Mutual regulation of Arabidopsis thaliana ethylene-responsive element binding protein and a plant floral homeotic gene, APETALA2

BACKGROUND AND AIMS: It has previously been shown that Arabidopsis thaliana ethylene-responsive element binding protein (AtEBP) contributed to resistance to abiotic stresses. Interestingly, it has also been reported that expression of ethylene-responsive factor (ERF) genes including AtEBP were regulated by the activity of APETALA2 (AP2), a floral homeotic factor. AP2 is known to regulate expression of several floral-specific homeotic genes such as AGAMOUS. The aim of this study was to clarify the relationship between AP2 and AtEBP in gene expression. METHODS: Northern blot analysis was performed on ap2 mutants, ethylene-related Arabidopsis mutants and transgenic Arabidopsis plants over-expressing AtEBP, and a T-DNA insertional mutant of AtEBP. Phenotypic analysis of these plants was performed. KEY RESULTS: Expression levels of ERF genes such as AtEBP and AtERF1 were increased in ap2 mutants. Over-expression of AtEBP caused upregulation of AP2 expression in leaves. AP2 expression was suppressed by the null-function of ethylene-insensitive2 (EIN2), although AP2 expression was not affected by ethylene treatment. Loss of AtEBP function slightly reduced the average number of stamens. CONCLUSIONS: AP2 and AtEBP are mutually regulated in terms of gene expression. AP2 expression was affected by EIN2 but was not regulated by ethylene treatment.     

A novel role for the floral homeotic gene APETALA2 during Arabidopsis fruit development

The majority of the Arabidopsis fruit comprises an ovary with three primary tissue types: the valves, the replum and the valve margins. The valves, which are derived from the ovary walls, are separated along their entire length by the replum. The valve margin, which consists of a separation layer and a lignified layer, forms as a narrow stripe of cells at the valve-replum boundaries. The valve margin identity genes are expressed at the valve-replum boundary and are negatively regulated by FUL and RPL in the valves and replum, respectively. In ful rpl double mutants, the valve margin identity genes become ectopically expressed, and, as a result, the entire outer surface of the ovary takes on valve margin identity. We carried out a genetic screen in this sensitized genetic background and identified a suppressor mutation that restored replum development. Surprisingly, we found that the corresponding suppressor gene was AP2, a gene that is well known for its role in floral organ identity, but whose role in Arabidopsis fruit development had not been previously described. We found that AP2 acts to prevent replum overgrowth by negatively regulating BP and RPL, two genes that normally act to promote replum formation. We also determined that AP2 acts to prevent overgrowth of the valve margin by repressing valve margin identity gene expression. We have incorporated AP2 into the current genetic network controlling fruit development in Arabidopsis.     

The Arabidopsis floral homeotic gene APETALA3 differentially regulates intercellular signaling required for petal and stamen development

Cell-cell signaling is crucial for the coordination of cell division and differentiation during plant organogenesis. We have developed a novel mosaic analysis method for Arabidopsis, based on the maize Ac/Ds transposable element system, to assess the requirements of individual genes in intercellular signaling. Using this strategy, we have shown that the floral homeotic APETALA3 (AP3) gene has distinct roles in regulating intercellular signaling in different tissues. In petals, AP3 acts primarily in a cell-autonomous fashion to regulate cell type differentiation, but its function is also required in a non-cell-autonomous fashion to regulate organ shape. In contrast, AP3-regulated intercellular interactions are required for conferring both cell type identity and organ shape and size in the stamens. Using antibodies raised against AP3, we have shown that the AP3 protein does not traffic between cells. These observations imply that AP3 acts by differentially regulating the production of intercellular signals in a whorl-specific manner.     

特异AT序列结合蛋白2(SATB2)的研究进展

特异AT序列结合蛋白2(SATB2)是与核基质结合区结合的转录因子,在转录调控和染色质重组等过程中发挥重要作用。SATB2是成骨细胞分化和骨基质形成过程中的关键调控分子,其突变可导致先天性颌面部畸形的发生;SATB2是中枢神经系统发育过程中的核心分子,在胼胝体、脑桥等正常发育中起重要作用;SATB2也参与肿瘤的发生,在乳腺癌等恶性肿瘤中的表达升高。文章就上述各领域中SATB2的研究进展进行了综述。     

The novel Mrf-2 DNA-binding domain recognizes a five-base core sequence through major and minor-groove contacts.

Recent NMR studies of the purified Mrf-2 DNA-binding domain peptide have shown that its structure differs significantly from previously characterized classes of DNA-binding domains. Here we report biochemical studies of the DNA-binding properties of this peptide. Binding interference and binding site selection assays indicated that Mrf-2 requires the core sequence AATA(C/T) for high affinity binding. Kinetic analyses of several selected sequences indicated that the core sequence alone is not sufficient for high affinity binding, however. Kinetic analyses were also performed using a series of synthetic oligonucleotides with single base analogues at each position in the core sequence. Base analogues that altered the major groove structure reduced or eliminated Mrf-2 binding when present in the second, third, and fourth base-pairs of the core sequence, but had little or no effect in the first and fifth positions. These results suggest that Mrf-2 contacts both the major and minor grooves of its target sequences.     

An RNA-binding protein recognizes a mammalian selenocysteine insertion sequence element required for cotranslational incorporation of selenocysteine.

In mammalian selenoprotein mRNAs, the recognition of UGA as selenocysteine requires selenocysteine insertion sequence (SECIS) elements that are contained in a stable stem-loop structure in the 3' untranslated region (UTR). In this study, we investigated the SECIS elements and cellular proteins required for selenocysteine insertion in rat phospholipid hydroperoxide glutathione peroxidase (PhGPx). We developed a translational readthrough assay for selenoprotein biosynthesis by using the gene for luciferase as a reporter. Insertion of a UGA or UAA codon into the coding region of luciferase abolished luciferase activity. However, activity was restored to the UGA mutant, but not to the UAA mutant, upon insertion of the PhGPx 3' UTR. The 3' UTR of rat glutathione peroxidase (GPx) also allowed translational readthrough, whereas the PhGPx and GPx antisense 3' UTRs did not. Deletion of two conserved SECIS elements in the PhGPx 3' UTR (AUGA in the 5' stem or AAAAC in the terminal loop) abolished readthrough activity. UV cross-linking studies identified a 120-kDa protein in rat testis that binds specifically to the sense strands of the PhGPx and GPx 3' UTRs. Direct cross-linking and competition experiments with deletion mutant RNAs demonstrated that binding of the 120-kDa protein requires the AUGA SECIS element but not AAAAC. Point mutations in the AUGA motif that abolished protein binding also prevented readthrough of the UGA codon. Our results suggest that the 120-kDa protein is a significant component of the mechanism of selenocysteine incorporation in mammalian cells.     

Cymbidium ringspot tombusvirus coat protein coding sequence acts as an avirulent RNA

Avirulent genes either directly or indirectly produce elicitors that are recognized by specific receptors of plant resistance genes, leading to the induction of host defense responses such as hypersensitive reaction (HR). HR is characterized by the development of a necrotic lesion at the site of infection which results in confinement of the invader to this area. Artificial chimeras and mutants of cymbidium ringspot (CymRSV) and the pepper isolate of tomato bushy stunt (TBSV-P) tombusviruses were used to determine viral factors involved in the HR resistance phenotype of Datura stramonium upon infection with CymRSV. A series of constructs carrying deletions and frameshifts of the CymRSV coat protein (CP) undoubtedly clarified that an 860-nucleotide (nt)-long RNA sequence in the CymRSV CP coding region (between nt 2666 and 3526) is the elicitor of a very rapid HR-like response of D. stramonium which limits the virus spread. This finding provides the first evidence that an untranslatable RNA can trigger an HR-like resistance response in virus-infected plants. The effectiveness of the resistance response might indicate that other nonhost resistance could also be due to RNA-mediated HR. It is an appealing explanation that RNA-mediated HR has evolved as an alternative defense strategy against RNA viruses.     

The design and analysis of a homeotic response element

We have shown that the 26 bp bx1 element from the regulatory region of Distal-less is capable of imposing control by the homeotic genes Ultrabithorax and abdominal-A on a general epidermal activator in Drosophila. This provides us with an assay to analyze the sequence requirements for specific repression by these Hox genes. Both the core Hox binding site, 5'-TAAT, and the adjacent EXD 5'-TGAT core site are required for repression by Ultrabithorax and abdominal-A. The Distal-less bx1 site thus fits with the model of Hox protein binding specificity based on the consensus PBX/HOX-family site TGATNNAT[g/t][g/a], where the key elements of binding specificity are proposed to lie in the two base pairs following the TGAT. A single base pair deletion in the bx1 sequence generates a site, bx1:A(-)mut, that on the consensus PBX/HOX model would be expected to be regulated by the Deformed Hox gene. We observed, however, that the bx1:A(-)mut site was regulated predominantly by Sex combs reduced, Ultrabithorax and abdominal-A. The analysis of this site indicates that the specificity of action of Hox proteins may depend not only on selective DNA binding but also on specific post-binding interactions.