Positive Factor 1 (PF1) from oat is an HMGY- and H1 histone-like protein that binds a functionally defined AT-rich DNA element in the oat phytochrome A gene (PHYA3) promoter.

Phytochrome A is a member of a plant regulatory photoreceptor family (1). The down-regulation by phytochrome of the transcription of its own PHYA3 gene, is one of the most rapid light-mediated effects on transcription reported to date in plants (2). Two positive elements designated PE1 (-367 bp to -346 bp) and PE3 (-111 bp to -81 bp) necessary for high level expression of the oat PHYA3 gene, were defined using linkerscan mutagenesis in a transient expression assay (3). We report the sequence of pC>2, a previously cloned cDNA encoding a protein that binds to the AT-rich PE1 region in the oat PHYA3 gene promoter (4). pO2 was originally obtained by screening a Xgtll cDNA expression library prepared from poly A RNA isolated from the shoots of etiolated oat seedlings. Screening was performed with double stranded synthetic oligonucleotides containing the sequence of the PE1 region of the oat PHYA3 gene promoter (4). pC>2 encodes a protein with the DNA binding characteristics expected for oat PF1, the nuclear factor that interacts in vivo with the PE1 region (4) and is therefore designated hereafter as PF1 (Figure 1). Oat PF1 is 79% similar and 69% identical to rice PF1, another PE1-binding protein encoded by a recently described cDNA (5). Oat PF1 contains three repeats of the nonapeptide: K/GRG/PRGRPP/AK. This sequence is very similar to the 'AT hook' motif, a DNA binding domain characteristic of all HMG I and HMG Y proteins (6, 7, 8) that are known to bind dAdTrich DNA, as oat and rice PF1 do (4, 5). Oat PF1 is also related to the protein encoded by the soybean SB 16 cDNA which contains three 'AT-hooks' (65% similarity, 51% identity) (9), as well as to the pea histone HI protein (50% similarity, 29% identity) (10), mostly in the NH2 terminal region. As in reported HMG Y proteins, oat PF1 lacks the distinctive hydrophobic region present in mammalian HMG I proteins (a.a. 110— 116 of the rice PF1 sequence) (Figure 1). The absence of acidic regions that may function as activation domains (11) is notable in the oat PF1 sequence. The role of PF1 in the transcriptional activation of the oat PHYA3 gene has yet to be determined. The analysis of the cis 1 60 Human HHG-I Human HHG-Y SoybeanHMGY-a U T I I vnKpq alPpTPUKr kt l taUHpn CSHKSAISKy n t t Y g e L . SoybaanHHGY-b aLnXad GSNKSAISXy IEttYgeL . FUc* PF1 MaTEKAatm auaadpKp* atPaYFEWl aailaLdDm GSKKtAISqb IEgkYegLlP O«t Prl MaTUvA Kjl UPpTTEHll aaitaLnDkv GSNXSAISKy IBgkYgd mP C O I M I I I I K-TH-A-— -I— —t-TTDtl I-L-I— H--Y--L-I