Introduction

Gene transcription requires the interplay between transcription factors with their cognate promoter elements. Structural and functional analyses of many of these transcription factors revealed a modular protein structure, composed of a DNA-binding domain and a transcriptional activation domain (Johnson & McKnight, 1989; Mitchell & Tjian, 1989). The DNA-binding domain of the b-Zip proteins is characterised by the presence of a basic region with an adjacent leucine zipper (Landschulz et al., 1988). Whereas the basic region is required for specific protein–DNA interaction and directly contacts the DNA, the leucine zipper facilitates homodimer and heterodimer formation (Hu et al., 1990 and references therein). Transcriptional activation domains are often enriched in acidic amino acids (Hope & Struhl, 1987; Ptashne, 1988), glutamines (Courey et al., 1989) or prolines (Mermod et al., 1989).

Although many DNA-binding proteins have been identified in plant nuclear extracts, only a few cDNA sequences encoding these proteins have been cloned. Plant b-Zip proteins for which cDNAs have been isolated include the wheat proteins, EmBP-1 (Guiltinan et al., 1990) and HBP-1 (Tabata et al., 1989), the maize proteins OCSBF-1 (Singh et al., 1990) and Opaque2 (Schmidt et al., 1990) and the tobacco proteins TGA1a and TGA1b (Katagiri et al., 1989). HBP-1, as originally identified in crude nuclear extract, was shown to interact with the hexamer (Hex) motif TGACGT found in several histone promoters (Mikami et al., 1987, 1989a,b,c). The cDNA identified as encoding HBP-1 was isolated by screening an expression library for specific DNA binding to an oligonucleotide derived from the wheat histone 3 promoter and containing the conserved Hex motif (Tabata et al., 1989).