Repression of the defense gene PR-10a by the single-stranded DNA binding protein SEBF

The potato pathogenesis-related gene PR-10a is transcriptionally activated in response to pathogen infection or elicitor treatment. Characterization of the cis-acting elements of the PR-10a promoter revealed the presence of a silencing element between residues -52 and -27 that contributes to transcriptional regulation. In this study, we have isolated a silencing element binding factor (SEBF) from potato tuber nuclei that binds to the coding strand of the silencing element in a sequence-specific manner. The consensus binding site of SEBF, PyTGTCNC, is present in a number of PR genes and shows striking similarity to the auxin response element. Mutational analysis of the PR-10a promoter revealed an inverse correlation between the in vitro binding of SEBF and the expression of PR-10a. SEBF was purified to homogeneity from potato tubers, and sequencing of the N terminus of the protein led to the isolation of a cDNA clone. Sequence analysis revealed that SEBF is homologous with chloroplast RNA binding proteins that possess consensus sequence-type RNA binding domains characteristic of heterogeneous nuclear ribonucleoproteins (hnRNPs). Overexpression of SEBF in protoplasts repressed the activity of a PR-10a reporter construct in a silencing element-dependent manner, confirming the role of SEBF as a transcriptional repressor.