The HutP protein family regulates the expression of Bacillus 'hut' structural genes by an anti-termination complex, which recognises three UAG triplet units, separated by four non-conserved nucleotides on the RNA terminator region. L-histidine and Mg2+ ions are also required. These proteins exhibit the structural elements of alpha/beta proteins, arranged in the order: alpha-alpha-beta-alpha-alpha-beta-beta-beta in the primary structure, and the four antiparallel beta-strands form a beta-sheet in the order beta1-beta2-beta3-beta4, with two alpha-helices each on the front (alpha1 and alpha2) and at the back (alpha3 and alpha4) of the beta-sheet [1].

1: Structural basis of HutP-mediated anti-termination and roles of the Mg2+ ion and L-histidine ligand. Kumarevel T, Mizuno H, Kumar PK; Nature. 2005;434:183-191. PMID:15758992