In vitro binding of the tomato bZIP transcriptional activator VSF-1 to a regulatory element that controls xylem-specific gene expression.
The bean grp1.8 gene is specifically expressed in vascular tissue. Monomers and multimers of a 28 bp regulatory element of the grp1.8 promoter (vs-1) specifically activated both the -82 CaMV 35S and the -76/grp1.8 minimal promoters in vascular tissue of transgenic tobacco plants. vs-1 partially overlaps with a negative regulatory element in the grp1.8 promoter that is necessary for restriction of gene expression to vascular tissue. Nuclear extracts from tobacco and tomato cells contain a factor that binds to vs-1 in vitro. To study the molecular basis of xylem-specific expression mediated by the vs-1 promoter element, a gene was isolated from tomato encoding a protein that binds to vs-1 in vitro. This protein, designated VSF-1, contains a bZIP motif close to the C-terminus. Mutated vs-1 elements were no longer bound by VSF-1 and also failed to activate the minimal -82 CaMV 35S promoter in vivo. Transient expression of VSF-1 in protoplasts stimulated vs-1 dependent activation of the -76/grp1.8 minimal promoter. Binding studies and use of a polyclonal antiserum against VSF-1 provided further evidence that vs-1 is a potential in vivo target site, as VSF-1 was a part of the observed complex formed between vs-1 and nuclear protein extract. vs-1 does not contain the 5'-ACGT-3' core sequence that is part of known plant bZIP protein binding sites or another palindromic sequence. Based on the unusual binding specificity and a characteristic amino acid sequence in the bZIP domain we propose that VSF-1 and the partially homologous PosF21, a bZIP protein from Arabidopsis, belong to a new family of plant bZIP proteins.
Torres-Schumann
S
S
Department of Resistance and Quality Breeding, Swiss Federal Research Station for Agroecology and Agriculture, Zürich, Switzerland.
Ringli
C
C
Heierli
D
D
Amrhein
N
N
Keller
B
B
D000595:Amino Acid Sequence; D001483:Base Sequence; D050976:Basic-Leucine Zipper Transcription Factors; D001665:Binding Sites / Q000235:genetics; D002874:Chromosome Mapping; D018076:DNA, Complementary / Q000235:genetics; D018744:DNA, Plant / Q000235:genetics; D004268:DNA-Binding Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D051790:G-Box Binding Factors; D018506:Gene Expression Regulation, Plant; D017343:Genes, Plant; D005809:Genes, Regulator; D016350:Leucine Zippers / Q000235:genetics; D018551:Solanum lycopersicum / Q000235:genetics / Q000378:metabolism; D008969:Molecular Sequence Data; D010940:Plant Proteins / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism; D010947:Plants, Toxic; D011401:Promoter Regions, Genetic; D017386:Sequence Homology, Amino Acid; D014026:Nicotiana / Q000235:genetics / Q000378:metabolism; D015534:Trans-Activators / Q000235:genetics / Q000302:isolation & purification / Q000378:metabolism*; D014157:Transcription Factors
D050976:Basic-Leucine Zipper Transcription Factors; D018076:DNA, Complementary; D018744:DNA, Plant; D004268:DNA-Binding Proteins; D051790:G-Box Binding Factors; D010940:Plant Proteins; D015534:Trans-Activators; D014157:Transcription Factors