Bacteria must rapidly respond to both intracellular and environmental changes to survive. One critical mechanism to rapidly detect and adapt to changes in environmental conditions is control of gene expression at the level of protein synthesis. At each of the three major steps of translation—initiation, elongation, and termination—cells use stimuli to tune translation rate and cellular protein concentrations. For example, changes in nutrient concentrations in the cell can lead to translational responses involving mechanisms such as dynamic folding of riboswitches during translation initiation or the synthesis of alarmones, which drastically alter cell physiology. Moreover, the cell can fine-tune the levels of specific protein products using programmed ribosome pausing or inducing frameshifting. Recent studies have improved understanding and revealed greater complexity regarding longstanding paradigms describing key regulatory steps of translation such as start-site selection and the coupling of transcription and translation. In this review, we describe how bacteria regulate their gene expression at the three translational steps and discuss how translation is used to detect and respond to changes in the cellular environment. Finally, we appraise the costs and benefits of regulation at the translational level in bacteria.

Elongation factor P (EF-P) is a ubiquitous translation factor that facilitates translation of polyproline motifs. In order to perform this function, EF-P generally requires posttranslational modification (PTM) on a conserved residue. Although the position of the modification is highly conserved, the structure can vary widely between organisms. In Bacillus subtilis, EF-P is modified at Lys32 with a 5-aminopentanol moiety. Here, we use a forward genetic screen to identify genes involved in 5-aminopentanolylation. Tandem mass spectrometry analysis of the PTM mutant strains indicated that ynbB, gsaB, and ymfI are required for modification and that yaaO, yfkA, and ywlG influence the level of modification. Structural analyses also showed that EF-P can retain unique intermediate modifications, suggesting that 5-aminopentanol is likely directly assembled on EF-P through a novel modification pathway. Phenotypic characterization of these PTM mutants showed that each mutant does not strictly phenocopy the efp mutant, as has previously been observed in other organisms. Rather, each mutant displays phenotypic characteristics consistent with those of either the efp mutant or wild-type B. subtilis depending on the growth condition. In vivo polyproline reporter data indicate that the observed phenotypic differences result from variation in both the severity of polyproline translation defects and altered EF-P context dependence in each mutant. Together, these findings establish a new EF-P PTM pathway and also highlight a unique relationship between EF-P modification and polyproline context dependence.

Translation elongation factor P (EF-P) in Bacillus subtilis is required for a form of surface migration called swarming motility. Furthermore, B. subtilis EF-P is post-translationally modified with a 5-aminopentanol group but the pathway necessary for the synthesis and ligation of the modification is unknown. Here we determine that the protein YmfI catalyzes the reduction of EF-P-5 aminopentanone to EF-P-5 aminopentanol. In the absence of YmfI, accumulation of 5-aminopentanonylated EF-P is inhibitory to swarming motility. Suppressor mutations that enhanced swarming in the absence of YmfI were found at two positions on EF-P, including one that changed the conserved modification site (Lys 32) and abolished post-translational modification. Thus, while modification of EF-P is thought to be essential for EF-P activity, here we show that in some cases it can dispensable. YmfI is the first protein identified in the pathway leading to EF-P modification in B. subtilis, and B. subtilis encodes the first EF-P ortholog that retains function in the absence of modification.

Elongation factor P (EF-P) is a universally conserved translation factor that alleviates ribosome pausing at polyproline (PPX) motifs by facilitating peptide bond formation. In the absence of EF-P, PPX peptide bond formation can limit translation rate, leading to pleotropic phenotypes including slowed growth, increased antibiotic sensitivity, and loss of virulence. In this study, we observe that many of these phenotypes are dependent on growth rate. Limiting growth rate suppresses a variety of detrimental phenotypes associated with ribosome pausing at PPX motifs in the absence of EF-P. Polysome levels are also similar to wild-type under slow growth conditions, consistent with global changes in ribosome queuing in cells without EF-P when growth rate is decreased. Inversely, under high protein synthesis demands, we observe that Escherichia coli lacking EF-P have reduced fitness. Our data demonstrate that EF-P-mediated relief of ribosome queuing is required to maintain proteome homeostasis under conditions of high translational demands.

Many antibiotics available in the clinic today directly inhibit bacterial translation. Despite the past success of such drugs, their efficacy is diminishing with the spread of antibiotic resistance. Through the use of ribosomal modifications, ribosomal protection proteins, translation elongation factors and mistranslation, many pathogens are able to establish resistance to common therapeutics. However, current efforts in drug discovery are focused on overcoming these obstacles through the modification or discovery of new treatment options. Here, we provide an overview for common mechanisms of resistance to translation-targeting drugs and summarize several important breakthroughs in recent drug development.