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.