Plant 45S rDNA Clusters Are Fragile Sites and Their Instability Is Associated with Epigenetic Alterations

Abstract

Our previous study demonstrated that 45S ribosomal DNA (45S rDNA) clusters were chromosome fragile sites expressed spontaneously in Lolium . In this study, fragile phenotypes of 45S rDNA were observed under aphidicolin (APH) incubation in several plant species. Further actinomycin D (ActD) treatment showed that transcriptional stress might interfere with chromatin packaging, resulting in 45S rDNA fragile expression. These data identified 45S rDNA sites as replication-dependent as well as transcription-dependent fragile sites in plants. In the presence of ActD, a dramatic switch to an open chromatin conformation and accumulated incomplete 5′ end of the external transcribed spacer (5′ETS) transcripts were observed, accompanied by decreased DNA methylation, decreased levels of histone H3, and increased histone acetylation and levels of H3K4me2, suggesting that these epigenetic alterations are associated with failure of 45S rDNA condensation. Furthermore, the finding that γ-H2AX was accumulated at 45S rDNA sites following ActD treatment suggested that the DNA damage signaling pathway was associated with the appearance of 45S rDNA fragile phenotypes. Our data provide a link between 45S rDNA transcription and chromatin-packaging defects and open the door for further identifying the molecular mechanism involved.

Translocations of Chromosome End-Segments and Facultative Heterochromatin Promote Meiotic Ring Formation in Evening Primroses

“…biennis strain suaveolens Grado ( Figures 1F and 1I) are difficult to be explained by whole-arm translocations. In eukaryotes, there is a strong association between breakpoint hot spots, condensation state, and repeated DNA sequences (Coghlan et al, 2005;Huang et al, 2012). The fact that all chromosomes are involved in translocations (Supplemental Table 1) argues for the evolution of conserved breakpoint hot spots on all Oenothera chromosomes.…”

“…The fact that all chromosomes are involved in translocations (Supplemental Table 1) argues for the evolution of conserved breakpoint hot spots on all Oenothera chromosomes. Although the active 45S rDNA triggers chromosome rearrangements in diverse organisms as a preferable breaking site (Huang et al, 2012, and references therein), it does not seem to be a likely candidate for a breakpoint-associated DNA repeat in Oenothera. If it played such a role, it should have been dispersed throughout the genome ( Figure 4B, translocation 2).…”

Dynamics of 45S rDNA sites in the cell cycle: fragile sites and chromosomal stability in Lolium and Festuca

“…This suggests that there is no loss of chromosome fragments in these genotypes. Rocha et al (2016) suggested that the chromatin fiber in FSs of Lolium, Festuca, and Festulolium hybrids have a high decondensation process related to an increased genic activity, thus, supporting the findings by Huang et al (2012), Bustamante et al (2014), and Rocha et al (2015).…”

“…In their study, the 45S rDNA sites of L. perenne were highly decondensed in more than 90% of the G2 nuclei and the authors suggested that failure in the chromatin folding complex of these sites could be responsible for the spontaneous formation of FSs (Huang et al, 2009). In another study, following induction with actinomicin D (a transcription inhibitor) and aphidicolin (a replication inhibitor), the interphasic nuclei of maize, rice, barley, shorgum, and ryegrass were highly decondensed and had fiber-like thread after FISH 45S in metaphasic chromosomes (Huang et al, 2012). Thus, it is likely that the replicative and transcriptional processes are involved with the spontaneous FSs in Lolium and Festuca.…”

“…In some plants, FSs are expressed in 45S rDNA sites (Huang et al, 2008(Huang et al, , 2009(Huang et al, , 2012Waminal et al, 2013;Bustamante et al, 2014;Grabowska-Joachimiak et al, 2015;Rocha et al, , 2016Lan et al, 2016). In Lolium multiflorum Lam.…”