FIG. 4.

Primary structure of the RecBCD enzyme. The RecB protein is modular. The N-terminal domain (red) contains seven motifs (numbered) characteristic of SF1 helicases. The C-terminal domain (magenta) contains motifs characteristic of a diverse family of nucleases. “Nuc” marks the position of nuclease motif 3, which contains key catalytic aspartate and lysine residues (D1067, D1080, and K1082) (308, 325). The RecC protein (blue) does not show significant homology to other known proteins. A region of RecC implicated in Chi recognition is indicated (see the text). The RecD protein (green) contains the seven conserved SF1 helicase motifs (numbered). The total number of amino acids (aa) in each polypeptide is indicated in parentheses. This color scheme is used in figures of the RecBCD-DNA crystal structure.