Repression of the CaMV 35S promoter by the octopine synthase enhancer element

Abstract
A 16 base‐pair palindrome upstream of the Agrobacterium tumefaciens octopine synthase (ocs) gene functions as a positive regulatory element in plant cells (Ellis et al. (1987) EMBO J. 6, 3203–3208; Fromm et al. (1989) Plant Cell 1, 977–984). We have converted it into a negative element by locating two copies flanking the TATA‐box of the constitutively expressed CaMV 35S promoter. The reduced promoter activity is very likely due to sterical hindrance of the ocs binding protein (OCSTF) × ocs complex with the transcription initiation complex. We propose that this type of constructs can be used for the identification of recognition sites for DNA‐binding proteins which are labile in vitro as well as for determining the DNA‐binding activity of a trans‐acting factor in vivo.