Phenotypic reversion in fas mutants of Arabidopsis thaliana by reintroduction of FAS genes: variable recovery of telomeres with major spatial rearrangements and transcriptional reprogramming of 45S rDNA genes.

Arabidopsis thaliana mutants dysfunctional in the evolutionarily conserved protein complex chromatin assembly factor-1 (CAF-1), which deposits the canonical histone H3 variant H3.1 during DNA synthesis-dependent chromatin assembly, display complex phenotypic changes including meristem and growth alterations, sensitivity to DNA-damaging agents, and reduced fertility. We reported previously that mutants in the FAS1 subunit of CAF-1 progressively lose telomere and 45S rDNA repeats.

Subnuclear partitioning of rRNA genes between the nucleolus and nucleoplasm reflects alternative epiallelic states.

Eukaryotes can have thousands of 45S ribosomal RNA (rRNA) genes, many of which are silenced during development. Using fluorescence-activated sorting techniques, we show that active rRNA genes in Arabidopsis thaliana are present within sorted nucleoli, whereas silenced rRNA genes are excluded. DNA methyltransferase (met1), histone deacetylase (hda6), or chromatin assembly (caf1) mutants that disrupt silencing abrogate this nucleoplasmic-nucleolar partitioning.

A telomerase-independent component of telomere loss in chromatin assembly factor 1 mutants of Arabidopsis thaliana.

Dysfunction of chromatin assembly factor 1 in FASCIATA mutants (fas) of Arabidopsis thaliana results in progressive loss of telomeric DNA. Although replicative telomere shortening is typically associated with incomplete resynthesis of their ends by telomerase, no change in telomerase activity could be detected in vitro in extracts from fas mutants. Besides a possible telomerase malfunction, the telomere shortening in fas mutants could presumably be due to problems with conventional replication of telomeres.

Three TERT genes in Nicotiana tabacum.

Telomerase is essential for proper functioning of telomeres in eukaryotes. We cloned and characterised genes for the protein subunit of telomerase (TERT) in the allotetraploid Nicotiana tabacum (tobacco) and its diploid progenitor species Nicotiana sylvestris and Nicotiana tomentosiformis with the aim of determining if allopolyploidy (hybridisation and genome duplication) influences TERT activity and divergence. Two of the three sequence variants present in the tobacco genome (NtTERT-C/s and NtTERT-D) revealed similarity to two sequence variants found in N.

Role of HMGB proteins in chromatin dynamics and telomere maintenance in Arabidopsis thaliana.

Chromosome stability is conditioned by functional chromatin structure of chromosome ends - telomeres. Organisation and regulation of telomere maintenance represent a complex process whose details still remain enigmatic, especially in plants. Several telomere-binding or telomere-associated proteins and distinct epigenetic marks have been shown to influence telomere length and telomerase activity.

Dysfunction of chromatin assembly factor 1 induces shortening of telomeres and loss of 45S rDNA in Arabidopsis thaliana.

Chromatin Assembly Factor 1 (CAF1) is a three-subunit H3/H4 histone chaperone responsible for replication-dependent nucleosome assembly. It is composed of CAC 1-3 in yeast; p155, p60, and p48 in humans; and FASCIATA1 (FAS1), FAS2, and MULTICOPY SUPPRESSOR OF IRA1 in Arabidopsis thaliana. We report that disruption of CAF1 function by fas mutations in Arabidopsis results in telomere shortening and loss of 45S rDNA, while other repetitive sequences (5S rDNA, centromeric 180-bp repeat, CACTA, and Athila) are unaffected.

ATM regulates the length of individual telomere tracts in Arabidopsis.

Telomeres have the paradoxical ability of protecting linear chromosome ends from DNA damage sensors by using these same proteins as essential components of their maintenance machinery. We have previously shown that the absence of ataxia telangiectasia mutated (ATM), a central regulator of the DNA damage response, accelerates the onset of genome instability in telomerase-deficient Arabidopsis, without increasing the rate of bulk telomere shortening. Here, we examine individual telomere tracts through successive plant generations using both fluorescence situ in hybridization (FISH) and primer extension telomere repeat amplification (PETRA).

Identification of chromosomal fusion sites in Arabidopsis mutants using sequential bicolour BAC-FISH.

Double stranded chromosomal breaks are repaired by homologous recombination or nonhomologous end joining (NHEJ). When broken chromosome ends are fused together by NHEJ, the resulting dicentric chromosomes can be detected as anaphase bridges during the subsequent mitosis. Telomeres in the absence of functional telomerase shorten, became unprotected, and are eventually recognized by the cell repair system as double stranded breaks.

Mre11 deficiency in Arabidopsis is associated with chromosomal instability in somatic cells and Spo11-dependent genome fragmentation during meiosis.

The Mre11/Rad50/Nbs1 complex is involved in many aspects of chromosome metabolism. Aberrant function of the complex is associated with defects in the DNA checkpoint, double-strand break repair, meiosis, and telomere maintenance. In this article, we report the consequences of Mre11 dysfunction for the stability of mitotic and meiotic chromosomes in Arabidopsis thaliana.