title: Transcriptional control of aspartate kinase expression during darkness and sugar depletion in Arabidopsis: involvement of bZIP transcription factors.
abstract: Initial steps of aspartate-derived biosynthesis pathway (Asp pathway) producing Lys, Thr, Met and Ile are catalyzed by bifunctional (AK/HSD) and monofunctional (AK-lys) aspartate kinase (AK) enzymes. Here, we show that transcription of all AK genes is negatively regulated under darkness and low sugar conditions. By using yeast one-hybrid assays and complementary chromatin immunoprecipitation analyses in Arabidopsis cells, the bZIP transcription factors ABI5 and DPBF4 were identified, capable of interacting with the G-box-containing enhancer of AK/HSD1 promoter. Elevated transcript levels of DPBF4 and ABI5 under darkness and low sugar conditions coincide with the repression of AK gene expression. Overexpression of ABI5, but not DPBF4, further increases this AK transcription suppression. Concomitantly, it also increases the expression of asparagines synthetase 1 (ASN1) that shifts aspartate utilization towards asparagine formation. However, in abi5 or dpbf4 mutant and abi5, dpbf4 double mutant the repression of AK expression is maintained, indicating a functional redundancy with other bZIP-TFs. A dominant-negative version of DPBF4 fused to the SRDX repressor domain of SUPERMAN could counteract the repression and stimulate AK expression under low sugar and darkness in planta. This effect was verified by showing that DPBF4-SRDX fails to recognize the AK/HSD1 enhancer sequence in yeast one-hybrid assays, but increases heterodimmer formation with DPBF4 and ABI5, as estimated by yeast two-hybrid assays. Hence it is likely that heterodimerization with DPBF4-SRDX inhibits the binding of redundantly functioning bZIP-TFs to the promoters of AK genes and thereby releases the repressing effect. These data highlight a novel transcription control of the chloroplast aspartate pathway that operates under energy limiting conditions.
authors.affiliation: Department of Plant Sciences, The George S. Wise Faculty of Life Sciences, Tel Aviv University, Tel Aviv 69978, Israel. shai.ufaz@gmail.com
authors.forename: Shai
authors.initials: S
authors.lastname: Ufaz
authors.forename: Vijaya
authors.initials: V
authors.lastname: Shukla
authors.forename: Yulia
authors.initials: Y
authors.lastname: Soloveichik
authors.forename: Yelena
authors.initials: Y
authors.lastname: Golan
authors.forename: Frank
authors.initials: F
authors.lastname: Breuer
authors.forename: Zsuzsa
authors.initials: Z
authors.lastname: Koncz
authors.forename: Gad
authors.initials: G
authors.lastname: Galili
authors.forename: Csaba
authors.initials: C
authors.lastname: Koncz
authors.forename: Aviah
authors.initials: A
authors.lastname: Zilberstein
mesh_terms: D000595:Amino Acid Sequence; D017360:Arabidopsis / Q000201:enzymology* / Q000235:genetics* / Q000378:metabolism; D029681:Arabidopsis Proteins / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D001222:Aspartate Kinase / Q000096:biosynthesis* / Q000235:genetics / Q000378:metabolism; D001483:Base Sequence; D050976:Basic-Leucine Zipper Transcription Factors / Q000235:genetics / Q000378:metabolism*; D003624:Darkness; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D014644:Genetic Variation; D008969:Molecular Sequence Data; D009154:Mutation; D011401:Promoter Regions, Genetic; D012045:Regulatory Sequences, Nucleic Acid; D013395:Sucrose / Q000378:metabolism*; D015533:Transcriptional Activation
chemical_list: D029681:Arabidopsis Proteins; D050976:Basic-Leucine Zipper Transcription Factors; D013395:Sucrose; D001222:Aspartate Kinase