GSBF1, A SEEDLING-SPECIFIC BZIP DNA-BINDING PROTEIN WITH PREFERENCE FOR A SPLIT G-BOX-RELATED ELEMENT IN BRASSICA-NAPUS RBCS PROMOTERS

Promoters of RbcS genes may contain a GS-box, which is a cis element w ith a core sequence related to the G-box, but split by a spacer of abo ut 14 bp. Here we describe GSBF1, a DNA-binding protein that specifica lly interacts with a GS-box element located proximal to the G-box in t he Brassica napus RbcS IV promoter. Sequence analysis of GSBF1 reveale d a basic region/leucine zipper (bZIP) domain that displays structural features distinct from that of G-box binding factors (GBFs). Gel shif t experiments showed that recombinant GSBF1 does not efficiently bind to the continuous G-box motif. RNA gel blot analysis indicated that GS BF1 transcripts are cotyledon-specific and accumulate to the highest l evels during late seedling development in a light-dependent manner. Du ring the same time period, RbcS IV transcript levels decreased simulta neously, suggesting that GSBF1 acts as a developmental, stage-specific , negative regulator of RbcS IV gene expression in rape seedlings.