Overexpression of tnaC of Escherichia coli inhibits growth by depleting tRNA2Pro availability

A mini-hairpin shaped nascent peptide blocks translation termination by a distinct mechanism

Protein synthesis by ribosomes produces functional proteins but also serves diverse regulatory functions, which depend on the coding amino acid sequences. Certain nascent peptides interact with the ribosome exit tunnel to arrest translation and modulate themselves or the expression of downstream genes. However, a comprehensive understanding of the mechanisms of such ribosome stalling and its regulation remains elusive. In this study, we systematically screen for unidentified ribosome arrest peptides through phenotypic evaluation, proteomics, and mass spectrometry analyses, leading to the discovery of the arrest peptides PepNL and NanCL in E. coli. Our cryo-EM study on PepNL reveals a distinct arrest mechanism, in which the N-terminus of PepNL folds back towards the tunnel entrance to prevent the catalytic GGQ motif of the release factor from accessing the peptidyl transferase center, causing translation arrest at the UGA stop codon. Furthermore, unlike sensory arrest peptides that require an arrest inducer, PepNL uses tryptophan as an arrest inhibitor, where Trp-tRNATrp reads through the stop codon. Our findings illuminate the mechanism and regulatory framework of nascent peptide-induced translation arrest, paving the way for exploring regulatory nascent peptides.

Evaluating cellular roles and phenotypes associated with trehalose degradation genes in Saccharomyces cerevisiae

In the yeast Saccharomyces cerevisiae, 2 types of trehalase activities have been described. Neutral trehalases (Nth1 and Nth2) are considered to be the main proteins that catalyze intracellular trehalose mobilization. In addition to Nth1 and Nth2, studies have shown that acid trehalase Ath1 is required for extracellular trehalose degradation. Although both neutral and acid-type trehalases have been predominantly investigated in laboratory strains of S. cerevisiae, we sought to examine the phenotypic consequences of disrupting these genes in wild strains. In this study, we constructed mutants of the trehalose degradation pathway (NTH1, NTH2, and ATH1) in 5 diverse S. cerevisiae strains to examine whether published lab strain phenotypes are also exhibited by wild strains. For each mutant, we assessed a number of phenotypes for comparison to trehalose biosynthesis mutants, including trehalose production, glycogen production, cell size, acute thermotolerance, high-temperature growth, sporulation efficiency, and growth on a variety of carbon sources in rich and minimal medium. We found that all trehalase mutants including single deletion nth1Δ, nth2Δ, and ath1Δ, as well as double deletion nth1nth2Δ, accumulated higher intracellular trehalose levels compared to their isogenic wild-type cells. Also, nth1Δ and nth1Δnth2Δ mutants exhibited mild thermal sensitivity, suggesting a potential minor role for trehalose mobilization when cells recover from stress. In addition, we evaluated phenotypes more directly relevant to trehalose degradation, including both extracellular and intracellular trehalose utilization. We discovered that intracellular trehalose hydrolysis is critical for typical spore germination progression, highlighting a role for trehalose in cell cycle regulation, likely as a storage carbohydrate providing glycolytic fuel. Additionally, our work provides further evidence suggesting Ath1 is indispensable for extracellular trehalose utilization as a carbon source, even in the presence of AGT1.

An exploratory in silico comparison of open-source codon harmonization tools

BACKGROUND

Not changing the native constitution of genes prior to their expression by a heterologous host can affect the amount of proteins synthesized as well as their folding, hampering their activity and even cell viability. Over the past decades, several strategies have been developed to optimize the translation of heterologous genes by accommodating the difference in codon usage between species. While there have been a handful of studies assessing various codon optimization strategies, to the best of our knowledge, no research has been performed towards the evaluation and comparison of codon harmonization algorithms. To highlight their importance and encourage meaningful discussion, we compared different open-source codon harmonization tools pertaining to their in silico performance, and we investigated the influence of different gene-specific factors.

RESULTS

In total, 27 genes were harmonized with four tools toward two different heterologous hosts. The difference in %MinMax values between the harmonized and the original sequences was calculated (ΔMinMax), and statistical analysis of the obtained results was carried out. It became clear that not all tools perform similarly, and the choice of tool should depend on the intended application. Almost all biological factors under investigation (GC content, RNA secondary structures and choice of heterologous host) had a significant influence on the harmonization results and thus must be taken into account. These findings were substantiated using a validation dataset consisting of 8 strategically chosen genes.

CONCLUSIONS

Due to the size of the dataset, no complex models could be developed. However, this initial study showcases significant differences between the results of various codon harmonization tools. Although more elaborate investigation is needed, it is clear that biological factors such as GC content, RNA secondary structures and heterologous hosts must be taken into account when selecting the codon harmonization tool.

Anaerobic Fungal Mevalonate Pathway Genomic Biases Lead to Heterologous Toxicity Underpredicted by Codon Adaptation Indices

Anaerobic fungi are emerging biotechnology platforms with genomes rich in biosynthetic potential. Yet, the heterologous expression of their biosynthetic pathways has had limited success in model hosts like E. coli. We find one reason for this is that the genome composition of anaerobic fungi like P. indianae are extremely AT-biased with a particular preference for rare and semi-rare AT-rich tRNAs in E coli, which are not explicitly predicted by standard codon adaptation indices (CAI). Native P. indianae genes with these extreme biases create drastic growth defects in E. coli (up to 69% reduction in growth), which is not seen in genes from other organisms with similar CAIs. However, codon optimization rescues growth, allowing for gene evaluation. In this manner, we demonstrate that anaerobic fungal homologs such as PI.atoB are more active than S. cerevisiae homologs in a hybrid pathway, increasing the production of mevalonate up to 2.5 g/L (more than two-fold) and reducing waste carbon to acetate by ~90% under the conditions tested. This work demonstrates the bioproduction potential of anaerobic fungal enzyme homologs and how the analysis of codon utilization enables the study of otherwise difficult to express genes that have applications in biocatalysis and natural product discovery.

A Physiological Basis for Nonheritable Antibiotic Resistance

Antibiotics constitute one of the cornerstones of modern medicine. However, individuals may succumb to a bacterial infection if a pathogen survives exposure to antibiotics. The ability of bacteria to survive bactericidal antibiotics results from genetic changes in the preexisting bacterial genome, from the acquisition of genes from other organisms, and from nonheritable phenomena that give rise to antibiotic tolerance. Nonheritable antibiotic tolerance can be exhibited by a large fraction of the bacterial population or by a small subpopulation referred to as persisters. Nonheritable resistance to antibiotics has been ascribed to the activity of toxins that are part of toxin-antitoxin modules, to the universal energy currency ATP, and to the signaling molecule guanosine (penta) tetraphosphate. However, these molecules are dispensable for nonheritable resistance to antibiotics in many organisms. By contrast, nutrient limitation, treatment with bacteriostatic antibiotics, or expression of genes that slow bacterial growth invariably promote nonheritable resistance. We posit that antibiotic persistence results from conditions promoting feedback inhibition among core cellular processes, resulting phenotypically in a slowdown or halt in bacterial growth.

The regulatory TnaC nascent peptide preferentially inhibits release factor 2-mediated hydrolysis of peptidyl-tRNA

The tnaC regulatory gene from the tna operon of Escherichia coli controls the transcription of its own operon through an attenuation mechanism relying on the accumulation of arrested ribosomes during inhibition of its own translation termination. This free l-Trp-dependent mechanism of inhibition of translation termination remains unclear. Here, we analyzed the inhibitory effects of l-Trp on the function of two known E. coli translation termination factors, RF1 and RF2. Using a series of reporter genes, we found that the in vivo l-Trp sensitivity of tnaC gene expression is influenced by the identity of its stop codon, with the UGA stop codon producing higher expression efficiency of the tnaA-lacZ gene construct than the UAG stop codon. In vitro TnaC-peptidyl-tRNA accumulation and toe-printing assays confirmed that in the presence of l-Trp, the UGA stop codon generates higher accumulation of both TnaC-peptidyl-tRNA and arrested ribosomes than does the UAG stop codon. RF-mediated hydrolysis assays corroborated that l-Trp blocks RF2 function more than that of RF1. Mutational analyses disclosed that amino acids substitutions at the 246 and 256 residue positions surrounding the RF2-GGQ functional motif reduce l-Trp-dependent expression of the tnaC(UGA) tnaA-lacZ construct and the ability of l-Trp to inhibit RF2-mediated cleavage of the TnaC-peptidyl-tRNA. Altogether, our results indicate that l-Trp preferentially blocks RF2 activity during translation termination of the tnaC gene. This inhibition depends on the identities of amino acid residues surrounding the RF2-GGQ functional motif.

"CodonWizard" - An intuitive software tool with graphical user interface for customizable codon optimization in protein expression efforts

Optimization of coding sequences to maximize protein expression yield is often outsourced to external service providers during commercial gene synthesis and thus unfortunately remains a black box for many researchers. The presented software program "CodonWizard" offers scientists a powerful but easy-to-use tool for customizable codon optimization: The intuitive graphical user interface empowers even scientists inexperienced in the art to straightforward design, modify, test and save complex codon optimization strategies and to publicly share successful otimization strategies among the scientific community. "Codon Wizard" provides highly flexible features for sequence analysis and completely customizable modification/optimization of codon usage of any given input sequence data (DNA/RNA/peptide) using freely combinable algorithms, allowing for implementation of contemporary, well-established optimization strategies as well as novel, proprietary ones alike. Contrary to comparable tools, "Codon Wizard" thus finally opens up ways for an empirical approach to codon optimization and may also >be used completely offline to protect resulting intellectual property. As a benchmark, the reliability, intuitiveness and utility of the application could be demonstrated by increasing the yield of recombinant TEV-protease expressed in E. coli by several orders of magnitude after codon optimization using "CodonWizard" - Permanently available for download on the web at http://schwalbe.org.chemie.uni-frankfurt.de/node/3324.

Estimating the protein burden limit of yeast cells by measuring the expression limits of glycolytic proteins

The ultimate overexpression of a protein could cause growth defects, which are known as the protein burden. However, the expression limit at which the protein-burden effect is triggered is still unclear. To estimate this limit, we systematically measured the overexpression limits of glycolytic proteins in Saccharomyces cerevisiae. The limits of some glycolytic proteins were up to 15% of the total cellular protein. These limits were independent of the proteins’ catalytic activities, a finding that was supported by an in silico analysis. Some proteins had low expression limits that were explained by their localization and metabolic perturbations. The codon usage should be highly optimized to trigger the protein-burden effect, even under strong transcriptional induction. The S–S-bond-connected aggregation mediated by the cysteine residues of a protein might affect its expression limit. Theoretically, only non-harmful proteins could be expressed up to the protein-burden limit. Therefore, we established a framework to distinguish proteins that are harmful and non-harmful upon overexpression.

If a cell makes too much of a given protein, it can sometimes cause problems and impair the cell’s growth. Overproducing some proteins may deplete the cell’s limited resources, meaning it does not have enough to make other more essential proteins. This phenomenon is known as the protein burden effect. Theoretically, only harmless proteins can be overproduced up to a level where growth would be impaired in this way. Conversely, if an overproduced protein causes harm before it becomes a burden on resources, scientists must consider other mechanisms to explain the cell’s problems, namely that the protein itself is harmful.

Knowing the ultimate level of protein production that could cause the protein burden effect – the protein burden limit – would allow scientists to distinguish between harmful and non-harmful proteins. However, to date, this limit had not been defined for any cell.

Eguchi et al. have now tried to estimate the protein burden limit for budding yeast – one of the best-studied experimental organisms. The experiments first focused on enzymes involved in alcoholic fermentation because they were expected to be non-harmful. Some of these enzymes were overproduced to the level were the made up 15% of all the cell’s proteins before they started to cause growth defects. The same results were seen with versions of the enzymes that had been mutated to be less active, leading Eguchi et al. to conclude that this level is the protein burden limit.

In other experiments, harmful enzymes could only be overproduced to levels that were far less than this proposed protein burden limit. These enzymes caused problems for the yeast in several ways, including interfering with biochemical reactions and forming large aggregates in the cell. Lastly, Eguchi et al. looked at the yeast’s genetic code and saw that most of its genes seemed to have evolved to specifically limit the production of proteins to a level that would avoid the unwanted protein burden effect.

Together these findings establish a framework to clearly distinguish between harmful and non-harmful proteins. This framework will be useful to understand the different reasons why the overproduction of certain proteins, which is seen in neurodegenerative diseases and cancer cells, can cause problems for cells.

ATGme: Open-source web application for rare codon identification and custom DNA sequence optimization

Background

Codon usage plays a crucial role when recombinant proteins are expressed in different organisms. This is especially the case if the codon usage frequency of the organism of origin and the target host organism differ significantly, for example when a human gene is expressed in E. coli. Therefore, to enable or enhance efficient gene expression it is of great importance to identify rare codons in any given DNA sequence and subsequently mutate these to codons which are more frequently used in the expression host.

Results

We describe an open-source web-based application, ATGme, which can in a first step identify rare and highly rare codons from most organisms, and secondly gives the user the possibility to optimize the sequence.

Conclusions

This application provides a simple user-friendly interface utilizing three optimization strategies: 1. one-click optimization, 2. bulk optimization (by codon-type), 3. individualized custom (codon-by-codon) optimization. ATGme is an open-source application which is freely available at: http://atgme.org

Expression of codon optimized genes in microbial systems: current industrial applications and perspectives

The efficient production of functional proteins in heterologous hosts is one of the major bases of modern biotechnology. Unfortunately, many genes are difficult to express outside their original context. Due to their apparent “silent” nature, synonymous codon substitutions have long been thought to be trivial. In recent years, this dogma has been refuted by evidence that codon replacement can have a significant impact on gene expression levels and protein folding. In the past decade, considerable advances in the speed and cost of gene synthesis have facilitated the complete redesign of entire gene sequences, dramatically improving the likelihood of high protein expression. This technology significantly impacts the economic feasibility of microbial-based biotechnological processes by, for example, increasing the volumetric productivities of recombinant proteins or facilitating the redesign of novel biosynthetic routes for the production of metabolites. This review discusses the current applications of this technology, particularly those regarding the production of small molecules and industrially relevant recombinant enzymes. Suggestions for future research and potential uses are provided as well.

Arrest peptides: cis-acting modulators of translation

Each peptide bond of a protein is generated at the peptidyl transferase center (PTC) of the ribosome and then moves through the exit tunnel, which accommodates ever-changing segments of ≈ 40 amino acids of newly translated polypeptide. A class of proteins, called ribosome arrest peptides, contains specific sequences of amino acids (arrest sequences) that interact with distinct components of the PTC-exit tunnel region of the ribosome and arrest their own translation continuation, often in a manner regulated by environmental cues. Thus, the ribosome that has translated an arrest sequence is inactivated for peptidyl transfer, translocation, or termination. The stalled ribosome then changes the configuration or localization of mRNA, resulting in specific biological outputs, including regulation of the target gene expression and downstream events of mRNA/polypeptide maturation or localization. Living organisms thus seem to have integrated potentially harmful arrest sequences into elaborate regulatory mechanisms to express genetic information in productive directions.

Engineering genes for predictable protein expression

The DNA sequence used to encode a polypeptide can have dramatic effects on its expression. Lack of readily available tools has until recently inhibited meaningful experimental investigation of this phenomenon. Advances in synthetic biology and the application of modern engineering approaches now provide the tools for systematic analysis of the sequence variables affecting heterologous expression of recombinant proteins. We here discuss how these new tools are being applied and how they circumvent the constraints of previous approaches, highlighting some of the surprising and promising results emerging from the developing field of gene engineering.

Physiological responses to folate overproduction in Lactobacillus plantarum WCFS1

BACKGROUND

Using a functional genomics approach we addressed the impact of folate overproduction on metabolite formation and gene expression in Lactobacillus plantarum WCFS1. We focused specifically on the mechanism that reduces growth rates in folate-overproducing cells.

RESULTS

Metabolite formation and gene expression were determined in a folate-overproducing- and wild-type strain. Differential metabolomics analysis of intracellular metabolite pools indicated that the pool sizes of 18 metabolites differed significantly between these strains. The gene expression profile was determined for both strains in pH-regulated chemostat culture and batch culture. Apart from the expected overexpression of the 6 genes of the folate gene cluster, no other genes were found to be differentially expressed both in continuous and batch cultures. The discrepancy between the low transcriptome and metabolome response and the 25% growth rate reduction of the folate overproducing strain was further investigated. Folate production per se could be ruled out as a contributing factor, since in the absence of folate production the growth rate of the overproducer was also reduced by 25%. The higher metabolic costs for DNA and RNA biosynthesis in the folate overproducing strain were also ruled out. However, it was demonstrated that folate-specific mRNAs and proteins constitute 8% and 4% of the total mRNA and protein pool, respectively.

CONCLUSION

Folate overproduction leads to very little change in metabolite levels or overall transcript profile, while at the same time the growth rate is reduced drastically. This shows that Lactobacillus plantarum WCFS1 is unable to respond to this growth rate reduction, most likely because the growth-related transcripts and proteins are diluted by the enormous amount of gratuitous folate-related transcripts and proteins.

RNase II is important for A-site mRNA cleavage during ribosome pausing

In Escherichia coli, translational arrest can elicit cleavage of codons within the ribosomal A site. This A-site mRNA cleavage is independent of RelE, and has been proposed to be an endonucleolytic activity of the ribosome. Here, we show that the 3'-->5' exonuclease RNase II plays an important role in RelE-independent A-site cleavage. Instead of A-site cleavage, translational pausing in DeltaRNase II cells produces transcripts that are truncated +12 and +28 nucleotides downstream of the A-site codon. Deletions of the genes encoding polynucleotide phosphorylase (PNPase) and RNase R had little effect on A-site cleavage. However, PNPase overexpression restored A-site cleavage activity to DeltaRNase II cells. Purified RNase II and PNPase were both unable to directly catalyse A-site cleavage in vitro. Instead, these exonucleases degraded ribosome-bound mRNA to positions +18 and +24 nucleotides downstream of the ribosomal A site respectively. Finally, a stable structural barrier to exoribonuclease activity inhibited A-site cleavage when introduced immediately downstream of paused ribosomes. These results demonstrate that 3'-->5' exonuclease activity is an important prerequisite for efficient A-site cleavage. We propose that RNase II degrades mRNA to the downstream border of paused ribosomes, facilitating cleavage of the A-site codon by an unknown RNase.

Small membrane proteins found by comparative genomics and ribosome binding site models

The correct annotation of genes encoding the smallest proteins is one of the biggest challenges of genome annotation, and perhaps more importantly, few annotated short open reading frames have been confirmed to correspond to synthesized proteins. We used sequence conservation and ribosome binding site models to predict genes encoding small proteins, defined as having 16-50 amino acids, in the intergenic regions of the Escherichia coli genome. We tested expression of these predicted as well as previously annotated genes by integrating the sequential peptide affinity tag directly upstream of the stop codon on the chromosome and assaying for synthesis using immunoblot assays. This approach confirmed that 20 previously annotated and 18 newly discovered proteins of 16-50 amino acids are synthesized. We summarize the properties of these small proteins; remarkably more than half of the proteins are predicted to be single-transmembrane proteins, nine of which we show co-fractionate with cell membranes.

Peptidyl-prolyl-tRNA at the ribosomal P-site reacts poorly with puromycin

Despite remarkable recent progress in our chemical and structural understanding of the mechanisms of peptide bond formation by the ribosome, only very limited information is available about whether amino acid side chains affect the rate of peptide bond formation. Here, we generated a series of peptidyl-tRNAs that end with different tRNA-attached amino acids in the P-site of the Escherichia coli ribosome and compared their reactivity with puromycin, a rapidly A-site-accessing analog of aminoacyl-tRNAs. Among the 20 amino acids examined, proline was found to receive exceptionally slow peptidyl transfer to puromycin. These results raise a possibility that the peptidyl transferase activity of the ribosome may have some specificity with regard to the P-site amino acids.

RNA-based regulation of genes of tryptophan synthesis and degradation, in bacteria

We are now aware that RNA-based regulatory mechanisms are commonly used to control gene expression in many organisms. These mechanisms offer the opportunity to exploit relatively short, unique RNA sequences, in altering transcription, translation, and/or mRNA stability, in response to the presence of a small or large signal molecule. The ability of an RNA segment to fold and form alternative hairpin secondary structures -- each dedicated to a different regulatory function -- permits selection of specific sequences that can affect transcription and/or translation. In the present paper I will focus on our current understanding of the RNA-based regulatory mechanisms used by Escherichia coli and Bacillus subtilis in controlling expression of the tryptophan biosynthetic operon. The regulatory mechanisms they use for this purpose differ, suggesting that these organisms, or their ancestors, adopted different strategies during their evolution. I will also describe the RNA-based mechanism used by E. coli in regulating expression of its operon responsible for tryptophan degradation, the tryptophanase operon.

Ribosome recycling factor and release factor 3 action promotes TnaC-peptidyl-tRNA Dropoff and relieves ribosome stalling during tryptophan induction of tna operon expression in Escherichia coli

Upon tryptophan induction of tna operon expression in Escherichia coli, the leader peptidyl-tRNA, TnaC-tRNA(2)(Pro), resists cleavage, resulting in ribosome stalling at the tnaC stop codon. This stalled ribosome blocks Rho factor binding and action, preventing transcription termination in the tna operon's leader region. Plasmid-mediated overexpression of tnaC was previously shown to inhibit cell growth by reducing uncharged tRNA(2)(Pro) availability. Which factors relieve ribosome stalling, facilitate TnaC-tRNA(2)(Pro) cleavage, and relieve growth inhibition were addressed in the current study. In strains containing the chromosomal tna operon and lacking a tnaC plasmid, the overproduction of ribosome recycling factor (RRF) and release factor 3 (RF3) reduced tna operon expression. Their overproduction in vivo also increased the rate of cleavage of TnaC-tRNA(2)(Pro), relieving the growth inhibition associated with plasmid-mediated tnaC overexpression. The overproduction of elongation factor G or initiation factor 3 did not have comparable effects, and tmRNA was incapable of attacking TnaC-tRNA(2)(Pro) in stalled ribosome complexes. The stability of TnaC-tRNA(2)(Pro) was increased appreciably in strains deficient in RRF and RF3 or deficient in peptidyl-tRNA hydrolase. These findings reveal the existence of a natural mechanism whereby an amino acid, tryptophan, binds to ribosomes that have just completed the synthesis of TnaC-tRNA(2)(Pro). Bound tryptophan inhibits RF2-mediated cleavage of TnaC-tRNA(2)(Pro), resulting in the stalling of the ribosome translating tnaC mRNA. This stalling results in increased transcription of the structural genes of the tna operon. RRF and RF3 then bind to this stalled ribosome complex and slowly release TnaC-tRNA(2)(Pro). This release allows ribosome recycling and permits the cleavage of TnaC-tRNA(2)(Pro) by peptidyl-tRNA hydrolase.

Gene Designer: a synthetic biology tool for constructing artificial DNA segments

Background

Direct synthesis of genes is rapidly becoming the most efficient way to make functional genetic constructs and enables applications such as codon optimization, RNAi resistant genes and protein engineering. Here we introduce a software tool that drastically facilitates the design of synthetic genes.

Results

Gene Designer is a stand-alone software for fast and easy design of synthetic DNA segments. Users can easily add, edit and combine genetic elements such as promoters, open reading frames and tags through an intuitive drag-and-drop graphic interface and a hierarchical DNA/Protein object map. Using advanced optimization algorithms, open reading frames within the DNA construct can readily be codon optimized for protein expression in any host organism. Gene Designer also includes features such as a real-time sliding calculator of oligonucleotide annealing temperatures, sequencing primer generator, tools for avoidance or inclusion of restriction sites, and options to maximize or minimize sequence identity to a reference.

Conclusion

Gene Designer is an expandable Synthetic Biology workbench suitable for molecular biologists interested in the de novo creation of genetic constructs.

Changes produced by bound tryptophan in the ribosome peptidyl transferase center in response to TnaC, a nascent leader peptide

Studies in vitro have established that free tryptophan induces tna operon expression by binding to the ribosome that has just completed synthesis of TnaC-tRNA(Pro), the peptidyl-tRNA precursor of the leader peptide of this operon. Tryptophan acts by inhibiting Release Factor 2-mediated cleavage of this peptidyl-tRNA at the tnaC stop codon. Here we analyze the ribosomal location of free tryptophan, the changes it produces in the ribosome, and the role of the nascent TnaC-tRNA(Pro) peptide in facilitating tryptophan binding and induction. The positional changes of 23S rRNA nucleotides that occur during induction were detected by using methylation protection and binding/competition assays. The ribosome-TnaC-tRNA(Pro) complexes analyzed were formed in vitro; they contained either wild-type TnaC-tRNA(Pro) or its nonfunctional substitute, TnaC(W12R)-tRNA(Pro). Upon comparing these two peptidyl-tRNA-ribosome complexes, free tryptophan was found to block methylation of nucleotide A2572 of wild-type ribosome-TnaC-tRNA(Pro) complexes but not of ribosome-TnaC(W12R)-tRNA(Pro) complexes. Nucleotide A2572 is in the ribosomal peptidyl transferase center. Tryptophanol, a noninducing competitor of tryptophan, was ineffective in blocking A2572 methylation; however, it did reverse the protective effect of tryptophan. Free tryptophan inhibited puromycin cleavage of TnaC-tRNA(Pro); it also inhibited binding of the antibiotic sparsomycin. These effects were not observed with TnaC(W12R)-tRNA(Pro) mutant complexes. These findings establish that Trp-12 of TnaC-tRNA(Pro) is required for introducing specific changes in the peptidyl transferase center of the ribosome that activate free tryptophan binding, resulting in peptidyl transferase inhibition. Free tryptophan appears to act at or near the binding sites of several antibiotics in the peptidyl transferase center.