MYB.PH3 TRANSCRIPTION FACTOR FROM PETUNIA-HYBRIDA INDUCES SIMILAR DNA-BENDING DISTORTIONS ON ITS 2 TYPES OF BINDING-SITE/

MYB.Ph3 from Petunia is a member of the MYB transcription factor famil y that recognizes two types of binding site, resembling direct and inv erted repeats of the motif GTTA, respectively, and flanked on their 5' side by an A-rich sequence. In this study, it is shown that MYB.Ph3 i nduces similar conformational changes in both of its DNA-binding sites . Circular permutation assays indicate that the center of the distorti ons induced by MYB.Ph3 maps near the center of either MYB.Ph3-binding site. The degree of the distortion induced by MYB.Ph3 appears to be gr eatly affected by regions of the protein other than the DNA-binding do main, and differs from the distortion caused by animal c-MYB. Phasing analysis reveals that part of the distortion induced by MYB.Ph3 is DNA bending, oriented towards the minor groove, as is the case of its ani mal counterpart, c-MYB. DNA-binding by both MYB.Ph3 and animal c-MYB i s more efficient in the presence of Ba2+, a divalent cation known to p romote/stabilize DNA bending, than in the presence of other cations wh ich do not favor this distortion, such as Na+ and Mg2+. In addition, b oth MYB proteins show higher affinity to selectively nicked DNA, which has increased DNA flexibility, strongly suggesting that DNA binding b y these MYB proteins and DNA structural properties are mutually influe nced.