BRU1, a novel link between responses to DNA damage and epigenetic gene silencing in Arabidopsis

Abstract

DNA repair associated with DNA replication is important for the conservation of genomic sequence information, whereas reconstitution of chromatin after replication sustains epigenetic information. We have isolated and characterized mutations in the BRU1 gene of Arabidopsis that suggest a novel link between these underlying maintenance mechanisms. Bru1 plants are highly sensitive to genotoxic stress and show stochastic release of transcriptional gene silencing. They also show increased intrachromosomal homologous recombination and constitutively activated expression of poly (ADP-ribose) polymerase-2 (AtPARP-2), the induction of which is associated with elevated DNA damage. Bru1 mutations affect the stability of heterochromatin organization but do not interfere with genome-wide DNA methylation. BRU1 encodes a novel nuclear protein with two predicted protein-protein interaction domains. The developmental abnormalities characteristic of bru1 mutant plants resemble those triggered by mutations in genes encoding subunits of chromatin assembly factor (CAF-1), the condensin complex, or MRE11. Comparison of bru1 with these mutants indicates cooperative roles in the replication and stabilization of chromatin structure, providing a novel link between chromatin replication, epigenetic inheritance, S-phase DNA damage checkpoints, and the regulation of meristem development. A dynamic chromatin structure contributes to the regulation of repair and transcription of DNA templates. Chromatin components involved in both processes have been described that imply shared molecular mechanisms modulating DNA accessibility for repair and transcription (Green and Almouzni 2002). The first molecular link between transcription and DNA repair was revealed during characterization of transcription factor IIH (TFIIH), which is required for initiation of RNA synthesis by RNA polymerase II and for efficient repair of DNA through nucleotide excision (Feaver et al. 1993;Schaeffer et al. 1993;Drapkin et al. 1994;Wang et al. 1994).Accessibility is determined by compaction of chromatin, which consists of loosely packaged, transcriptionally active euchromatin, and heterochromatin, which is condensed and transcriptionally silent and consists mainly of transposable elements and repetitive sequences. Chromatin states are inherited during DNA replication, providing a scaffold for epigenetic information that influences transcriptional gene regulation.Several chromatin components determining heritable features of chromatin also have an influence on epigenetic regulation of gene activity and efficiency of DNA repair or genome stability. For example, SIR proteins in yeast mediate formation of a compact chromatin structure similar to heterochromatin in multicellular eukaryotes (Gross 2001) and are required for transcriptional gene silencing (TGS) and for suppression of homologous recombination of rDNA repeats (Guarente 2000). They

Chromatin assembly factor 1 ensures the stable maintenance of silent chromatin states in Arabidopsis

“…Recently, bru1 mutants (also reported as tsk or mgo3) have also been shown to exhibit phenotypic similarities to fas mutants (Guyomarc'h et al 2004;Suzuki et al 2004;Takeda et al 2004). These include variable degrees of developmental abnormalities, accompanied by misregulation of meristematic genes, and in particular, the stochastic release of TGS.…”

“…However, the gross level of DNA methylation ( Supplementary Fig. S1) and the formation of chromocenters (Takeda et al 2004) are not changed in fas mutants. Stochastic release of TGS in fas mutants might be caused mainly by disturbance of speed or efficiency of nucleosome formation during DNA replication.…”

MGO3 and GIP1 act synergistically for the maintenance of centromeric cohesion

“…15 Such flattened shoots, as observed in mgo3/bru1/tsk, fasciata 1, 2 and tebichi mutants, were functionally linked either to defects in chromatin assembly, DNA damage repair, DNA replication, or to meristem organization. 11,12,15,[17][18][19] Thus, the increased fasciation observed in gip1mgo3 suggests that GIP1 may contribute, together with MGO3, to these processes.…”

“…6 TSA1 was also shown to interact with MGO3/BRU1/TSK. 9 As BRU1 is involved in genomic maintenance, 12 we investigated the relationships between MGO3/BRU1/TSK and GIPs. Among the several allelic mutants described in the literature, we used herein mgo3-2, further described as mgo3.…”

“…9 TONSOKU (TSK) is specifically expressed during S phase. 10 Allelic mutations in the epigenetic regulator TSK 11 also named BRUSHY1 (BRU1) [12][13][14] and MGOUN3 (MGO3) 15 led to a deregulation of various biologic processes: DNA damage response during DNA replication, cell cycle progression, heterochromatin organization, chromosomal subdomain architecture as well as meristem organization and flowering transition in Arabidopsis. TSA1 was also described as a partner of the COP9 signalosome subunit 1 (CSN1).…”