Ribosomes not only decode mRNAs and stitch amino acids together, they also control early protein folding events. In this issue of Biotechnology Journal, Contreras-Martin et al.  report that the ribosomal tunnel exit protein L29 exerts a pronounced influence on the production of T7-transcribed gene products.
Over fifty years ago , ribosomes were recognized as remarkably robust, speedy, and accurate molecular machines for decoding mRNAs and catalyzing peptide bond formation between incoming amino acids attached to the A (aminoacyl)-site tRNA and the growing peptide chain tethered to the P (peptidyl) site tRNA. In the past decade, it has also become apparent that ribosomes play a key role in early protein folding events by controlling co-translational folding through translational pausing, promoting intra-ribosomal acquisition of α-helical structure, and serving as a scaffold onto which molecular chaperones and modifying enzymes transiently latch to better interact with nascent chains [3–5].
Prokaryotic ribosomes synthesize proteins at an average (but non-uniform) rate of 15–20 amino acids per second and release growing chains into a tube-like structure – the ribosomal tunnel – that is approximately 80 to 100 Å in length and varies in width between 10 and 20 Å . This is sufficient time and space to allow for the formation of an α-helix at discrete “folding zones” of the tunnel, depending on the propensity of a particular stretch of amino acids to adopt helical structure . Nascent chains emerge from the tunnel into an enlarged vestibule consisting of rRNA and a protein ring made of four evolutionarilied conserved ribosomal proteins (L22, L23, L24 and L29) and other kingdom-specific polypeptides (Fig. 1). Among these, L23 is unique because – at least in vitro  – it serves as a docking site for both trigger factor (TF; a molecular chaperone that interacts with partially folded domains in certain nascent proteins to favor their proper folding in the cytoplasm) and signal recognition particle (SRP; a ribonucleoprotein that captures the hydrophobic signal anchor sequence of membrane proteins and targets ribosome nascent chain complexes to the inner membrane for their co-translational insertion). The neighboring ribosomal protein L29 has also been implicated in the attachment of TF and SRP to the ribosome [9, 10], but little is known about its precise function.
In this issue, DeLisa and coworkers  show that although Escherichia coli BL21(DE3) lacking the L29 protein exhibits normal growth and host protein biogenesis patterns, it fails to accumulate a variety of plasmid-encoded autologous, heterologous and fusion proteins placed under transcriptional control of the T7 promoter. Rather than finding that misfolding, degradation, inefficient chaperoning, or alterations in gene copy number are responsible for the phenotype, the authors track the decrease in recombinant protein expression to a 5 to 10-fold drop in transcript levels. They also identify gain of function L29 variants that boost the expression of green fluorescent protein in L29-deficient cells by 2.5–3.5 fold relative to the native protein. While the mechanisms at play remain for now unclear, it will be interesting to see if these effects are generic or specific to genes transcribed by the highly processive T7 RNA polymerase and to examine the very intriguing hypothesis that L29 directly or indirectly influences transcript binding and/or stability.