Due to its long half-life compared to messenger RNA, bacterial transfer RNA is known as stable RNA. Here, we show that tRNAs become highly unstable as part of Escherichia coli's response to amino acid starvation. Degradation of the majority of cellular tRNA occurs within twenty minutes of the onset of starvation for each of several amino acids. Both the non-cognate and cognate tRNA for the amino acid that the cell is starving for are degraded, and both charged and uncharged tRNA species are affected. The alarmone ppGpp orchestrates the stringent response to amino acid starvation. However, tRNA degradation occurs in a ppGpp-independent manner, as it occurs with similar kinetics in a relaxed mutant. Further, we also observe rapid tRNA degradation in response to rifampicin treatment, which does not induce the stringent response. We propose a unifying model for these observations, in which the surplus tRNA is degraded whenever the demand for protein synthesis is reduced. Thus, the tRNA pool is a highly regulated, dynamic entity. We propose that degradation of surplus tRNA could function to reduce mistranslation in the stressed cell, because it would reduce competition between cognate and near-cognate charged tRNAs at the ribosomal A-site.

Transfer RNA (tRNA) is an essential part of the translational machinery in any organism. tRNAs bind and transfer amino acids to the translating ribosome. The relative levels of different tRNAs, and the ratio of aminoacylated tRNA to total tRNA, known as the charging level, are important factors in determining the accuracy and speed of translation. Therefore, the abundance and charging levels of tRNAs are important variables to measure when studying protein synthesis, for example under various stress conditions. Here, we describe a method for harvesting tRNA and directly measuring both the relative abundance and the absolute charging level of specific tRNA species in Escherichia coli. The tRNA is harvested in such a way that the labile bond between the tRNA and its amino acid is preserved. The RNA is then subjected to gel electrophoresis and Northern blotting, which results in separation of the charged and uncharged tRNAs. The levels of specific tRNAs in different samples can be compared due to the addition of spike-in cells for normalization. Prior to RNA purification, we add 5% of E. coli cells that overproduce the rare tRNA to each sample. The amount of the tRNA species of interest in a sample is then normalized to the amount of tRNA in the same sample. Addition of spike-in cells prior to RNA purification has the advantage over addition of purified spike-in RNAs that it also accounts for any differences in cell lysis efficiency between samples.

RNA-binding proteins (RPBs) are deeply involved in fundamental cellular processes in bacteria and are vital for their survival. Despite this, few studies have so far been dedicated to direct and global identification of bacterial RBPs. We have adapted the RNA interactome capture (RIC) technique, originally developed for eukaryotic systems, to globally identify RBPs in bacteria. RIC takes advantage of the base pairing potential of poly(A) tails to pull-down RNA–protein complexes. Overexpressing poly(A) polymerase I in Escherichia coli drastically increased transcriptome-wide RNA polyadenylation, enabling pull-down of crosslinked RNA–protein complexes using immobilized oligo(dT) as bait. With this approach, we identified 169 putative RBPs, roughly half of which are already annotated as RNA-binding. We experimentally verified the RNA-binding ability of a number of uncharacterized RBPs, including YhgF, which is exceptionally well conserved not only in bacteria, but also in archaea and eukaryotes. We identified YhgF RNA targets in vivo using CLIP-seq, verified specific binding in vitro, and reveal a putative role for YhgF in regulation of gene expression. Our findings present a simple and robust strategy for RBP identification in bacteria, provide a resource of new bacterial RBPs, and lay the foundation for further studies of the highly conserved RBP YhgF.

Post‐transcriptional regulatory networks in Gammaproteobacteria are to a large extent built around the two globally acting RNA‐binding proteins (RBPs) CsrA and Hfq. Both RBPs interact with small regulatory RNAs (sRNAs), but the functional outcomes of these interactions are generally distinct. Whereas Hfq both stabilizes sRNAs and promotes their base‐pairing to target mRNAs, the sRNAs bound by CsrA act as sequestering molecules that titrate the RBP away from its mRNA targets. In this issue of Molecular Microbiology, Lai et al. reveal that CsrA interacts with the Hfq‐associated and base‐pairing sRNA Spot 42. In this case, CsrA increases Spot 42 stability by masking a cleavage site for endoribonuclease RNase E, thereby promoting Spot 42‐dependent regulation of srlA mRNA. Interestingly, the effect of CsrA on srlA expression is two‐fold. In addition to affecting Spot 42‐dependent regulation, CsrA directly inhibits translation of SrlM, an activator of srlA transcription. Together, this study reveals a new function for CsrA and indicates more intricate connections between the CsrA and Hfq networks than previously anticipated. Several recent studies have identified additional RBPs that interact with sRNAs. With new RBP identification methods at hand, it will be intriguing to see how many more sRNA‐binding proteins will be uncovered.

Three out of the seven ribosomal RNA operons in Escherichia coli end in dual terminator structures. Between the two terminators of each operon is a short sequence that we report here to be an sRNA gene, transcribed as part of the ribosomal RNA primary transcript by read-through of the first terminator. The sRNA genes (rrA, rrB and rrF) from the three operons (rrnA, rrnB and rrnD) are more than 98% identical, and pull-down experiments show that their transcripts interact with Hfq and CsrA. Deletion of rrA, B, F, as well as overexpression of rrB, only modestly affect known CsrA-regulated phenotypes like biofilm formation, pgaA translation and glgC translation, and the role of the sRNAs in vivo may not yet be fully understood. Since RrA, B, F are short-lived and transcribed along with the ribosomal RNA components, their concentration reflect growth-rate regulation at the ribosomal RNA promoters and they could function to fine-tune other growth-phase-dependent processes in the cell. The primary and secondary structure of these small RNAs are conserved among species belonging to different genera of Enterobacteriales.

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