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

Abstract

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. To distinguish between the possible contribution of suboptimal function of telomerase in fas mutants under in vivo conditions and problems in conventional telomere replication, we crossed fas and tert (telomerase reverse transcriptase) knockout mutants and analyzed telomere shortening in segregated fas mutants, tert mutants, and double fas tert mutants in parallel. We demonstrate that fas tert knockouts show greater replicative telomere shortening than that observed even in the complete absence of telomerase (tert mutants). While the effect of tert and fas mutations on telomere lengths in double mutants is additive, manifestations of telomere dysfunction in double fas tert mutants (frequency of anaphase bridges, onset of chromosome end fusions, and common involvement of 45S rDNA in chromosome fusion sites) are similar to those in tert mutants. We conclude that in addition to possible impairment of telomerase action, a further mechanism contributes to telomere shortening in fas mutants.

The Absence of the Arabidopsis Chaperone Complex CAF-1 Produces Mitotic Chromosome Abnormalities and Changes in the Expression Profiles of Genes Involved in DNA Repair

“…However, there is a loss of telomere sequences, although it is produced by a different mechanism from that responsible for the loss of 45S rDNA repeats (Muchová et al, 2015). Jaške et al (2013), after analyzing single tert (telomerase reverse transcriptase) and fas mutants, and the corresponding double mutants, concluded that the progressive loss of telomeric DNA along generations in fas mutants is partially due to a suboptimal function of telomerase, but they also assumed that a further mechanism that contributes to telomere shortening in fas mutants must exist. The existence of telomeric acentric fragments in fas1-4 anaphases ( Figure 1N ) and the enhanced frequency of these fragments in the double mutant fas1-4 rad51 ( Figure 3 ) suggest the involvement of a RAD51-independent mechanism in telomere shortening.…”

“…They also present high levels of H2AX phosphorylation, a marker of DNA double-strand breaks (DSBs), and increased RAD51 expression compared with wild-type (WT) (Takeda et al, 2004; Endo et al, 2006; Kirik et al, 2006; Ono et al, 2006; Schönrock et al, 2006; Ramirez-Parra and Gutierrez, 2007b). Recently, it has been reported that in fas mutants there is also a specific and transgenerational loss of 45S rDNA and telomeric sequences (Mozgová et al, 2010; Jaške et al, 2013; Muchová et al, 2015; Pavlištová et al, 2016). The loss of these sequences is produced during the cell division and it does not occur during meiosis (Muchová et al, 2015; Varas et al, 2015).…”

Histone H3 Dynamics in Plant Cell Cycle and Development

“…In addition, fas1 mutants also show an immediate telomere shortening and dysfunction, although telomerase activity is not altered. Surprisingly, the fas1;tert double mutants show even more telomere shortening than the single tert mutant [Jaske et al, 2013], suggesting a complex mechanism to explain the role of CAF-1 in telomere maintenance.…”

Looking at plant cell cycle from the chromatin window

“…Although plants are more tolerant to defects in CAF-1 function than mammals, alteration in the H3.1/H3.3 balance seems to be highly deleterious for plant development, as revealed by the pleiotropic phenotype of fas1 , fas2 , and msi1 mutants, encoding each of the three CAF-1 subunits (Kaya et al, 2001; Hennig et al, 2003; Ramirez-Parra and Gutierrez, 2007a). Thus, fas1 mutants show increased homologous recombination, limited TE silencing, telomere shortening, and loss of 45S rDNA repeats (Endo et al, 2006; Kirik et al, 2006; Ono et al, 2006; Schonrock et al, 2006; Mozgova et al, 2010; Jaske et al, 2013). Likewise, asf1a, b double mutants exhibit a S-phase delay and up-regulation of checkpoint genes, such as ATM , ATR , and PARP1 (Zhu et al, 2011).…”