Two tobacco DNA-binding proteins with homology to the nuclear factor CREB

Abstract

THE 35S promoter of the cauliflower mosaic virus (CaMV) contains a tandem repeat of the sequence TGACG in the region −83 to −63. This 21-base pair (bp) sequence, called as-1, is involved in root expression of the 35S promoter. When inserted in a promoter of a gene expressed specifically in photosynthetic tissues, as-1 confers high level expression in roots¹³. We have described a factor, ASF-1, that binds specifically to as-1 in vitro. There is a good correlation between ASF-1 binding affinity to as-1 related sequences in vitro and the function of these sequences in vivo. These results strongly suggest that ASF-1 is responsible for the function of as-1 (ref. 13). Here we report the isolation of tobacco complementary DNA clones encoding two TGACG-sequence-specific binding-proteins (TGAla and TGAlb). Sequence analysis of the cDNA clones shows that both proteins contain a basic region that shows high homology to a stretch of basic amino acids in the nuclear factors CREB, GCN4, and c-Jun^1-4 to a 'leucine-zipper' region⁵. On the basis of binding specificity we propose TGAla to be a good candidate for ASF-1.