RESUMO
Plants have as many as 20 heat shock factors (Hsfs) grouped into three classes, A, B and C, based on sequence similarity and modular structures. Through screening for cell death-inducing factor(s) in Nicotiana benthamiana, we identified Arabidopsis HsfB2b and thus subjected all other members of Arabidopsis Hsf class B (HsfB1, HsfB2a, HsfB2b, HsfB3 and HsfB4) to the same cell death assay. When expressed in N. benthamiana leaves, only HsfB1 and HsfB2b elicited mild cell death. Simultaneously we found that HsfB1 has a post-transcriptional control mechanism, in which a sequence-conserved upstream open-reading frame (sc-uORF) is involved. The known repressor function of the respective HsfBs was confirmed and the difference in cell death-inducing activity of HsfBs was explained by the fact that HsfB1 and HsfB2b are transcriptional repressors but the others are not. Indeed, the cell death symptom by HsfB1 and HsfB2b required not only their repression activity but also their nuclear localization activity. HsfB1 expression was drastically and transiently induced by heat shock (HS) and the intactness of sc-uORF was required for its HS response. Based on the results, the physiological significance of cell death-inducing activity of HsfB1 and HsfB2b and the sc-uORF in the HsfB1 transcript during HS response is discussed.
Assuntos
Proteínas de Arabidopsis/genética , Arabidopsis/genética , Proteínas de Ligação a DNA/genética , Proteínas de Choque Térmico/genética , Proteínas de Plantas/genética , Biossíntese de Proteínas , RNA Mensageiro/metabolismo , RNA de Plantas/metabolismo , Fatores de Transcrição/genética , Sequência de Aminoácidos , Apoptose/genética , Proteínas de Arabidopsis/metabolismo , Sequência Conservada , Proteínas de Ligação a DNA/metabolismo , Glucuronidase/análise , Proteínas de Fluorescência Verde/análise , Fatores de Transcrição de Choque Térmico , Proteínas de Choque Térmico/metabolismo , Dados de Sequência Molecular , Sinais de Localização Nuclear , Cebolas/genética , Fases de Leitura Aberta , Proteínas de Plantas/metabolismo , Proteínas Recombinantes de Fusão/análise , Sequências Reguladoras de Ácido Nucleico/fisiologia , Alinhamento de Sequência , Fatores de Transcrição/metabolismoRESUMO
Sucrose is known to repress the translation of Arabidopsis thaliana AtbZIP11 transcript which encodes a protein belonging to the group of S (S--stands for small) basic region-leucine zipper (bZIP)-type transcription factor. This repression is called sucrose-induced repression of translation (SIRT). It is mediated through the sucrose-controlled upstream open reading frame (SC-uORF) found in the AtbZIP11 transcript. The SIRT is reported for 4 other genes belonging to the group of S bZIP in Arabidopsis. Tobacco tbz17 is phylogenetically closely related to AtbZIP11 and carries a putative SC-uORF in its 5'-leader region. Here we demonstrate that tbz17 exhibits SIRT mediated by its SC-uORF in a manner similar to genes belonging to the S bZIP group of the Arabidopsis genus. Furthermore, constitutive transgenic expression of tbz17 lacking its 5'-leader region containing the SC-uORF leads to production of tobacco plants with thicker leaves composed of enlarged cells with 3-4 times higher sucrose content compared to wild type plants. Our finding provides a novel strategy to generate plants with high sucrose content.