Genetic screening identified a suppressor of ros1-1, a mutant of REPRESSOR OF SILENCING1 (ROS1; encoding a DNA demethylation protein). The suppressor is a mutation in the gene encoding the largest subunit of replication factor C (RFC1). This mutation of RFC1 reactivates the unlinked 35S-NPTII transgene, which is silenced in ros1 and also increases expression of the pericentromeric Athila retrotransposons named transcriptional silent information in a DNA methylationindependent manner. rfc1 is more sensitive than the wild type to the DNA-damaging agent methylmethane sulphonate and to the DNA inter-and intra-cross-linking agent cisplatin. The rfc1 mutant constitutively expresses the G2/M-specific cyclin CycB1;1 and other DNA repair-related genes. Treatment with DNA-damaging agents mimics the rfc1 mutation in releasing the silenced 35S-NPTII, suggesting that spontaneously induced genomic instability caused by the rfc1 mutation might partially contribute to the released transcriptional gene silencing (TGS). The frequency of somatic homologous recombination is significantly increased in the rfc1 mutant. Interestingly, ros1 mutants show increased telomere length, but rfc1 mutants show decreased telomere length and reduced expression of telomerase. Our results suggest that RFC1 helps mediate genomic stability and TGS in Arabidopsis thaliana.
DNA and histone methylation coregulate heterochromatin formation and gene silencing in animals and plants. To identify factors involved in maintaining gene silencing, we conducted a forward genetic screen for mutants that release the silenced transgene in the transgenic Arabidopsis () line We identified, which encodes methionine (Met) adenosyltransferase 4 (MAT4)/-adenosyl-Met synthetase 3 that catalyzes the synthesis of -adenosyl-Met (SAM) in the one-carbon metabolism cycle. mostly decreases CHG and CHH DNA methylation and histone H3K9me2 and reactivates certain silenced transposons. The exogenous addition of SAM partially rescues the epigenetic defects of SAM content and DNA methylation were reduced more in than in three other mutants. knockout mutations generated by CRISPR/Cas9 were lethal, indicating that is an essential gene in Arabidopsis. MAT1, 2, and 4 proteins exhibited nearly equal activity in an in vitro assay, whereas MAT3 exhibited higher activity. The native promoter driving ,, and cDNA complemented the mutant. However, most transgenic lines carrying native, , and promoters driving cDNA did not complement the mutant because of their lower expression in seedlings. Genetic analyses indicated that the double mutant is dwarfed and the double mutant was nonviable, while showed normal growth and fertility. These results indicate that MAT4 plays a predominant role in SAM production, plant growth, and development. Our findings provide direct evidence of the cooperative actions between metabolism and epigenetic regulation.
No abstract
Due to the introduction of the intensified image detector, the dynamic performance of the intensified star tracker is effectively improved. However, its attitude update rate is still seriously restricted by the transmission and processing of pixel data. In order to break through the above limitation, a multiexposure imaging approach for intensified star trackers is proposed in this paper. One star image formed by this approach actually records N different groups of star positions, and then N corresponding groups of attitude information can be acquired. Compared with the existing exposure imaging approach, the proposed approach improves the attitude update rate by N times. Furthermore, for a dim star, the proposed approach can also accumulate the energy of its N positions and then effectively improve its signal-to-noise ratio. Subsequently, in order to obtain the optimal performance of the proposed approach, parameter optimization is carried out. First, the motion model of the star spot in the image plane is established, and then based on it, all the key parameters are optimized. Simulations and experiments demonstrate the feasibility and effectiveness of the proposed approach and parameter optimization.
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