Structures of the activator of K. pneumonia biofilm formation， MrkH， indicates PilZ domains involved in c-di-GMP and DNA binding.

The pathogenesis of Klebsiella pneumonia is linked to the bacteria's ability to form biofilms. Mannose-resistant Klebsiella-like (Mrk) hemagglutinins are critical for K pneumonia biofilm development， and the expression of the genes encoding these proteins is activated by a 3'，5'-cyclic diguanylic acid (c-di-GMP)-regulated transcription factor， MrkH. To gain insight into MrkH function， we performed structural and biochemical analyses. Data revealed MrkH to be a monomer with a two-domain architecture consisting of a PilZ C-domain connected to an N domain that unexpectedly also harbors a PilZ-like fold. Comparison of apo- and c-di-GMP-bound MrkH structures reveals a large 138° interdomain rotation that is induced by binding an intercalated c-di-GMP dimer. c-di-GMP interacts with PilZ C-domain motifs 1 and 2 (RxxxR and D/NxSxxG) and a newly described c-di-GMP-binding motif in the MrkH N domain. Strikingly， these c-di-GMP-binding motifs also stabilize an open state conformation in apo MrkH via contacts from the PilZ motif 1 to residues in the C-domain motif 2 and the c-di-GMP-binding N-domain motif. Use of the same regions in apo structure stabilization and c-di-GMP interaction allows distinction between the states. Indeed， domain reorientation by c-di-GMP complexation with MrkH， which leads to a highly compacted structure， suggests a mechanism by which the protein is activated to bind DNA. To our knowledge， MrkH represents the first instance of specific DNA binding mediated by PilZ domains. The MrkH structures also pave the way for the rational design of inhibitors that target K pneumonia biofilm formation.