The far-upstream region of the Arabidopsis thaliana plastocyanin (Pc) promoter acts positively on transcription. This -1580 to -710 region (relative to the translation start site) has enhancer-like properties since it is also functional when situated downstream of the gene. Using tobacco nuclear extracts, this region was tested for protein-binding sites. Two short binding sequences were identified. The AT-rich sequence separating these binding sites shows extensive homology to the sequences separating the paired GT-1-binding sites of the pea rbcS-3A promoter. The requirements for complex formation strongly suggest that a GT-1-like protein binds to the two identified boxes in the Pc promoter. Sequence comparisons revealed that both boxes fit within the moderate consensus sequence needed for GT-1-binding. This GT-1-like DNA-binding activity is present in light-grown as well as in dark-adapted plants. Therefore, the possible role for GT-1 in light regulation of transcription does not depend upon its de novo synthesis. In some of the gel mobility shift assays, an additional DNA-protein complex was formed. The formation of this complex was only observed if the heteropolymer poly(dAdT).poly(dAdT) was used as a non-specific competitor and was dependent on the CpG density of the probe used.
(Rcceivcd X Aug~i~tI30 Octoher 1995) -E3R 95 1321)/2 Thc methylatinn of cytosine rcsidues in CpG dinucleotides of cukaryolic DNA is an important inechunistn for the regulation of gene expression. Higher plants havc a high content of inethylated cytosine residues in CpG as well as CpNpC sites, and experimental evidence suggests a role in gene expression for DNA methylation. In this article, we dcscribe ii tobacco nuclear protcin whose binding to various DNA sequences is positively correlated with the CpG density of thc probes. This protein, CpG-binding protein 1 (CGBP-1). has reduced affinity for DNA when the CpC; sites are methylated. Ribonucleasc treatment also reduces thc formation of the CGRP-I complex. The hinding characteristics of' CGBP-1 make it an interesting protein with respect to methylation-meJiated gene expression in pl tits.
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