Fig. 3. Analysis of the σ537-binding surface by NMR ‘footprinting’. (A) AsiA homodimer (black) and the AsiA–σ537 complex (red) were analyzed by ¹⁵N–¹H HSQC spectroscopy with ¹⁵N-labeled AsiA and unlabeled σ537. The positions of several residues that move substantially upon addition of σ537 are shown in black for the AsiA homodimer and in red for the AsiA–σ537 complex. (B) A histogram of the composite chemical shift changes (see Materials and methods) defines a broad ‘footprint’ (>0.8 p.p.m. change) of σ537 on AsiA that encompasses many of residues 3–20 and residues 33, 36, 39–40 and 44. These residues reside along the homodimer interface of AsiA.