Yuki Goto

Flexizymes for genetic code reprogramming

Genetic code reprogramming is a method for the reassignment of arbitrary codons from proteinogenic amino acids to nonproteinogenic ones; thus, specific sequences of nonstandard peptides can be ribosomally expressed according to their mRNA templates. Here we describe a protocol that facilitates genetic code reprogramming using flexizymes integrated with a custom-made in vitro translation apparatus, referred to as the flexible in vitro translation (FIT) system. Flexizymes are flexible tRNA acylation ribozymes that enable the preparation of a diverse array of nonproteinogenic acyl-tRNAs. These acyl-tRNAs read vacant codons created in the FIT system, yielding the desired nonstandard peptides with diverse exotic structures, such as N-methyl amino acids, D-amino acids and physiologically stable macrocyclic scaffolds. The facility of the protocol allows a wide variety of applications in the synthesis of new classes of nonstandard peptides with biological functions. Preparation of flexizymes and tRNA used for genetic code reprogramming, optimization of flexizyme reaction conditions and expression of nonstandard peptides using the FIT system can be completed by one person in ∼1 week. However, once the flexizymes and tRNAs are in hand and reaction conditions are fixed, synthesis of acyl-tRNAs and peptide expression is generally completed in 1 d, and alteration of a peptide sequence can be achieved by simply changing the corresponding mRNA template.

Reprogramming the Translation Initiation for the Synthesis of Physiologically Stable Cyclic Peptides

The initiation codon dictates that the translation initiation event exclusively begins with methionine. We report here a new technology to reprogram the initiation event, where various amino acids and those bearing N (alpha)-acyl groups can be used as an initiator for peptide synthesis. The technology is built upon the concept of genetic code reprogramming, where methionine is depleted from the translation system and the initiation codon is reassigned to the desired amino acid. We have applied this technology to the synthesis of an antitumor cyclic peptide, G7-18NATE, closed by a physiologically stable bond, and it is also extended to the custom synthesis of its analogues with various ring sizes. Significantly, cyclization occurs spontaneously upon translation of the precursor linear peptides. To demonstrate the practicality of this methodology, we also prepared a small cyclic peptide library designated by 160 distinct mRNAs. Thus, this technology offers a new means to prepare a wide array of in vivo compatible cyclic peptide libraries for the discovery of peptidic drug candidates against various therapeutic targets.

Discovery of unique lanthionine synthetases reveals new mechanistic and evolutionary insights

Identification of a new class of lanthionine synthetases provides insight into the mechanism and evolution of cyclic peptide biosynthesis.

Selection-Based Discovery of Druglike Macrocyclic Peptides

Macrocyclic peptides are an emerging class of therapeutics that can modulate protein-protein interactions. In contrast to the heavily automated high-throughput screening systems traditionally used for the identification of chemically synthesized small-molecule drugs, peptide-based macrocycles can be synthesized by ribosomal translation and identified using in vitro selection techniques, allowing for extremely rapid (hours to days) screening of compound libraries comprising more than 10(13) different species. Furthermore, chemical modification of translated peptides and engineering of the genetic code have greatly expanded the structural diversity of the available peptide libraries. In this review, we discuss the use of these technologies for the identification of bioactive macrocyclic peptides, emphasizing recent developments.

Lantibiotics from Geobacillus thermodenitrificans

The lantibiotic nisin has been used as an effective food preservative to combat food-borne pathogens for over 40 y. Despite this successful use, nisin’s stability at pH 7 is limited. Herein, we describe a nisin analog encoded on the genome of the thermophilic bacterium Geobacillus thermodenitrificans NG80-2. This analog termed geobacillin I was obtained by heterologous expression in Escherichia coli and subsequent purification. Extensive NMR characterization demonstrated that geobacillin I contains seven thioether cross-links, two more than the five cross-links found in nisin and the most cross-links found in any lantibiotic to date. The antimicrobial spectrum of geobacillin I was generally similar to that of nisin A, with increased activity against Streptococcus dysgalactiae, one of the causative agents of bovine mastitis. Geobacillin I demonstrated increased stability compared to nisin A. In addition to geobacillin I, the genome of G. thermodenitrificans NG80-2 also contains a class II lantibiotic biosynthetic gene cluster. The corresponding compound was produced in E. coli, and has a ring topology different than that of any known lantibiotic as determined by tandem mass spectrometry. Interestingly, geobacillin II only demonstrated antimicrobial activity against Bacillus strains. Seven Geobacillus strains were screened for production of the geobacillins using whole-cell MALDI-MS and five were shown to produce geobacillin I, but none produced geobacillin II.

Ribosomal synthesis of peptidase-resistant peptides closed by a nonreducible inter-side-chain bond.

Here we report a new enabling technology for the synthesis of peptidase-resistant cyclic peptides by means of genetic code reprogramming involving the flexizyme (a tRNA acylation ribozyme) and PURE (a reconstituted cell-free translation) systems. In this work, we have developed a new nonproteinogenic amino acid bearing a chloroacetyl group in the side chain, which forms a physiologically stable thioether bond by intramolecular reaction with the sulfhydryl group of a Cys residue in the peptide chain upon translation. Significantly, this chemistry takes place spontaneously in situ of the translation solution, giving the corresponding cyclic peptides independent of ring sizes. We have used this method to convert human urotensin II, known as a potent vasoconstrictor, to its analogue containing a thioether bond, showing that this new analogue retains biological activity. Moreover, this peptide exhibits remarkable resistance against peptidases under reducing conditions. Thus, this technology offers a new means to accelerate the discovery of therapeutic peptidic drugs.

Initiating translation with D-amino acids

Here we report experimental evidence that the translation initiation apparatus accepts D-amino acids ((D)aa), as opposed to only L-methionine, as initiators. Nineteen (D)aa, as the stereoisomers to their natural L-amino acids, were charged onto initiator tRNA(fMet)(CAU) using flexizyme technology and tested for initiation in a reconstituted Escherichia coli translation system lacking methionine, i.e., the initiator was reprogrammed from methionine to (D)aa. Remarkably, all (D)aa could initiate translation while the efficiency of initiation depends upon the type of side chain. The peptide product initiated with (D)aa was generally in a nonformylated form, indicating that methionyl-tRNA formyltransferase poorly formylated the corresponding (D)aa-tRNA(fMet)(CAU). Although the inefficient formylation of (D)aa-tRNA(fMet)(CAU) resulted in modest expression of the corresponding peptide, preacetylation of (D)aa-tRNA(fMet)(CAU) dramatically increased expression level, implying that the formylation efficiency is one of the critical determinants of initiation efficiency with (D)aa. Our findings provide not only the experimental evidence that translation initiation tolerates (D)aa, but also a new means for the mRNA-directed synthesis of peptides capped with (D)aa or acyl-(D)aa at the N terminus.

Translation Initiation with Initiator tRNA Charged with Exotic Peptides

In the prokaryotic translation system formylmethionyl-tRNA(fMet)(CAU) acts as an exclusive initiator to yield peptides bearing formylmethionine at the N-terminus. We herein report a new methodology to initiate the translation reaction with peptidyl-tRNA(fMet)(CAU) in which the peptidyl group consists of unusual building blocks, such as D-amino acids, beta-amino acids, and N-methyl amino acids, and express peptides containing a stretch of exotic peptides at the N-terminus. Synthesis of the initiator peptidyl-tRNAs was facilitated by flexizyme, a flexible tRNA aminoacylation ribozyme, and the start codon was reprogrammed by withdrawing methionine from a reconstituted E. coli cell-free translation system to reassign the initiator from formylmethionine to exotic peptides. This represents the first demonstration for initiating the ribosomal peptide synthesis with peptides, which offers us a new tool for the synthesis of a wide variety of unusual peptides and also the mechanistic studies on the initiation and/or elongation events in translation.

Reevaluation of the d-Amino Acid Compatibility with the Elongation Event in Translation

The compatibility of D-amino acids with peptide elongation during translation has been examined in several studies. However, some of the studies have reported that D-amino acids are incompatible with translation, whereas others have reported that D-amino acids are incorporated into polypeptides. Here, we have reevaluated the incorporation of a series of D-amino acids into the nascent chain of short peptides with a reprogrammed genetic code by using the flexible in vitro translation (FIT) system. The FIT system enables the compatibility of each D-amino acid with elongation to be assessed quantitatively in the absence of potential competitors. The incorporation efficiencies were determined by Tricine-SDS-PAGE and the full-length peptide was detected by MALDI-TOF-MS. The D-amino acids were categorized into three groups based on their incorporation efficiencies relative to the corresponding L-amino acid. The D-isomers in group I showed efficiencies of 40% or higher (Ala, Ser, Cys, Met, Thr, His, Phe, and Tyr), and those in group II showed efficiencies of 10-40% (Asn, Gln, Val, and Leu). The D-amino acids in group III produced truncated peptides or no detectable full-length peptides (Arg, Lys, Asp, Glu, Ile, Trp, and Pro). When group I D-amino acids were used consecutively or were alternated with L-amino acids, this completely inhibited their elongation. However, when two or three L-amino acids were inserted between the D-amino acids, the double-incorporation efficiency was restored. Our results quantitatively reveal the compatibility of D-amino acids with peptide elongation and raise new questions about the mechanism of D-amino acid selection and incorporation by the ribosome.

Ribosomal Synthesis of Cyclic Peptides with a Fluorogenic Oxidative Coupling Reaction

Ring around the peptides: We demonstrate a new method for the cyclization of peptides that involves the oxidative coupling of 5‐hydroxyindole and benzylamine. After two nonproteinogenic amino acids were incorporated into peptides by reprogramming the genetic code, cyclization took place rapidly upon the addition of K3Fe(CN)6 and generated a conjugated, fluorescent, heterocyclic structure.