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 fu nctionally 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-el ement being an AT-rich sequence, designated PE1, whereas GT-2 is a transcri ptional activator with twin DNA binding domains that recognize a triplet of GT-boxes in a complex motif designated GTE. To further define the DNA-bind ing activity of PF1 and to explore potential inter-relationships between th e two factors, we have performed a series of in vitro DNA-binding experimen ts 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 t o PE1. In addition, PF1 can bind promiscuously, with varying affinity, to o ther 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 re port of such activity for a plant HMG-I/Y protein. This enhancement takes p lace without demonstrable physical contact between the two proteins, sugges ting the possibility of a novel, indirect mechanism of recruitment involvin g DNA target-site pre-conditioning. The evidence indicates therefore that P F1 and GT-2 do not perform functionally equivalent roles in positively regu lating oat and rice PHYA gene expression. However, the data suggest the pos sibility that PF1 may act as an architectural factor, promiscuously recogni zing a spectrum of AT-containing elements in plant promoters, with the gene ral function of catalyzing enhanced binding of conventional cognate transcr iptional regulators to these elements via DNA bending.