CRYSTAL STRUCTURE OF PU.1 ETS DOMAIN-DNA COMPLEX: A NEW PATTERN FOR HELIX-TURN-HELIX RECOGNITION: K. R. Ely, R. Kodandapani, F. Pio, C.-Z. Ni, G. Piccialli, S. McKercher, M. Klemsz and R. A. Maki, La Jolla Cancer Research Foundation, La Jolla, CA 92037 USA

The ets family of transcription factors regulate gene expression during growth and development and share a conserved ETS DNA-binding domain that binds as a monomer to the sequence 5'-C/AGGAA/T-3'. The crystal structure of the PU.1 ETS domain complexed with a 16 bp DNA oligonucleotide has been determined at 2.3-Å resolution. Crystals formed in space group C2 with a=89.1, b=101.9, c=55.6 Å, [[beta]]=111.2deg. with two complexes in the asymmetric unit. The structure was solved by the MIRAS method. The current R-factor=23.7 (R_free=29.9). The domain is similar to [[alpha]]+[[beta]] ("winged") helix-turn-helix DNA-binding proteins and contacts a ten bp region of duplex DNA that is bent (8deg.) but uniformly curved without distinct kinks. Four conserved amino acids contact DNA from a novel loop-helix-loop architecture. Arg232 and Arg235 from the recognition helix are hydrogen-bonded to the GGA bases in the major groove. These interactions represent the paradigm for ets recognition. Two loops contact the DNA backbone in the minor groove: Lys245 in the "wing" between the third and fourth [[beta]]-strands contacts the phosphate backbone of the GGAA strand upstream from the core sequence while Lys219 from the "turn" of the HTH motif contacts the phosphate backbone of the opposite strand downstream of the GGAA core. This work was supported by grants USAMRDC-DAMD17-94J-4439, NIH-CA63489-01 and NIH Al20194.