Deciphering how the chromatin factor RCC1 recognizes the nucleosome: the importance of individuals in the scientific discovery process

Nuclear formation induced by DNA-conjugated beads in living fertilised mouse egg

Reformation of a functional nucleus at the end of mitosis is crucial for normal cellular activity. Reconstitution approaches using artificial beads in frog egg extracts have clarified the molecules required for nuclear formation in vitro. However, the spatiotemporal regulation of these components, which is required for the formation of a functional nucleus in living embryos, remains unknown. Here we demonstrate that exogenous DNA introduced in the form of DNA-conjugated beads induces the assembly of an artificial nucleus in living mouse cleavage-stage embryos. Live-cell imaging and immunofluorescence studies revealed that core histones and regulator of chromosome condensation 1 (RCC1) assembled on the DNA, suggesting that nucleosomes were formed. Electron microscopy showed that double-membrane structures, partly extended from annulate lamellae, formed around the beads. Nuclear pore complex-like structures indistinguishable from those of native nuclei were also formed, suggesting that this membranous structure resembled the normal nuclear envelope (NE). However, the reconstituted NE had no nuclear import activity, probably because of the absence of Ras-related nuclear protein (Ran). Thus, DNA is necessary for NE reassembly in mouse embryos but is insufficient to form a functional nucleus. This approach provides a new tool to examine factors of interest and their spatiotemporal regulation in nuclear formation.

MtZR1, a PRAF protein, is involved in the development of roots and symbiotic root nodules in Medicago truncatula

PRAF proteins are present in all plants, but their functions remain unclear. We investigated the role of one member of the PRAF family, MtZR1, on the development of roots and nitrogen-fixing nodules in Medicago truncatula. We found that MtZR1 was expressed in all M. truncatula organs. Spatiotemporal analysis showed that MtZR1 expression in M. truncatula roots was mostly limited to the root meristem and the vascular bundles of mature nodules. MtZR1 expression in root nodules was down-regulated in response to various abiotic stresses known to affect nitrogen fixation efficiency. The down-regulation of MtZR1 expression by RNA interference in transgenic roots decreased root growth and impaired nodule development and function. MtZR1 overexpression resulted in longer roots and significant changes to nodule development. Our data thus indicate that MtZR1 is involved in the development of roots and nodules. To our knowledge, this work provides the first in vivo experimental evidence of a biological role for a typical PRAF protein in plants.

From beads on a string to the pearls of regulation: the structure and dynamics of chromatin

The assembly of eukaryotic chromatin, and the bearing of its structural organization on the regulation of gene expression, were the central topics of a recent conference organized jointly by the Biochemical Society and Wellcome Trust. A range of talks and poster presentations covered topical aspects of this research field and illuminated recent advances in our understanding of the structure and function of chromatin. The two-day meeting had stimulating presentations complemented with lively discourse and interactions of participants. In the present paper, we summarize the topics presented at the meeting, in particular highlighting subjects that are reviewed in more detail within this issue of Biochemical Society Transactions. The reports bring to life the truly fascinating molecular and structural biology of chromatin.