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 o f the cis-acting elements of the PR-10a promoter revealed the presence of a silencing element between residues -52 and -27 that contributes to transcr iptional regulation. In this study, we have isolated a silencing element bi nding factor (SEBF) from potato tuber nuclei that binds to the coding stran d of the silencing element in a sequence-specific manner. The consensus bin ding site of SEBF1 PyTGTCNC, is present in a number of PR genes and shows s triking similarity to the auxin response element. Mutational analysis of th e PR-10a promoter revealed an inverse correlation between the in vitro bind ing 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 homo logous with chloroplast RNA binding proteins that possess consensus sequenc e-type RNA binding domains characteristic of heterogenous nuclear ribonucle oproteins (hnRNPs). Overexpression of SEBF in protoplasts repressed the act ivity of a PR-10a reporter construct in a silencing element-dependent manne r, confirming the role of SEBF as a transcriptional repressor.