The HMG-I/Y protein PF1 stimulates binding of the transcriptional activator GT-2 to the PHYA gene promoter.

The DNA-binding proteins PF1 and GT-2 are factors that bind to different functionally defined, positively acting cis-elements in the PHYA genes of oat and rice, respectively. PF1 is an HMG-I/Y protein, with its cognate cis-element being an AT-rich sequence, designated PE1, whereas GT-2 is a transcriptional activator with twin DNA binding domains that recognize a triplet of GT-boxes in a complex motif designated GTE. To further define the DNA-binding activity of PF1 and to explore potential inter-relationships between the two factors, we have performed a series of in vitro DNA-binding experiments with both PE1 and GTE target sites. The data show that, consistent with its membership of the HMG-I/Y protein family, PF1 can bend DNA when bound to PE1. In addition, PF1 can bind promiscuously, with varying affinity, to other AT-containing motifs, including GTE. When co-incubated with GT-2, PF1 enhances the specific DNA-binding activity of GT-2 toward GTE, the first report of such activity for a plant HMG-I/Y protein. This enhancement takes place without demonstrable physical contact between the two proteins, suggesting the possibility of a novel, indirect mechanism of recruitment involving DNA target-site pre-conditioning. The evidence indicates therefore that PF1 and GT-2 do not perform functionally equivalent roles in positively regulating oat and rice PHYA gene expression. However, the data suggest the possibility that PF1 may act as an architectural factor, promiscuously recognizing a spectrum of AT-containing elements in plant promoters, with the general function of catalyzing enhanced binding of conventional cognate transcriptional regulators to these elements via DNA bending.