Human asparaginyl-tRNA synthetase: molecular cloning and the inference of the evolutionary history of Asx-tRNA synthetase family

Using CRISPR/Cas9 to identify genes required for mechanosensory neuron development and function

Tunicates are marine, non-vertebrate chordates that comprise the sister group to the vertebrates. Most tunicates have a biphasic lifecycle that alternates between a swimming larva and a sessile adult. Recent advances have shed light on the neural basis for the tunicate larva's ability to sense a proper substrate for settlement and initiate metamorphosis. Work in the highly tractable laboratory model tunicate Ciona robusta suggests that sensory neurons embedded in the anterior papillae transduce mechanosensory stimuli to trigger larval tail retraction and initiate the process of metamorphosis. Here, we take advantage of the low-cost and simplicity of Ciona by using tissue-specific CRISPR/Cas9-mediated mutagenesis to screen for genes potentially involved in mechanosensation and metamorphosis, in the context of an undergraduate 'capstone' research course. This small screen revealed at least one gene, Vamp1/2/3, which appears crucial for the ability of the papillae to trigger metamorphosis. We also provide step-by-step protocols and tutorials associated with this course, in the hope that it might be replicated in similar CRISPR-based laboratory courses wherever Ciona are available.

Using CRISPR/Cas9 to identify genes required for mechanosensory neuron development and function

Tunicates are marine, non-vertebrate chordates that comprise the sister group to the vertebrates. Most tunicates have a biphasic lifecycle that alternates between a swimming larva and a sessile adult. Recent advances have shed light on the neural basis for the tunicate larva's ability to sense a proper substrate for settlement and initiate metamorphosis. Work in the highly tractable laboratory model tunicate Ciona robusta suggests that sensory neurons embedded in the anterior papillae of transduce mechanosensory stimuli to trigger larval tail retraction and initiate the process of metamorphosis. Here, we take advantage of the low-cost and simplicity of Ciona by using tissue-specific CRISPR/Cas9-mediated mutagenesis to screen for genes potentially involved in mechanosensation and metamorphosis, in the context of an undergraduate "capstone" research course. This small screen revealed at least one gene, Vamp1/2/3 , that appears crucial for the ability of the papillae to trigger metamorphosis. We also provide step-by-step protocols and tutorials associated with this course, in the hope that it might be replicated in similar CRISPR-based laboratory courses wherever Ciona are available.

Did Amino Acid Side Chain Reactivity Dictate the Composition and Timing of Aminoacyl-tRNA Synthetase Evolution?

The twenty amino acids in the standard genetic code were fixed prior to the last universal common ancestor (LUCA). Factors that guided this selection included establishment of pathways for their metabolic synthesis and the concomitant fixation of substrate specificities in the emerging aminoacyl-tRNA synthetases (aaRSs). In this conceptual paper, we propose that the chemical reactivity of some amino acid side chains (e.g., lysine, cysteine, homocysteine, ornithine, homoserine, and selenocysteine) delayed or prohibited the emergence of the corresponding aaRSs and helped define the amino acids in the standard genetic code. We also consider the possibility that amino acid chemistry delayed the emergence of the glutaminyl- and asparaginyl-tRNA synthetases, neither of which are ubiquitous in extant organisms. We argue that fundamental chemical principles played critical roles in fixation of some aspects of the genetic code pre- and post-LUCA.

Noncanonical functions of aminoacyl-tRNA synthetases

Aminoacyl-tRNA synthetases, together with their main function of covalent binding of an amino acid to a corresponding tRNA, also perform many other functions. They take part in regulation of gene transcription, apoptosis, translation, and RNA splicing. Some of them function as cytokines or catalyze different reactions in living cells. Noncanonical functions can be mediated by additional domains of these proteins. On the other hand, some of the noncanonical functions are directly associated with the active center of the aminoacylation reaction. In this review we summarize recent data on the noncanonical functions of aminoacyl-tRNA synthetases and on the mechanisms of their action.

[Analysis of the aminoacyl-tRNA synthetase genes of silkworm (Bombyx mori)]

For further research on number, type, composition and origin of Bombyx mori aminoacyl-tRNA synthetase (BmaaRS) genes, in silico cloning was performed with Bombyx mori genomic and EST databases. There might be two different sets of aaRS nuclear gene in Bombxy nori genome, which encode mitochondrial BmaaRS and cytoplasmic BmaaRS, respectively. Among BmaaRS genes, there were 2 genes encoding mitochondrial BmSerRS, but no genes encoding cytoplasmic BmHisRS and mitochondrial BmGlnRS, BmLysRS, BmGlyRS, and BmThrRS. The functions of these absent genes could be directly replaced by other proteins with similar functions, or might undergo their distinct BmaaRS functions based on the alternative splice of one certain BmaaRS mRNA. Evidence of EST indicated that BmaaRS performed different alternative splicing patterns. The homology comparison and advanced structural analysis of BmaaRS demonstrated the existence of extended domains of BmaaRS. This is because some different BmaaRSs contained similar domain. Moreover, BmaaRSs with similar functions possessed the similar tertiary structure. Phylogenetic analysis revealed that BmaaRS encoded by two various sources of BmaaRS genes. Mitochondrial and cytoplasmic BmaaRS had different origin.

Crystal structure of Pyrococcus horikoshii tryptophanyl-tRNA synthetase and structure-based phylogenetic analysis suggest an archaeal origin of tryptophanyl-tRNA synthetase

The ancient and ubiquitous aminoacyl-tRNA synthetases constitute a valuable model system for studying early evolutionary events. So far, the evolutionary relationship of tryptophanyl- and tyrosyl-tRNA synthetase (TrpRS and TyrRS) remains controversial. As TrpRS and TyrRS share low sequence homology but high structural similarity, a structure-based method would be advantageous for phylogenetic analysis of the enzymes. Here, we present the first crystal structure of an archaeal TrpRS, the structure of Pyrococcus horikoshii TrpRS (pTrpRS) in complex with tryptophanyl-5′ AMP (TrpAMP) at 3.0 Å resolution which demonstrates more similarities to its eukaryotic counterparts. With the pTrpRS structure, we perform a more complete structure-based phylogenetic study of TrpRS and TyrRS, which for the first time includes representatives from all three domains of life. Individually, each enzyme shows a similar evolutionary profile as observed in the sequence-based phylogenetic studies. However, TyrRSs from Archaea/Eucarya cluster with TrpRSs rather than their bacterial counterparts, and the root of TrpRS locates in the archaeal branch of TyrRS, indicating the archaeal origin of TrpRS. Moreover, the short distance between TrpRS and archaeal TyrRS and that between bacterial and archaeal TrpRS, together with the wide distribution of TrpRS, suggest that the emergence of TrpRS and subsequent acquisition by Bacteria occurred at early stages of evolution.

WHEP domains direct noncanonical function of glutamyl-Prolyl tRNA synthetase in translational control of gene expression

The heterotetrameric GAIT complex suppresses translation of selected mRNAs in interferon-gamma-activated monocytic cells. Specificity is dictated by glutamyl-prolyl tRNA synthetase (EPRS) binding to a 3'UTR element in target mRNAs. EPRS consists of two synthetase cores joined by a linker containing three WHEP domains of unknown function. Here we show the critical role of EPRS WHEP domains in targeting and regulating GAIT complex binding to RNA. The upstream WHEP pair directs high-affinity binding to GAIT element-bearing mRNAs, while the overlapping, downstream pair binds NSAP1, which inhibits mRNA binding. Interaction of EPRS with ribosomal protein L13a and GAPDH induces a conformational switch that rescues mRNA binding and restores translational control. Total reconstitution from purified components indicates that the four GAIT proteins are necessary and sufficient for self-assembly of a functional complex. Our results establish the essentiality of WHEP domains in the noncanonical function of EPRS in regulating inflammatory gene expression.

Structural elements defining elongation factor Tu mediated suppression of codon ambiguity

In most prokaryotes Asn-tRNA^Asn and Gln-tRNA^Gln are formed by amidation of aspartate and glutamate mischarged onto tRNA^Asn and tRNA^Gln, respectively. Coexistence in the organism of mischarged Asp-tRNA^Asn and Glu-tRNA^Gln and the homologous Asn-tRNA^Asn and Gln-tRNA^Gln does not, however, lead to erroneous incorporation of Asp and Glu into proteins, since EF-Tu discriminates the misacylated tRNAs from the correctly charged ones. This property contrasts with the canonical function of EF-Tu, which is to non-specifically bind the homologous aa-tRNAs, as well as heterologous species formed in vitro by aminoacylation of non-cognate tRNAs. In Thermus thermophilus that forms the Asp-tRNA^Asn intermediate by the indirect pathway of tRNA asparaginylation, EF-Tu must discriminate the mischarged aminoacyl-tRNAs (aa-tRNA). We show that two base pairs in the tRNA T-arm and a single residue in the amino acid binding pocket of EF-Tu promote discrimination of Asp-tRNA^Asn from Asn-tRNA^Asn and Asp-tRNA^Asp by the protein. Our analysis suggests that these structural elements might also contribute to rejection of other mischarged aa-tRNAs formed in vivo that are not involved in peptide elongation. Additionally, these structural features might be involved in maintaining a delicate balance of weak and strong binding affinities between EF-Tu and the amino acid and tRNA moieties of other elongator aa-tRNAs.

The nondiscriminating aspartyl-tRNA synthetase from Helicobacter pylori: anticodon-binding domain mutations that impact tRNA specificity and heterologous toxicity

Divergent tRNA substrate recognition patterns distinguish the two distinct forms of aspartyl-tRNA synthetase (AspRS) that exist in different bacteria. In some cases, a canonical, discriminating AspRS (D-AspRS) specifically generates Asp-tRNA(Asp) and usually coexists with asparaginyl-tRNA synthetase (AsnRS). In other bacteria, particularly those that lack AsnRS, AspRS is nondiscriminating (ND-AspRS) and generates both Asp-tRNA(Asp) and the noncanonical, misacylated Asp-tRNA(Asn); this misacylated tRNA is subsequently repaired by the glutamine-dependent Asp-tRNA(Asn)/Glu-tRNA(Gln) amidotransferase (Asp/Glu-Adt). The molecular features that distinguish the closely related bacterial D-AspRS and ND-AspRS are not well-understood. Here, we report the first characterization of the ND-AspRS from the human pathogen Helicobacter pylori (H. pylori or Hp). This enzyme is toxic when heterologously overexpressed in Escherichia coli. This toxicity is rescued upon coexpression of the Hp Asp/Glu-Adt, indicating that Hp Asp/Glu-Adt can utilize E. coli Asp-tRNA(Asn) as a substrate. Finally, mutations in the anticodon-binding domain of Hp ND-AspRS reduce this enzyme's ability to misacylate tRNA(Asn), in a manner that correlates with the toxicity of the enzyme in E. coli.

Structural basis of the water-assisted asparagine recognition by asparaginyl-tRNA synthetase

Asparaginyl-tRNA synthetase (AsnRS) is a member of the class-II aminoacyl-tRNA synthetases, and is responsible for catalyzing the specific aminoacylation of tRNA(Asn) with asparagine. Here, the crystal structure of AsnRS from Pyrococcus horikoshii, complexed with asparaginyl-adenylate (Asn-AMP), was determined at 1.45 A resolution, and those of free AsnRS and AsnRS complexed with an Asn-AMP analog (Asn-SA) were solved at 1.98 and 1.80 A resolutions, respectively. All of the crystal structures have many solvent molecules, which form a network of hydrogen-bonding interactions that surrounds the entire AsnRS molecule. In the AsnRS/Asn-AMP complex (or the AsnRS/Asn-SA), one side of the bound Asn-AMP (or Asn-SA) is completely covered by the solvent molecules, which complement the binding site. In particular, two of these water molecules were found to interact directly with the asparagine amide and carbonyl groups, respectively, and to contribute to the formation of a pocket highly complementary to the asparagine side-chain. Thus, these two water molecules appear to play a key role in the strict recognition of asparagine and the discrimination against aspartic acid by the AsnRS. This water-assisted asparagine recognition by the AsnRS strikingly contrasts with the fact that the aspartic acid recognition by the closely related aspartyl-tRNA synthetase is achieved exclusively through extensive interactions with protein amino acid residues. Furthermore, based on a docking model of AsnRS and tRNA, a single arginine residue (Arg83) in the AsnRS was postulated to be involved in the recognition of the third position of the tRNA(Asn) anticodon (U36). We performed a mutational analysis of this particular arginine residue, and confirmed its significance in the tRNA recognition.

The C-terminal appended domain of human cytosolic leucyl-tRNA synthetase is indispensable in its interaction with arginyl-tRNA synthetase in the multi-tRNA synthetase complex

Human cytosolic leucyl-tRNA synthetase is one component of a macromolecular aminoacyl-tRNA synthetase complex. This is unlike prokaryotic and lower eukaryotic LeuRSs that exist as free soluble enzymes. There is little known about it, since the purified enzyme has been unavailable. Herein, human cytosolic leucyl-tRNA synthetase was heterologously expressed in a baculovirus system and purified to homogeneity. The molecular mass (135 kDa) of the enzyme is close to the theoretical value derived from its cDNA. The kinetic constants of the enzyme for ATP, leucine, and tRNA(Leu) in the ATP-PP(i) exchange and tRNA leucylation reactions were determined, and the results showed that it is quite active as a free enzyme. Human cytosolic leucyl-tRNA synthetase expressed in human 293 T cells localizes predominantly to the cytosol. Additionally, it is found to have a long C-terminal extension that is absent from bacterial and yeast LeuRSs. A C-terminal 89-amino acid truncated human cytosolic leucyl-tRNA synthetase was constructed and purified, and the catalytic activities, thermal stability, and subcellular location were found to be almost identical to native enzyme. In vivo and in vitro experiments, however, show that the C-terminal extension of human cytosolic leucyl-tRNA synthetase is indispensable for its interaction with the N-terminal of human cytosolic arginyl-tRNA synthetase in the macromolecular complex. Our results also indicate that the two molecules interact with each other only through their appended domains.

Calcium regulates the expression of a Dictyostelium discoideum asparaginyl tRNA synthetase gene

In a screen for calcium-regulated gene expression during growth and development of Dictyostelium discoideum we have identified an asparaginyl tRNA synthetase (ddAsnRS) gene, the second tRNA synthetase gene identified in this organism. The ddAsnRS gene shows many unique features. One, it is repressed by lowering cellular calcium, making it the first known calcium-regulated tRNA synthetase. Two, despite the calcium-dependence, its expression is unaltered during the cell cycle, making this the first D. discoideum gene to show a calcium-dependent but cell cycle phase-independent expression. Finally, the N-terminal domain of the predicted ddAsnRS protein shows higher sequence similarity to Glutaminyl tRNA synthetases than to other Asn tRNA synthetases. These unique features of the AsnRS from this primitive eukaryote not only point to a novel mechanism regulating the components of translation machinery and gene expression by calcium, but also hint at a link between the evolution of GlnRS and AsnRS in eukaryotes.

Expanding tRNA recognition of a tRNA synthetase by a single amino acid change

Aspartyl-tRNA synthetase (AspRS) occurs in two types: the discriminating enzyme (D-AspRS) forms only Asp-tRNA(Asp), whereas the nondiscriminating enzyme (ND-AspRS) also synthesizes Asp-tRNA(Asn), which is a required intermediate for protein synthesis in many organisms. We attempted to expand the tRNA recognition of the discriminating Thermococcus kodakaraensis AspRS to that of a ND-AspRS by in vitro mutagenesis. An alignment of 26 archaeal AspRS proteins revealed two positions (26 and 85 in the T. kodakaraensis sequence) whose amino acid identity changes according to the enzymes' tRNA specificity. In their anticodon-binding domain, D-AspRS proteins contain W26 (or Q26) and K85, compared with H26 and P85 in the ND-AspRSs. T. kodakaraensis AspRS gained the ability to form Asp-tRNA(Asn) in vitro when the W26H or K85P changes were introduced independently or in combination. In the aminoacylation of tRNA(Asn) or tRNA(Asp) transcripts, the mutant enzymes displayed at least a 100- to 500-fold change in tRNA specificity, as judged by the ratio of the k(cat)K(m) values of Asp-tRNA(Asp) vs. Asp-tRNA(Asn) formation. That T. kodakaraensis mutant AspRSs mischarge tRNA(Asn) was also manifested in the higher level (1.7%) of aspartylation of unfractionated Pyrococcus tRNA compared with that achieved by the wild-type enzyme (0.9%). Northern blot analysis of the Asp-tRNA separated by acidurea gel electrophoresis confirmed the in vitro synthesis of Asp-tRNA(Asn). A structure-based model points to a direct interaction of K85 in T. kodakaraensis AspRS with the anticodon nucleotide C36 of tRNA(Asp). Thus, a switch between D-AspRS and ND-AspRS enzymes could have evolved with only limited amino acid changes.

Evolutionary divergence of the archaeal aspartyl-tRNA synthetases into discriminating and nondiscriminating forms

Asparaginyl-tRNA (Asn-tRNA) is generated in nature via two alternate routes, either direct acylation of tRNA with asparagine by asparaginyl-tRNA synthetase (AsnRS) or in a two-step pathway that requires misacylated Asp-tRNA(Asn) as an intermediate. This misacylated aminoacyl-tRNA is formed by a nondiscriminating aspartyl-tRNA synthetase (AspRS), an enzyme that in addition to forming Asp-tRNA(Asp) also misacylates tRNA(Asn). In contrast, a discriminating AspRS cannot acylate tRNA(Asn). It has been suggested that the archaeal AspRS enzymes are nondiscriminating, whereas the bacterial ones discriminate. The archaeal and bacterial AspRS proteins are indeed distinct in sequence and structure. However, we show that both discriminating and nondiscriminating forms of AspRS exist among the archaea. Using unfractionated methanobacterial and pyrococcal tRNA, the Methanothermobacter thermautotrophicus AspRS acylated approximately twice as much tRNA as did AspRS from Pyrococcus kodakaraensis or Ferroplasma acidarmanus. Proof that Asp-tRNA(Asn) was generated by the methanogen synthetase was the conversion of Asp-tRNA formed by M. thermautotrophicus AspRS to Asn-tRNA by M. thermautotrophicus Asp-tRNA(Asn) amidotransferase. In contrast, Asp-tRNA formed by the Pyrococcus or Ferroplasma enzymes was not a substrate for the amidotransferase. Also, although all three AspRS enzymes charged tRNA(Asp) transcripts, only M. thermautotrophicus AspRS aspartylated the tRNA(Asn) transcript. Genomic analysis provides a rationale for the nature of these enzymes. The mischarging AspRS correlates with the absence in the genome of AsnRS and the presence of Asp-tRNA(Asn) amidotransferase, employed by the transamidation pathway. In contrast, the discriminating AspRS correlates with the absence of the amidotransferase and the presence of AsnRS, forming Asn-tRNA by direct aminoacylation. The high sequence identity, up to 60% between discriminating and nondiscriminating archaeal AspRSs, suggests that few mutational steps may be necessary to convert the tRNA-discriminating ability of a tRNA synthetase.

Aminoacyl-tRNA synthesis

Aminoacyl-tRNAs are substrates for translation and are pivotal in determining how the genetic code is interpreted as amino acids. The function of aminoacyl-tRNA synthesis is to precisely match amino acids with tRNAs containing the corresponding anticodon. This is primarily achieved by the direct attachment of an amino acid to the corresponding tRNA by an aminoacyl-tRNA synthetase, although intrinsic proofreading and extrinsic editing are also essential in several cases. Recent studies of aminoacyl-tRNA synthesis, mainly prompted by the advent of whole genome sequencing and the availability of a vast body of structural data, have led to an expanded and more detailed picture of how aminoacyl-tRNAs are synthesized. This article reviews current knowledge of the biochemical, structural, and evolutionary facets of aminoacyl-tRNA synthesis.

Aminoacyl-tRNA synthetases, the genetic code, and the evolutionary process

The aminoacyl-tRNA synthetases (AARSs) and their relationship to the genetic code are examined from the evolutionary perspective. Despite a loose correlation between codon assignments and AARS evolutionary relationships, the code is far too highly structured to have been ordered merely through the evolutionary wanderings of these enzymes. Nevertheless, the AARSs are very informative about the evolutionary process. Examination of the phylogenetic trees for each of the AARSs reveals the following. (i) Their evolutionary relationships mostly conform to established organismal phylogeny: a strong distinction exists between bacterial- and archaeal-type AARSs. (ii) Although the evolutionary profiles of the individual AARSs might be expected to be similar in general respects, they are not. It is argued that these differences in profiles reflect the stages in the evolutionary process when the taxonomic distributions of the individual AARSs became fixed, not the nature of the individual enzymes. (iii) Horizontal transfer of AARS genes between Bacteria and Archaea is asymmetric: transfer of archaeal AARSs to the Bacteria is more prevalent than the reverse, which is seen only for the "gemini group. " (iv) The most far-ranging transfers of AARS genes have tended to occur in the distant evolutionary past, before or during formation of the primary organismal domains. These findings are also used to refine the theory that at the evolutionary stage represented by the root of the universal phylogenetic tree, cells were far more primitive than their modern counterparts and thus exchanged genetic material in far less restricted ways, in effect evolving in a communal sense.

Evolution of Aminoacyl-tRNA Synthetases—Analysis of Unique Domain Architectures and Phylogenetic Trees Reveals a Complex History of Horizontal Gene Transfer Events

Phylogenetic analysis of aminoacyl-tRNA synthetases (aaRSs) of all 20 specificities from completely sequenced bacterial, archaeal, and eukaryotic genomes reveals a complex evolutionary picture. Detailed examination of the domain architecture of aaRSs using sequence profile searches delineated a network of partially conserved domains that is even more elaborate than previously suspected. Several unexpected evolutionary connections were identified, including the apparent origin of the β-subunit of bacterial GlyRS from the HD superfamily of hydrolases, a domain shared by bacterial AspRS and the B subunit of archaeal glutamyl-tRNA amidotransferases, and another previously undetected domain that is conserved in a subset of ThrRS, guanosine polyphosphate hydrolases and synthetases, and a family of GTPases. Comparison of domain architectures and multiple alignments resulted in the delineation of synapomorphies—shared derived characters, such as extra domains or inserts—for most of the aaRSs specificities. These synapomorphies partition sets of aaRSs with the same specificity into two or more distinct and apparently monophyletic groups. In conjunction with cluster analysis and a modification of the midpoint-rooting procedure, this partitioning was used to infer the likely root position in phylogenetic trees. The topologies of the resulting rooted trees for most of the aaRSs specificities are compatible with the evolutionary “standard model” whereby the earliest radiation event separated bacteria from the common ancestor of archaea and eukaryotes as opposed to the two other possible evolutionary scenarios for the three major divisions of life. For almost all aaRSs specificities, however, this simple scheme is confounded by displacement of some of the bacterial aaRSs by their eukaryotic or, less frequently, archaeal counterparts. Displacement of ancestral eukaryotic aaRS genes by bacterial ones, presumably of mitochondrial origin, was observed for three aaRSs. In contrast, there was no convincing evidence of displacement of archaeal aaRSs by bacterial ones. Displacement of aaRS genes by eukaryotic counterparts is most common among parasitic and symbiotic bacteria, particularly the spirochaetes, in which 10 of the 19 aaRSs seem to have been displaced by the respective eukaryotic genes and two by the archaeal counterpart. Unlike the primary radiation events between the three main divisions of life, that were readily traceable through the phylogenetic analysis of aaRSs, no consistent large-scale bacterial phylogeny could be established. In part, this may be due to additional gene displacement events among bacterial lineages. Argument is presented that, although lineage-specific gene loss might have contributed to the evolution of some of the aaRSs, this is not a viable alternative to horizontal gene transfer as the principal evolutionary phenomenon in this gene class.

[Complete multiple alignments of all aaRSs from complete genomes as well as the alignments of conserved regions used for phylogenetic tree construction are available at ftp://ncbi.nlm.nih.gov/pub/koonin/aaRS]

Genetic dissection of protein-protein interactions in multi-tRNA synthetase complex

Cytoplasmic aminoacyl-tRNA synthetases of higher eukaryotes acquired extra peptides in the course of their evolution. It has been thought that these appendices are related to the occurrence of the multiprotein complex consisting of at least eight different tRNA synthetase polypeptides. This complex is believed to be a signature feature of metazoans. In this study, we used multiple sequence alignments to infer the locations of the peptide appendices from human cytoplasmic tRNA synthetases found in the multisynthetase complex. The selected peptide appendices ranged from 22 aa of aspartyl-tRNA synthetase to 267 aa of methionyl-tRNA synthetase. We then made genetic constructions to investigate interactions between all 64 combinations of these peptides that were individually fused to nonsynthetase test proteins. The analyses identified 11 (10 heterologous and 1 homologous) interactions. The six peptide-dependent interactions paralleled what had been detected by crosslinking methods applied to the isolated multisynthetase complex. Thus, small peptide appendices seem to link together different synthetases into a complex. In addition, five interacting pairs that had not been detected previously were suggested from the observed peptide-dependent complexes.