Stress response as implemented by hibernating ribosomes: a structural overview

Protein synthesis is one of the most energy demanding cellular processes. The ability to regulate protein synthesis is essential for cells under normal as well as stress conditions, such as nutrient deficiencies. One mechanism for protein synthesis suppression is the dimerization of ribosomes into hibernation complexes. In most cells, this process is promoted by the hibernating promoting factor (HPF) and in a small group of Gram-negative bacteria (γ-proteobacteria), the dimer formation is induced by a shorter version of HPF (HPFshort ) and by an additional protein, the ribosome modulation factor. In most bacteria, the product of this process is the 100S ribosome complex. Recent advances in cryogenic electron microscopy methods resulted in an abundance of detailed structures of near atomic resolutions 100S complexes that allow for a better understanding of the dimerization process and the way it inhibits protein synthesis. As ribosomal dimerization is vital for cell survival, this process is an attractive target for the development of novel antimicrobial substances that might inhibit or stabilize the complex formation. As different dimerization processes exist among bacteria, including pathogens, this process may provide the basis for species-specific design of antimicrobial agents. Here, we review in detail the various dimerization mechanisms and discuss how they affect the overall dimer structures of the bacterial ribosomes.