Nosiheptide

TipA is a transcriptional regulator found in diverse bacteria. It constitutes a minimal autoregulated multidrug resistance system against numerous thiopeptide antibiotics. Here we report the structures of its drug-binding domain TipAS in complexes with promothiocin A and nosiheptide, and a model of the thiostrepton complex. Drug binding induces a large transition from a partially unfolded to a globin-like structure. The structures rationalize the mechanism of promiscuous, yet specific, drug recognition: (i) a four-ring motif present in all known TipA-inducing antibiotics is recognized specifically by conserved TipAS amino acids; and (ii) the variable part of the antibiotic is accommodated within a flexible cleft that rigidifies upon drug binding. Remarkably, the identified four-ring motif is also the major interacting part of the antibiotic with the ribosome. Hence the TipA multidrug resistance mechanism is directed against the same chemical motif that inhibits protein synthesis.

Nosiheptide is a member of the thiopeptide family of antibiotics which demonstrates potent activities against various bacterial pathogens. The formation of its C-terminal amide is catalysed by NosA in an unusual strategy for maturating certain thiopeptides by processing precursor peptides featuring a serine extension. Here, a recombinant C-terminally truncated selenomethionine-derivatized NosA1-111 variant from Streptomyces actuosus consisting of residues 1-111, named SeMet NosA1-111, was crystallized using the sitting-drop vapour-diffusion method. Diffraction data were collected to 2.40 Å resolution using synchrotron radiation. The crystals belonged to the primitive cubic space group P4132, with unit-cell parameters a = b = c = 143.3 Å. Assuming the presence of three molecules in the asymmetric unit, the calculated Matthews coefficient was 3.94 Å(3) Da(-1) and the corresponding solvent content was 40.3%.

Nosiheptide is a parent compound of thiopeptide family that exhibit potent activities against various bacterial pathogens. Its C-terminal amide formation is catalyzed by NosA, which is an unusual strategy for maturating certain thiopeptides by processing their precursor peptides featuring a serine extension. We here report the crystal structure of truncated NosA1-111 variant, revealing three key elements, including basic lysine 49 (K49), acidic glutamic acid 101 (E101) and flexible C-terminal loop NosA112-151, are crucial to the catalytic terminal amide formation in nosiheptide biosynthesis. The side-chain of residue K49 and the C-terminal loop fasten the substrate through hydrogen bonds and hydrophobic interactions. The side-chain of residue E101 enhances nucleophilic attack of H2O to the methyl imine intermediate, leading to Cα-N bond cleavage and nosiheptide maturation. The sequence alignment of NosA and its homologs NocA, PbtH, TpdK and BerI, and the enzymatic assay suggest that the mechanistic studies on NosA present an intriguing paradigm about how NosA family members function during thiopeptide biosynthesis.

A determination method of nosiheptide in formula feeds by HPLC-FL was developed and validated, including an inter-laboratory study. Formula feeds were extracted with acetone after adding acetic acid. Liquid chromatographic separation was performed using a ZORBAX Eclipse XDB-C18 column, with acetonitrile and water containing acetic acid as the mobile phase. Detection of NH was carried out with a fluorescence detector. Recovery tests and an inter-laboratory study were conducted using chicken and swine formula feeds fortified with nosiheptide at 0.5-27 mg-potency/kg. Mean recoveries in recovery tests ranged from 91.4 to 103%, and the repeatability in terms of relative standard deviation was within 7.8%. Mean recoveries of the inter-laboratory study ranged from 98.4 to 108%, the repeatability and reproducibility in terms of the relative standard deviations were within 8.1% and 13% respectively, and the HorRat values ranged from 0.21 to 0.75.