A NOVEL [G-(G.C)] BASE-TRIPLET: MODEL FOR BASE PAIR RECOGNITION DURING HOMOLOGOUS RECOMBINATION. Blaine H. M. Mooers & P. Shing Ho, Dept. of Biochemistry & Biophysics, Oregon State Univ., Corvallis, OR 97331

In the crystal structure of the nonamer d(GCGTACGCG), the 3'-terminal guanine forms a base-triplet in the minor groove of B-DNA that is consistent with recent biochemical evidence about the structure of RecA-DNA triplex complexes. During homologous recombination, RecA protein polymerizes on single-stranded DNA, and the resulting nucleoprotein filament incorporates a homologous region of double-stranded DNA into a RecA protein coated DNA triplex, otherwise known as "R-DNA". R-DNA has an extended and unwound conformation. The third strand in this enzymatically formed triplex is parallel to the homologous strand in the Watson-Crick duplex, and it has been generally thought to lie in the major groove of the DNA duplex. However, a recent study by Baliga et al., Proc. Natl. Acad. Sci., USA, 92, 10393-10397 (1995), indicates that the third DNA strand lies in the minor groove of the parent duplex. None of the available structures for DNA triplets, however, provide a model for how the third strand can recognize the homologous duplex in the minor groove. We present the 2.5 Angstrom structure of a base-triplet which has features common to R-DNA in that the third base sits in the minor groove parallel to the homologous strand in the Watson-Crick duplex. In the crystal lattice, the first eight nucleotides of the nonamer sequence form a standard B-DNA duplex with a complimentary strand. These duplexes stack end-to-end but fail to form continuous helices because the terminal base pairs of adjacent stacks are underwound with respect to each other. This underwinding places the orphaned 3U-terminal end in the minor groove of the adjacent duplex where it symmetrically pairs with the 5'-terminal guanine to form a d[G*(G.C)] base-triplet. Our findings extend to the minor groove a DNA hydrogen bonding pattern which permits base-pair recognition during homologous recombination.