The 70S Escherichia coli ribosome dimerizes to form an inactive 100S ribosome during stationary phase, which is called "ribosome hibernation". The hibernation promoting factor HPF plays a crucial role in 100S ribosome formation. However, YfiA, a known paralog of HPF inhibits 100S formation, although it shares high sequence similarity. Here, we report the first solution structure of HPF as determined by multidimensional NMR. HPF adopts beta alpha beta beta beta alpha-fold and the overall structure is similar to YfiA as expected. However, detailed structure comparison based on the determined structure in this study revealed that there are remarkable differences around the C-terminal portion of helix alpha 2, which is not predicted by homology modeling. Furthermore, some acidic residues conserved only in HPF are located at the rim of the common basic patch. (C) 2009 Elsevier Inc. All rights reserved.