Structural basis of HutP-mediated anti-termination and roles of the Mg2+ ion and L-histidine ligand

HutP regulates the expression of the hut structural genes of Bacillus subtilis by an anti-termination mechanism and requires two components, Mg2+ ions and L-histidine. HutP recognizes three UAG triplet units, separated by four non-conserved nucleotides on the terminator region. Here we report the 1.60-Angstrom resolution crystal structure of the quaternary complex ( HutP - L- histidine - Mg2+ 21-base single-stranded RNA). In the complex, the RNA adopts a novel triangular fold on the hexameric surface of HutP, without any base-pairing, and binds to the protein mostly by specific protein - base interactions. The structure explains how the HutP and RNA interactions are regulated critically by the L- histidine and Mg2+ ion through the structural rearrangement. To gain insights into these structural rearrangements, we solved two additional crystal structures ( uncomplexed HutP and HutP - L- histidine - Mg2+) that revealed the intermediate structures of HutP ( before forming an active structure) and the importance of the Mg2+ ion interactions in the complexes.