DNA transposition – a major contributor to plant chromosome structure

Higher plant nuclear genomes contain many families of dispersed repeats that change during evolution. Recent evidence from studies on genetically defined transposable elements raises the possibility that many of the dispersed repeats are remnants of such elements. Transposition of DNA has also occurred between mitochondria, chloroplasts and nuclei, a fact that underlines the major role played by DNA transposition in determining the structure of plant genomes.     

Does CD40 ligation induce B cell negative selection?

Binding of CD154 to its receptor, CD40, provides costimulation for mature B cell activation and differentiation in response to Ag receptor signals. In mice, early B cell precursors express CD40, but its function at this stage is unknown. We examined the effects of CD40 ligation during B cell ontogeny in transgenic mice constitutively expressing CD154 on B cells (kappaEP-CD154). Precursors beyond pro-B cells were absent in adult bone marrow but were increased in the fetal liver. Newborn kappaEP-CD154 mice had largely increased numbers of peripheral B cells, which were CD154+, and that 36 h after birth expressed high surface levels of CD23 and MHC class II, resembling activated mature B cells. Nevertheless, kappaEP-CD154 mice were hypogammaglobulinemic, indicating that the expanded population of apparently activated B cells was nonfunctional. Further analysis revealed that soon after birth, kappaEP-CD154 mice-derived B cells became CD5+/Fas+, after which progressively decreased in the periphery in a CD154-CD40-dependent manner. These results indicate that CD40 ligation during B cell ontogeny induces negative selection characterized by either hyporesponsiveness or an arrest in maturation depending on the time of analysis and the anatomic site studied.