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A tobacco bZip transcription activator (TAF‐1) binds to a G‐box‐like motif conserved in plant genes.

Tobacco nuclear extract contains a factor that binds specifically to the motif I sequence (5′‐GTACGTGGCG‐3′) conserved among rice rab genes and cotton lea genes. We isolated from a tobacco cDNA expression library, a partial cDNA clone encoding a truncated derivative of a protein designated as TAF‐1. The truncated TAF‐1 (Mr = 26,000) contains an acidic region at its N‐terminus and a bZip motif at its C‐terminus. Using a panel of motif I mutants as probes, we showed that the truncated TAF‐1 and the tobacco nuclear factor for motif I have similar, it not identical, binding specificities. In particular, both show high‐affinity binding to the perfect palindrome 5′‐GCCACGTGGC‐3′ which is also known as the G‐box motif. TAF‐1 mRNA is highly expressed in root, but the level is at least 10 times lower in stem and leaf. Consistent with this observation, we found that a motif I tetramer, when fused to the −90 derivative of the CaMV 35S promoter, is inactive in leaf of transgenic tobacco. The activity, however, can be elevated by transient expression of the truncated TAF‐1. We conclude from these results that TAF‐1 can bind to the G‐box and related motifs and that it functions as a transcription activator.