BRU1 Maintains Configuration of the Euchromatic Subchromosomal Domain in the Nucleus of Arabidopsis

Chromatin states are inseparably associated with regulation of genes. In yeast and animals, chromatin states also correlate with the configuration and spatial localization pattern of chromosomal domains in the nucleus. In plants, however, the dynamics of such domains associated with gene regulation is poorly understood except for heterochromatic domains. We have previously reported several euchromatic regions of Arabidopsis chromosomes where genes are preferentially upregulated by a defect in BRU1—a nuclear factor involved in DNA damage responses and epigenetic gene regulation. In this study, we present a cytogenetic characterization of one of these subchromosomal regions, SCR1. In nuclei of wild-type leaf cells, the 174-kb SCR1 region was moderately condensed near nucleolus organizing region 4 (NOR4). In bru1 mutants, the confined localization pattern of SCR1 was stochastically disrupted. In contrast, bru1 defects did not affect the localization patterns of NOR4 and another 149-kb euchromatic region in which gene activity was not altered in bru1. The degree of confinement of SCR1 in the nucleus varied between leaves, hypocotyls, and undifferentiated calli, but not drastically. These results suggest that BRU1 plays a role in maintaining the configuration of a euchromatic subchromosomal domain that is a potential determinant in the control of gene activity.