The E. coli RecA NTD exhibits significant amino acid sequence homology with the NTDs of the homologous proteins Rad51/Dmc1/RadA. (A) Sequence alignment of the NTDs of RecA proteins from S. solfataricus (Sso RadA), M. voltae (Mv RadA), P. furiosus (Pf Rad51), H. sapiens (Hs Rad51 and Hs Dmc1), S. cerevisiae (Sc Dmc1 and Sc Rad51), and E. coli (Ec RecA). Secondary structural features of the left-handed Sso RadA helical filament are indicated in cyan (α-helices). Functional motifs are indicated under their corresponding amino acid sequences: the first HhH motif (H'h'H2'), core helix (Hc the), and the second HhH motif (H1Hh2). (B) The NTD carries out a large conformational change in response to ssDNA and ATPγS. Shown are ribbon diagrams of the monomeric RecA structures in the RecA-ssDNA-ADP-AlF4- presynaptic filament  and in the inactive RecA protein helical filament . The ssDNA and RecA polypeptides are shown in gold and green, respectively. The side chains of Glu18, Lys23, and Arg33 are depicted as ball-and-stick models.