Summary We characterize the interaction of RecA with membranes in vivo and in vitro and demonstrate that RecA binds tightly to the anionic phospholipids (aPLs) cardiolipin (CL) and phosphatidylglycerol (PG). Using computational models, we identify two regions of RecA that interact with PG and CL: 1) the N-terminal helix and 2) loop L2. Mutating these regions decreased the affinity of RecA to PG and CL in vitro. Using 3D-super-resolution microscopy (3D-SIM), we demonstrate that depleting Escherichia coli PG and CL altered the localization of RecA foci and hindered the formation of RecA filament bundles. In consequence, E. coli cells lacking aPLs fail to initiate a robust SOS response after DNA damage, indicating that the membrane acts as a scaffold for nucleating the formation of RecA filament bundles and plays an important role in the SOS response.

The crystal structures of three compounds involving (o-pheny1enedioxy)diacetic acid (H,L) and sodium, ammonium, and magnesium ions have been determined by X-ray diffraction.[{Na,L,(H,O),},](5) is triclinic, with a = 7.206(1), b = 14.306(5), c = 14.545(4) A, a = 101.86(2), p = 92.1 9(2), y = 90.32(2)", Z = 2, and space group P i . Ionic [NH,],[L] ( 6) is orthorhombic, with a = 11.833(3), b = 9.199(2), c = 10.932(1) A, Z = 4, and space group Pbcn.[MgL( H,0),]-4H20 ( 7 ) is monoclinic, with a = 29.1 7(1), b = 6.758(2), c = 17.347(8) A, p = 108.99(3)", Z = 8, and space group C2/c. (5), (6), and (7) were refined to residuals 0.044,0.033, and 0.071 for 3 034, 1 024, and 1 448 observed reflections respectively. Complex (5) has a polymeric structure comprising four independent and different sodium centres, one five-, two six-, and one seven-co-ordinate. Each planar L2-ligand is bonded to a sodium ion via its four 'inner' oxygen donor atoms, with two waters in the axial sites of a pentagonal bipyramid. However, in one case the fifth pentagonal planar site is occupied by a bridging water molecule while for the other there is no water in this site. One of the axial waters in each also forms bridges to the third and fourth sodiums, together with a carboxyl oxygen of each acid ligand. The co-ordination about these two sodium ions is completed by the remaining water ligands, including two which bridge the centres and another which forms a polymer link. Stereochemistry is distorted octahedral in one, distorted square pyramidal in the other, with a Na-0 range and mean of 2.304(2)-2.693(4) A and 2.420(2) A respectively. In contrast, complex ( 6) is discretely ionic with the ammonium ion located outside the ionophore cavity of L2-but hydrogen bonded to the carboxylate oxygens (2.89-3.1 3 A). Complex ( 7) is isostructural and isomorphous with the zinc(ii) and cobalt(ti) complexes of L2-. It is monomeric with pentagonal-bipyramidal stereochemistry about Mg, comprising unequal bonds to the four 'inner' oxygens of the ligand [Mg-O(ether) (mean) 2.41 7(5) A, Mg-O(carboxy1ate) (mean) 2.1 26(5) A] and three to water molecules [two axial, 2.048(6) A (mean); one equatorial, 2.052(5) A].

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The structures of the polymeric sodium compound (I) (space group P1, Z=2), the ionic ammonium salt (II) (Pbcn, Z=4) containing H bridges between NH4+ and the carboxylate O atoms, and the Mg complex (III)·4 H2O (C2/c, Z = 8) are determined by X‐ray analysis.