A FEEDBACK-CONTROL ELEMENT NEAR THE TRANSCRIPTION START SITE OF THE MAIZE SHRUNKEN GENE DETERMINES PROMOTER ACTIVITY

The transcriptional activity of the Shrunken (Sh) promoter of Zea mays was monitored in transient expression assays using the neomycin phosphotransferase (NPT) II gene as a reporter in maize suspension protoplasts. Shortly after transfection, expression of this chimeric NPTII gene was negatively affected by high extracellular sucrose concentrations in the protoplast cultivation medium. However, 3-5 days after transfection an up to 405-fold increase in NPTII activity was observed. This could be blocked by dichlorobenzonitril (DCB) an inhibitor of cellulose biosynthesis. In the analysis promoter deletions 20 bp upstream of the Sh transcription start site were sufficient to reproduce the expression profile and the activity of the full promoter. Surprisingly this start sequence does not include the natural TATA-box.