S2F, A LEAF-SPECIFIC TRANS-ACTING FACTOR, BINDS TO A NOVEL CIS-ACTINGELEMENT AND DIFFERENTIALLY ACTIVATES THE RPL21 GENE

Tissue-specific factors control the differential expression of nuclear genes encoding plastid proteins. To identify some of these factors, t he light-independent spinach RPL21 gene encoding the plastid ribosomal protein L21 was chosen as a model. The RPL21 promoter organization wa s defined by transient and stable transfections of RPL21 promoter dele tion mutants fused to a reporter gene. The following results were obta ined. (1) We identified a strong core promoter, spanning the transcrip tion start site region, sufficient to drive high levels of gene expres sion, (2) We identified two nonoverlapping positive and negative domai ns, located upstream from the core promoter region, that modulate core promoter activity independently of light. (3) We found that the posit ive domain contains a new cis-acting element, the S2 site, related to but different from the light-responsive GT-1 binding site, We show tha t the S2 site binds a leaf-specific nuclear factor (named S2F). The S2 site is conserved in the promoter region of many nuclear genes encodi ng plastid proteins. Experiments with transgenic tobacco plants confir med that the S2 site is critical for positive domain activity in leaf tissues. The S2 site is thus identified as a new tissue-specific, ligh t-independent regulatory element.