Multifaceted Stoichiometry Control of Bacterial Operons Revealed by Deep Proteome Quantification

Decoding stoichiometric protein synthesis in E. coli through translation rate parameters

E. coli is one of the most widely used organisms for understanding the principles of cellular and molecular genetics. However, we are yet to understand the origin of several experimental observations related to the regulation of gene expression in E. coli. One of the prominent examples in this context is the proportional synthesis in multiprotein complexes where all of their obligate subunits are produced in proportion to their stoichiometry. In this work, by combining the next-generation sequencing data with the stochastic simulations of protein synthesis, we explain the origin of proportional protein synthesis in multicomponent complexes. We find that the estimated initiation rates for the translation of all subunits in those complexes are proportional to their stoichiometry. This constraint on protein synthesis kinetics enforces proportional protein synthesis without requiring any feedback mechanism. We also find that the translation initiation rates in E. coli are influenced by the coding sequence length and the enrichment of A and C nucleotides near the start codon. Thus, this study rationalizes the role of conserved and nonrandom features of genes in regulating the translation kinetics and unravels a key principle of the regulation of protein synthesis.

The novel type II toxin-antitoxin PacTA modulates Pseudomonas aeruginosa iron homeostasis by obstructing the DNA-binding activity of Fur

Type II toxin–antitoxin (TA) systems are widely distributed in bacterial and archaeal genomes and are involved in diverse critical cellular functions such as defense against phages, biofilm formation, persistence, and virulence. GCN5-related N-acetyltransferase (GNAT) toxin, with an acetyltransferase activity-dependent mechanism of translation inhibition, represents a relatively new and expanding family of type II TA toxins. We here describe a group of GNAT-Xre TA modules widely distributed among Pseudomonas species. We investigated PacTA (one of its members encoded by PA3270/PA3269) from Pseudomonas aeruginosa and demonstrated that the PacT toxin positively regulates iron acquisition in P. aeruginosa. Notably, other than arresting translation through acetylating aminoacyl-tRNAs, PacT can directly bind to Fur, a key ferric uptake regulator, to attenuate its DNA-binding affinity and thus permit the expression of downstream iron-acquisition-related genes. We further showed that the expression of the pacTA locus is upregulated in response to iron starvation and the absence of PacT causes biofilm formation defect, thereby attenuating pathogenesis. Overall, these findings reveal a novel regulatory mechanism of GNAT toxin that controls iron-uptake-related genes and contributes to bacterial virulence.

Multimodal cotranslational interactions direct assembly of the human multi-tRNA synthetase complex

Amino acid ligation to cognate transfer RNAs (tRNAs) is catalyzed by aminoacyl-tRNA synthetases (aaRSs)-essential interpreters of the genetic code during translation. Mammalian cells harbor 20 cytoplasmic aaRSs, out of which 9 (in 8 proteins), with 3 non-aaRS proteins, AIMPs 1 to 3, form the ∼1.25-MDa multi-tRNA synthetase complex (MSC). The function of MSC remains uncertain, as does its mechanism of assembly. Constituents of multiprotein complexes encounter obstacles during assembly, including inappropriate interactions, topological constraints, premature degradation of unassembled subunits, and suboptimal stoichiometry. To facilitate orderly and efficient complex formation, some complexes are assembled cotranslationally by a mechanism in which a fully formed, mature protein binds a nascent partner as it emerges from the translating ribosome. Here, we show out of the 121 possible interaction events between the 11 MSC constituents, 15 are cotranslational. AIMPs are involved in the majority of these cotranslational interactions, suggesting they are not only critical for MSC structure but also for assembly. Unexpectedly, several cotranslational events involve more than the usual dyad of interacting proteins. We show two modes of cotranslational interaction, namely a "multisite" mechanism in which two or more mature proteins bind the same nascent peptide at distinct sites and a second "piggy-back" mechanism in which a mature protein carries a second fully formed protein and binds to a single site on an emerging peptide. Multimodal mechanisms of cotranslational interaction offer a diversity of pathways for ordered, piecewise assembly of small subcomplexes into larger heteromultimeric complexes such as the mammalian MSC.

Bacterial chemotaxis to saccharides is governed by a trade-off between sensing and uptake

To swim up gradients of nutrients, E. coli senses nutrient concentrations within its periplasm. For small nutrient molecules, periplasmic concentrations typically match extracellular concentrations. However, this is not necessarily the case for saccharides, such as maltose, which are transported into the periplasm via a specific porin. Previous observations have shown that, under various conditions, E. coli limits maltoporin abundance so that, for extracellular micromolar concentrations of maltose, there are predicted to be only nanomolar concentrations of free maltose in the periplasm. Thus, in the micromolar regime, the total uptake of maltose from the external environment into the cytoplasm is limited not by the abundance of cytoplasmic transport proteins but by the abundance of maltoporins. Here we present results from experiments and modeling suggesting that this porin-limited transport enables E. coli to sense micromolar gradients of maltose despite having a high-affinity ABC transport system that is saturated at these micromolar levels. We used microfluidic assays to study chemotaxis of E. coli in various gradients of maltose and methyl-aspartate and leveraged our experimental observations to develop a mechanistic transport-and-sensing chemotaxis model. Incorporating this model into agent-based simulations, we discover a trade-off between uptake and sensing: although high-affinity transport enables higher uptake rates at low nutrient concentrations, it severely limits the range of dynamic sensing. We thus propose that E. coli may limit periplasmic uptake to increase its chemotactic sensitivity, enabling it to use maltose as an environmental cue.

The Classical, Yet Controversial, First Enzyme of Lipid Synthesis: Escherichia coli Acetyl-CoA Carboxylase

Escherichia coli acetyl-CoA carboxylase (ACC), the enzyme responsible for synthesis of malonyl-CoA, the building block of fatty acid synthesis, is the paradigm bacterial ACC. Many reports on the structures and stoichiometry of the four subunits comprising the active enzyme as well as on regulation of ACC activity and expression have appeared in the almost 20 years since this subject was last reviewed. This review seeks to update and expand on these reports.

A comprehensive spectral assay library to quantify the Escherichia coli proteome by DIA/SWATH-MS

Data-Independent Acquisition (DIA) is a method to improve consistent identification and precise quantitation of peptides and proteins by mass spectrometry (MS). The targeted data analysis strategy in DIA relies on spectral assay libraries that are generally derived from a priori measurements of peptides for each species. Although Escherichia coli (E. coli) is among the best studied model organisms, so far there is no spectral assay library for the bacterium publicly available. Here, we generated a spectral assay library for 4,014 of the 4,389 annotated E. coli proteins using one- and two-dimensional fractionated samples, and ion mobility separation enabling deep proteome coverage. We demonstrate the utility of this high-quality library with robustness in quantitation of the E. coli proteome and with rapid-chromatography to enhance throughput by targeted DIA-MS. The spectral assay library supports the detection and quantification of 91.5% of all E. coli proteins at high-confidence with 56,182 proteotypic peptides, making it a valuable resource for the scientific community. Data and spectral libraries are available via ProteomeXchange (PXD020761, PXD020785) and SWATHAtlas (SAL00222-28).