Suppressor of gamma response 1 (SOG1) encodes a putative transcription factor governing multiple responses to DNA damage

Abstract

The Arabidopsis sog1-1 (suppressor of gamma response) mutant was originally isolated as a second-site suppressor of the radiosensitive phenotype of seeds defective in the repair endonuclease XPF. Here, we report that SOG1 encodes a putative transcription factor. This gene is a member of the NAC domain [petunia NAM (no apical meristem) and Arabidopsis ATAF1, 2 and CUC2] family (a family of proteins unique to land plants). Hundreds of genes are normally up-regulated in Arabidopsis within an hour of treatment with ionizing radiation; the induction of these genes requires the damage response protein kinase ATM, but not the related kinase ATR. Here, we find that SOG1 is also required for this transcriptional up-regulation. In contrast, the SOG1-dependent checkpoint response observed in xpf mutant seeds requires ATR, but does not require ATM. Thus, phenotype of the sog1-1 mutant mimics aspects of the phenotypes of both atr and atm mutants in Arabidopsis, suggesting that SOG1 participates in pathways governed by both of these sensor kinases. We propose that, in plants, signals related to genomic stress are processed through a single, central transcription factor, SOG1.

Fig. 1.

Structure of the SOG1 gene. (A) Intron-exon organization of the SOG1 gene. Numbered gray boxes represent exons, lines indicate introns. (B) cDNA structure shows the location of the highly conserved NAC domain. Conserved subdomains are indicated by a–d. A conserved Gly was changed into Arg in the sog1-1 mutant.

Fig. 2.

Expression levels of G2 (CDKB2;1) and M (KNOLLE) phase transcripts after gamma-irradiation. Five-day-old seedlings were irradiated and then harvested for RNA preparation at the indicated times. All values were normalized to the expression level of ElF4A-1. Results are reported relative to the expression level in the 0-h untreated wild-type sample. Error bars indicate the standard deviation for technical triplicates. (A) CDKB2;1 expression. (B) KNOLLE expression. White, gray, and black symbols indicate wild-type, xpf-2, and xpf-2 sog1-1, respectively. Triangles and squares show biological repeats #1 and #2. Unirradiated controls are shown in Fig. S5.

Fig. 3.

Loss of heterozygosity in response to gamma irradiation. Wild-type, xpf-2, xpf-2 sog1-1, and sog1-1 seeds carrying a heterozygous albino mutation were gamma irradiated (100 Gy) and scored for the number of plants with sector(s) (because of the loss of the wild-type APG3 allele). Error bars indicate the standard deviation for biological triplicates of 100 plants each for each genotype. No sectors were observed in unirradiated controls or in APG3^+/+ plants.