Crystal Structure of Activated HutP: An RNA Binding Protein that Regulates Transcription of the hut Operon in Bacillus subtilis

HutP is an L-histidine-activated RNA binding protein that regulates the expression of the histidine utilization ( hut) operon in Bacillus subtilis by binding to cis-acting regulatory sequences on the hut mRNA. The crystal structure of HutP complexed with an L-histidine analog showed a novel fold; there are four antiparallel β strands in the central region of each monomer, with two α helices each on the front and back. Two HutP monomers form a dimer, and three dimers are arranged in crystallographic 3-fold symmetry to form a hexamer. A histidine analog was located in between the two monomers of HutP, with the imidazole group of L-histidine hydrogen bonded to Glu81. An activation mechanism is proposed based on the identification of key residues of HutP. The HutP binding region in hut mRNA was defined: it consists of three UAG trinucleotide motifs separated by four spacer nucleotides. Residues of HutP potentially important for RNA binding were identified.