA and chromatin.ConclusionH1 subtypes freely exchange in vitro between chromatin binding sites in physiological salt (0.14 M NaCl). The large differences in relative affinity of the H1 subtypes for chromatin suggest that differential affinity could be functionally relevant and thus contribute to the functional differentiation of the subtypes. The conservation of the relative affinities for SAR and non-SAR DNA, in spite of a strong preference for SAR sequences, indicates that differential affinity alone cannot be responsible for the heterogeneous distribution of some subtypes in cell nuclei.

ResumenSe realizó un estudio comprensivo sobre las características del discurso escrito de 37 estudiantes universitarios, acerca de los procesos que se desencadenan en el sistema inmune como respuesta a la acción de microagresores. Para la recolección de la información se aplicó un cuestionario tipo Likert y preguntas abiertas sobre casos clínicos. A partir del análisis de las oraciones con sentido u oraciones nucleares (Chomsky, 2004) dadas por los estudiantes sobre la inmunidad, ubicamos los textos en tres modelos explicativos, relacionados con la historia de la inmunología y de allí caracterizamos el discurso escrito de los estudiantes a partir de dos categorías: coherencia y estructura discursiva.Dentro de los principales resultados se destaca la elaboración de textos con coherencia local y el uso de conectores causales, tanto en expresiones cortas como en los textos más largos. Asimismo, es frecuente el uso de lenguajes tautológicos, que reflejan poca comprensión y evidencian el empleo de discursos descriptivos, los cuales pueden constituirse en un obstáculo para el aprendizaje.Palabras clave: inmunidad, discurso, causalidad, modelos explicativos.