Regulation of protein synthesis during quiescence in bacteria

Project Summary Protein synthesis is subject to elaborate transcriptional and post-transcriptional regulation in growing bacterial cells. Such mechanisms ensure that protein synthesis is efficiently coupled to the needs of a rapidly dividing cell when nutrients are not limiting. However, most microbial life exists in a non- proliferating state of quiescence that enables survival during nutrient limitation and in stressful environments. Thus, the needs of quiescent cells are rather different from growing cells as they must minimize energy consumption so as to maximize available resources over a potentially extended period. Protein synthesis is the most energy intensive metabolic process in a cell, accounting for as much as ~70% of total energy consumption in bacteria. Consistently, many bacteria such as Bacillus subtilis are known to substantially reduce protein synthesis when they exit exponential growth. However, quiescent cells need to effectively exploit the emergence of favorable conditions and undergo resuscitation, so this attenuation needs to be rapidly reversible. In addition, as the ribosome is itself the most energetically costly macromolecular machine to synthesize, it must be protected from any degradative processes. And, as with translational attenuation, this protection must be compatible with efficient re-initiation of protein synthesis when conditions improve. Thus, both the inhibitory and protective mechanisms need to be quickly reversible. How the cell balances these two goals is the subject of this research proposal. First, we examine how ribosomes are protected from degradation under metabolic conditions where de novo ribosome biosynthesis is limited. Second, we investigate a reversible mechanism of translation inactivation with particular focus on the role of the nucleotide (p)ppGpp.