Translation factors that keep the ribosome Pro-active

SUMMARYRibosomes produce the staggering array of proteins that perform the structural and enzymatic feats of the cell. Therefore, most of the cell's energy goes toward producing ribosomes and the work performed by them. The work of the ribosome is relatively simple-decode mRNA codons and catalyze the formation of peptide bonds. By marching iteratively along the length of an open reading frame, a complete peptide is produced. Ribosomes catalyze the formation of peptide bonds at a rate of approximately 15 amino acids per second. However, bonds between amino acids do not form with equal efficiency, and ribosomes can become stalled at difficult-to-translate sequences. Proline is unique among the amino acids in that its side chain is covalently bonded to the peptide backbone to form a rigid ring. The rigidity of proline, and especially tracts of proline, makes it a difficult substrate for peptide bond formation, but it is also an essential motif in many protein structures. Elongation factor P (EF-P) is the star player for facilitating translation of polyproline tracts. However, recently identified factors play an important supporting role, and loss of these factors incurs a severe fitness defect in the absence of EF-P. These factors include an EF-P paralog, EfpL, as well as the ABCF ATPase YfmR/Uup and YebC2. The abundance and partial redundancy of factors that prevent ribosome stalling at polyprolines highlights the structural importance of polyproline tracts and the need to facilitate their translation. Here, we review recently identified translation factors that prevent ribosome stalling at polyprolines in bacteria.