Structural and functional relationships between aminoacyl-tRNA synthetases

TIBS 17 - APRIL 1992

References Because of TIBS policy of short reference lists, not all the important primary data could be cited. Interested readers should consult the reference lists of papers cited here. 1 Blaschuk, O. W., Burdzy, K. and Fritz, I. B. (1983) J. Biol. Chem. 258, 7714-7720 2 Kissinger, C., Skinner, M. K. and Griswold, M. D. (1982) Biol. Reprod. 27,233-240 3 Collard, M. W. and Griswold, M. D. (1987) Biochemistry 26, 3297-3303 4 Cheng, C. Y., Mathur, P. P. and Grima, J. (1988) Biochemistry 27, 4079-4088 5 Murphy, B. F., Kirszbaum, L., Walker, I. D. and d'Apice, J. F. (1988) J. Clin. Invest. 81, 1858-1864 6 Jenne, D. E. and Tschopp, J. (1989) Proc. Natl Acad. Sci. USA 86, 7123-7127 7 Kirszbaum, L. et al. (1989) EMBO J. 8, 711-718 8 Leger, J. G., Montpetit, M. L. and Tenniswood, M. P. (1987) Biochero. Biophys. Res. Coromun. 147,196-203 9 Bettuzzi, S., Hiipakka, R. A., Gilna, P. and Liao, S. T. (1989) Biochero. J. 257, 293-296 10 May, P. C. et al. (1990) Neuron 5, 831-839 11 Danik, M. et al. (1991) Proc. Natl Acad. Sci. USA 88, 8577-8581 12 Michel, D. et al. (1989) Oncogene Res. 4, 127-136 13 Hochstrasser, A. C. et al. (1988) Appl. Theor. Electrophor. 1, 73-76 14 de Silva, H. V. et al. (1990) J. Biol. Chem. 265, 14292-14297 15 de Silva, H. V. et al. (1990) Biocheroistry 29,

THE TRANSLATION of genetic information relies upon the correct charging of transfer RNAs (tRNAs), the adaptor molecules, by aminoacyl-tRNA synthetases (aaRS), the key actors in the process. In each living cell this family of at least 20 enzymes, one for each amino acid, performs a two-step reaction that involves three substratesl: aaRS + aa + ATP ~ aaRS-(aa-AMP) + PPi aaRS-(aa-AMP) + tRNA ~ aa-tRNA + aaRS + AMP A more detailed examination of this highly specific reaction shows that even when they share a common frame the molecular mechanisms are complex and can vary from one enzyme to another 2. For example, in some systems, binding of tRNA is required for the binding of ATP and the first step of the reaction, while in most others adenylate formation can be performed in the

5380-5389 16 de Silva, H. V. et al. (1990) J. Biol. Chem. 265, 13240-13247 17 James, R. W. et al. (1991) Arterioscler. Throorob. 11, 645-652 18 Jenne, D. E. et al. (1991) J. Biol. Chem. 266, 11030-11036 19 Palmer, D. J. and Christie, D. L. (1990) J. Biol. Chem. 265, 6617-6623 20 Krisch, K. et al. (1988) Lab. Invest. 58, 411-420 21 Huttner, W. B., Gerdes, H. H. and Rosa, P. (1991) Trends. Biochero. Sci. 16, 27-30 22 Hartmann, K. et al. (1991) J. Biol. Chem. 266, 9924-9931 23 Slawin, K., Sawczuk, I. S., Olsson, C. A. and Buttyan, R. (1990) Biochero. Biophys. Res. Cororoun. 172, 160-164 24 Tsuruta, J. K., Wong, K., Fritz, I. B. and Griswold, M. D. (1990) Biochero. J. 268, 571-578 25 Griswold, M. D., Roberts, K. and Bishop, P. (1986) Biochemistry 25, 7265-7270 26 Burkey, B. F., de Silva, H. V. and Harmony, J. A. (1991) J. Lipid Res. 32, 1039-1048 27 Urban, J. et al. (1987) J. Cell Biol. 105, 2735-2743 28 Fischer Colbrie, R. et al. (1984) J. Neurochero. 42, 1008-1016 29 Margolis, R. U., Fischer Colbrie, R. and Margolis, R. K. (1988) J. Neurochero. 51, 1819-1824 30 Buttyan, R. et al. (1989) Mol. Cell Biol. 9, 3473-3481 31 Morales, C., Hugly, S. and Griswold, M. D. (1987) Biol. Reprod. 36, 1036-1046

32 Krisch, K. et al. (1986) Am. J. Pathol. 123, 100-108 33 Sylvester, S. R., Morales, C., Oko, R. and Griswold, M. D. (1991) Biol. Reprod. 45, 195-207 34 Tung, P. S. and Fritz, I. B. (1985) Biol. Reprod. 33, 177-186 35 Kyprianou, N., English, H. F. and Isaacs, J. T. (1990) Cancer Res. 50, 3748-3753 36 Rennie, P. S. et al. (1988) Cancer Res. 48, 6309-6312 37 Rosenberg, M. E. and Paller, M. S. (1991) Kidney Int. 39, 1156-1161 38 Sensibar, J. A. et al. (1991) Endocrinology 128, 2091-2102 39 Duguid, J. R., Bohmont, C. W., Liu, N. and Tourtellotte, W. W. (1989) Proc. Natl Acad. Sci. USA 86, 7260-7264 40 Day, J. R. et al. (1990) Mol. Endocrinol. 4, 1995-2002 41 Bettuzzi, S. et al. (1991) Biochero. Biophys. Res. Cororoun. 175, 810-815 42 Kyprianou, N., Alexander, R. B. and Isaacs, J. T. (1991) J. Natl Cancer Inst. 83, 346-350 43 Sylvester, S. R., Skinner, M. K. and Griswold, M. D. (1984) Biol. Reprod. 31, 1087-1101 44 Murphy, B. F. et al. (1989) Int. Iromunol. 1, 551-554 45 Choi, N-H. et al. (1990) Int. Irorounol. 2, 413-417 46 Eddy, A. A. and Fritz, I. B. (1991) Kidney Interoat. 39, 247-252

A workshop on clusterin is to be held this September. See Job Trends pages for details.

Structural and functional relationships between aminoacyltRNA synthetases

Aminoacyl-tRNA synthetases can be divided in two groups of equal size on the basis of differences in the structure of their active sites. The core of class I synthetases is the classical nucleotide-binding domain with its characteristic Rossmann fold. In contrast, the active site of class II synthetases is built around an antiparallel p-sheet, to which the substrates bind. This classification, which is based on structural data (amino acid sequences and tertiary structures), can be rationalized in functional terms.

absence of tRNA. It has also been shown in some but not all systems that the fidelity of the reaction can be controlled by hydrolytic proofreading mechanisms. D. Moras is at the Institut de Biologie Aminoacyl-tRNA synthetases bind Mol6culaire et Cellulaire du CNRS, Laboratoire one common substrate (ATP) and de Cristallographie Biologique, attach the amino acid to the same ter15 rue Ren6 Descartes, 6 7 0 8 4 Strasbourg Cedex, France. minal adenosine of the conserved CCA © 1992,ElsevierScience Publishers, (UK) 0376-5067/92/$05.00

end of tRNAs. Despite these consistencies this protein family is characterized by its large structural diversity: the polypeptide chains extend from as little as 334 residues for TrpRS to as much as 1112 for PheRS4; and their oligomeric states include single monomers, homotetramers and heterodimers, but the majority of synthetases are

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crystallography was also analysis of the significance of this disdeterminant, with the covery was presented in TIBS January Class I II structure determination of 19919. three additional enzymes, Class I. The first consensus sequence, Aminoacylation 2'OH 3'OH site on terminal two of them in their HIGH, where I can be leucine, valine or ribose complexed state with methionine, was first pointed out in Characteristic HIGH (1) ...P... their cognate tRNA5-7. 1982 by Barker and Winter when they motifs a KMSKS (2) ...FRXE... (3) ...GXGXGXER... These data have sig- analysed the structure of tyrosyl and Subclass nificantly increased o u r methionyl tRNA synthetases ~°. A strucmembers a b c a b c understanding of struc- tural correlation was noted later u. The Leu Tyr Arg His Asp Gly Phe ture-function relation- almost perfect match of an l 1-amino lie Trp Gin Pro Asn Ala Val Glu Ser Lys ships in aminoacyl-tRNA acid motif which included the HIGH Cys Thr synthetases. tetrapeptide between sequences of Met isoleucine and methionine led to the aBold type in the motifs signifies conserved residues. Sequence similarities and proposal of a 'signature sequence 'j2. class definition The same motif was then found in eight At the outset, sequence other aminoacyl-tRNA synthetases. Howhomodimers. For a long time attempts analyses of the synthetase families ever, now that 24 primary sequences of to unify the family through structural emphasized the lack of homology class I synthetases are available, it can correlations were largely unsuccessful. among their members. Significant but be seen that none of the four residues In the last two years the situation has limited homologies could be detected making for the consensus sequence are changed drastically with the availability between some enzymes. However, even strictly conserved. Only the first histiof new information on the sequence for the most favorable cases the per- dine and the glycine are almost invariand tertiary structure of these proteins. centage of identity in the conserved ant (respectively 22 and 23 times out of The development of genetic tools was domains is usually below 30%. Only 24) 13. The two histidine residues were the key both for the discovery of many recently did the methods of sequence shown to be involved in ATP binding. The second motif, KMSKS, was first new primary sequences and the pro- analysis permit a comprehensive classiduction of large quantities of material fication of synthetases with a clear par- identified by affinity labelling and then which in turn accelerated structural tition in two families of equal size 8 and a found in all other class I aminoacyl-tRNA studies. The contribution of X-ray functional correlation (Table I). A first synthetases by sequence analysis ~4J~. For this second pattern the strict conservation is even more limited and only the second lysine is strictly conserved (a) HIGH KMSKS among the 24 known sequences. This ArgRS lysine was shown to be essential for the GluRS amino acid activation catalysed by GInRS methionyl tRNA-synthetase in the E. coil JI ~I [ I I TrpRS system ~6. TyrRS It is clear that these two consensus IBB ,, MetRS motifs observed in all class I synII ,,~ I : I CysRS thetases are too small and not sufII I I I i I IleRS ~ I : ' I ' I I ficiently conserved to be significant sigLeuRS ValRS nature motifs in the conventional terms of sequence analysis. These motifs could only be picked up experimentally. Subsequently, the combination of se(b) motif I motif 2 motif 3 quence analysis with the comparison of AlaRS i i the three-dimensional structures of the GlyRS(A) I, I three known class I enzymes led to a - eroRS i = i I ThFRS i .... ~ml further refinement of the class notion. HisRS ~m~ ~m3 Three subclasses of class I synthetases SeraS [ ml sharing a more extended similarity can AspRS i : m I thus be defined (Landes, pets. comAsnRS I ...... I ii mun.; Ref. 4). Class la contains all LysRS(S) I ...... , ' t ! I 1,: I hydrophobic and sulfur-containing resiPheRS(S) I I ,I I I I ii dues. The close relationship of these five synthetases has already been noted 17,~s. Figure 1 Class Ib contains the aromatic tyrosine Sequence alignments of (a) class.I synthetases and (b) class II synthetases. Conserved and tryptophan synthetases and class patterns are shown in black and the rest of the polypeptide chain is shaded. Gaps (white Ic is formed by the group of charged part) were introduced for the purpose of aligning the conserved fragments. Note that the residues arginine, glutamic acid and signature motifs of the class I synthetases are located in the amino-terminal part of the glutaminyi. Fortuitously, the threepolypeptide chain. The three conserved motifs characteristic of class II synthetase are dimensional structure of one member essentially located in the carboxy-terminal part of the proteins with the exception of Alaand GlyRS. of each subclass is known. Table I. Classification of aminoacyl-tRNA synthetases

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Alignment of the primary sequences of all E. coil class 1 aminoacyl-tRNA synthetases reveals that the signature motifs are located in the amino-terminal part of the protein (Fig. la). The large monomeric synthetases (lle-Leu-Val) are characterized by a large insertion domain between the two conserved motifs. Class II. At first, no similarities could be found between those synthetases not in class I. Significant homologies between some synthetases were detected as soon as their sequences became available (i.e. between aspRS, asnRS and lysRS as well as between serRS and thrRS) but no general pattern could be picked out. Only after the elucidation of the primary sequence of E. coil ProRS could homology between all remaining synthetases be discerned 9. This last sequence established a link between

the two previously unrelated groups. Three motifs specific for this set of aminoacyl-tRNA synthetases could be detected and proved significant and specific of class 11 aminoacyl-tRNA synthetases, when all known protein sequences were searched. When considering the three sequence motifs in the light of the 19 known sequences of class I1 synthetases, very few residues are strictly conserved u. Motif 1, a signature peptide of about 20 residues, has only one invariant proline. Motifs 2 and 3 both exhibit a single invariant arginine. However, these two motifs do have a few semi-invariant residues at key positions. For example, a set of conserved glycines are present before the invariant arginine of motif 3. All these residues are directly involved in the active site of class I1 enzymes.

Even if sequence analysis alone was sufficient to correlate the ten members of class II synthetases, it is only with the three-dimensional structures of two members of this class that the real significance of these motifs could be understood. As for class I synthetases, the existence of three-dimensional structures enables the extension of primary sequence alignments and the refinement of some interesting details 7,~9.These structural correlations confirm the first alignment in contrast with that presented in a more recent tentative report 2°. Motif 1 is associated with the dimeric interface and is only observed in those RS that form ~2 dimers. Motifs 2 and 3, essential constituents of the active site, are located in the carboxy-terminal part of the proteins (note that the conserved motifs of class I synthetases are found in the

(b)

(e)

Figure 2 Polypeptide chain folding of five aminoacyl-tRNA synthetases: (a) B. stearothermophilus TyrRS, (b) E. coil MetRS, (c) E. coil GInRS, (d) E. coil SerRS, and (e) yeast AspRS. The three class I and the two class II enzymes are all viewed along the central ~sheet active site. GInRS is a monomer of 553 amino acids. The structure of MetRS is that of a tryptic fragment of 534 amino acids. TyrRS is an c~2 dimer (2 × 418), but only the first 318 residues of each subunit are seen in the crystal. AspRS is an (x2 dimer (2 x 564 residues, but the ordered structure starts at residue 70). SerRS is an (~2 dimer (2 x 430). The views (d) and (e) emphasize the different relative position of the amino-terminal parts in the three-dimensional structure.

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(a)

Insertion sequence

rim1

347

Insertion sequence

)

~

v Carboxylterminus Toaminoterminus

C°nsensussequenceKMSKS~ / / Insertionsequence

j # ~ ~ ~

/ "h B ~ ~

ConsensussequenceHIGH

different ATP analogs 22. Tubercidine 5'triphosphate activity provides a perfect match with the present classification. However, the complexity of the results obtained with the seven other ATP analogs hid this observation and led the authors to propose a larger number of subgroups. It is remarkable and significant that none of these subgroups contains synthetases of different classes (I and ll). This study confirms our previous conclusion that the class distribution is essentially based on the ATPbinding mode. In addition, it suggests that the subclasses defined from sequence analyses are related to another function. This could be the amino acid recognition, since the subclasses relate groups of amino acids with similar size and/or chemical properties (hydrophobicity, electrostatic status). This suggested relationship was first pointed out by Eriani et al) It may not be coincidental that the smallest residues are associated with class II synthetases in which, according to the crystal structures of AspRS and SerRS, the active sites form deeply buried pockets. In contrast, the bulkier amino acids bind preferentially to class I synthetases with their active site located at the bottom of a more open cleft.

Figure 3 Schematic diagram of the ATP-binding domain of class I synthetases (a) as found in the three known crystal structures, and the active site domain of the seven (/-2 dimers of class II synthetases (b) built around an antiparallel ~sheet as inferred from the combined comparisons of the two known three-dimensional structures and primary sequence analysis.

amino-terminal part), with the exception of AlaRS and GlyRS (Fig. lb). Class 1I enzymes can also be subdivided into three subclasses. Class lla (His, Pro, Ser, Thr) and lib (Asn, Asp, Lys) include the seven dimeric enzymes, each subgroup sharing significantly larger homologies than the conserved motifs. AlaRS and GlyRS could constitute a third subclass, with PheRS being in a special position as will be discussed later. This last group of synthetases is also characterized by a large variation in their oligomeric structure between species (i.e. GlyRS and PheRS a r e 0(.2]32 heterodimers in E. coli, an organism in which AlaRS is an ¢x4 homotetramer, but in Bombix morii AiaRS is a monomer, like PheRS in yeast mitochondria).

Functional correlations There have been many attempts to

arrange aminoacyl-tRNA synthetases into families on the basis of functions, ranging from their known mischarging

162

Tertiary structures

High-resolution crystal structures of five different aminoacyl-tRNA synthetases are known: TyrRS24, MetRS 2~, GInRS26 from B. stearothermophilus, SerRS 6 from E. coil, and AspRS 7 from capacities z~to their behavior toward ATP yeast. Each one belongs to a different subclass as described above. Two of analogs=; none were very convincing. The current partition of synthetases them, GInRS and AspRS, are seen in into two families can be almost exactly their functional state, complexed to correlated with a known property of their cognate tRNA. Figure 2 shows a these enzymes: the primary site of at- ribbon representation of their C, chain. tachment of the activated amino acid For the class I synthetases, although on the terminal ribose. All class 1 syn- both TyrRS and MetRS are (~2 dimers in thetases aminoacylate on the 2' OH, vivo, only TyrRS is a functional dimer whereas class II enzymes charge the 3' with known anti-cooperativity. Its OH. PheRS is the only exception to this active site was extensively studied by rule. The position of tRNA aminoacyl- site-directed mutagenesis 27. The crystal ation was experimentally determined in structure of E. coli MetRS is that of the the 1970s for most of the synthetases z3. fully functional tryptic digest. The two Since the structural partition of syn- class II synthetases are functional ec2 thetases is now so clearly related to dimers. The most obvious common feathis functional difference, it is import- ture of all these enzymes is the apparent subdivision in distinct structural ant to understand the relationship. Since the two families are essentially and functional domains. That syndefined by their different active sites, thetases are built out of distinct funcanalysis of the behavior of the various tional domains was first shown in synthetases toward ATP analogs is par- E. coli AlaRS by genetic engineering ticularly interesting. In 1981 the amino- experiments 28. For each synthetase the essential acylation reaction was analysed for the entire family of synthetases with eight domain is the active site built around

TIBS 17 - APRIL 1 9 9 2

the ATP-binding domain. The existence of two different structural motifs for the same function is the most interesting and a totally unexpected recent discovery in this field. In class I synthetases the central core domain is built around a parallel ~-sheet surrounded by a-helices with a topology first discovered in dehydrogenases 29 (Fig. 3a). This domain contains the conserved residues which have been shown to be involved in the binding of ATP. The HIGH peptide is located in the loop connecting the first ~strand to the following a-helix. For all known enzymes which possess the 'nucleotide-binding domain' (dehydrogenases, kinases, GTPbinding proteins, etc.) the pyrophosphate group of the coenzymes always bind to the same loop. However, it must be noted that the size of the loop and the nature and location of the conserved residues vary from one family of enzymes to another, according to the nature of the coenzyme (NAD, ATP, GTP, FAD, etc.) or even with the same ligand (ATP). Insertion of different 'building blocks' at conserved positions in the three-dimensional structure (indicated by dashed lines in Fig. 3) contributes the specific character of each individual system. In two of the known structures (GlnRS and MetRS) these large insertion domains are involved in tRNA positioning and recognition. The three conserved motifs of class II synthetases make up the central core of the protein and form the essential part of the active site and the interface domain of the dimer (Fig. 3b). The active site domain is formed by an antiparallel ~-sheet flanked by ahelices, with a unique folding topology. The conserved part of the domain comprises aproximately 250 amino acids. The rest of the protein is formed by insertions of variably sized polypeptides. Addition or deletion of 'building blocks' to an essential backbone can also be used~to differentiate the same enzyme in various species (e.g.E. coli AspRS and eukaryotic AspRS) 3°. For example, the comparison of the seryl and aspartic acid specific enzymes emphasizes the topologically different location of their specific amino-terminal domains on opposite sides of the central core (Fig. 2b). Assuming a similar positioning of the tRNA molecule in the conserved active site it is easy to explain the totally different behavior of these enzymes toward the anticodon of their cognate tRNA. The observation that synthetases are made from the insertion or addition of

g~

Accet)tor

~01

(It

Figure 4 Molecular structure (C, backbone) of E. coli GInRS5 (a) and yeast AspRS7 (b) complexed with their cognate tRNA (phosphate backbone). This figure emphasizes the different sides of approach of the acceptor stem by each synthetase. The class I enzyme (GInRS) binds to the minor groove side whereas AspRS, a class II synthetase, approaches tRNAASpfrom the major groove side. The resulting conformation of the CCA end of each bound tRNA is totally different.

structural domains of variable structure and size to a central core containing the active site discards the possibility for the subclasses to be correlated with some patterns of functionally related domains. Indeed, within each subclass different patterns are observed. It is thus tempting to suggest that this domain distribution is a late event in evolution and is essentially associated with specific tRNA recognition.

tRNA recognition If the basic partitioning into two different classes seems to be clearly related to their different ATP binding modes, another striking difference characterizes the two classes of aminoacyltRNA synthetases: their mode of recognition of the CCA stem. Two different binding modes are observed in the glutamine and aspartic acid systems respectively5,7. Figure 4 shows the molecular structure of the two acid complexes as ~een in their crystal structures. GlnRS approaches the tRNA acceptor stem from the minor groove side where the identity elements of the molecules lie. In order to reach the active site the CCA end of the tRNA molecule has to bend backwards to make a hairpin loop. In the aspartic acid system, the synthetase approaches the tRNA acceptor stem from the major groove side. The long variable loop of sequence motif 2 is in direct interaction with the discriminator base G and the first base pair of the acceptor stem UA.

The adenine end dips into a deep pocket where ATP and most probably the amino acid lie. Indirect information from sequence analysis and chemical experiments in solution suggest that these recognition patterns could be generalized to their respective classes of synthetases. It is clear just by comparing the sequence of tRNATM and tRNAAsp, which are very similar in their CCA ends, that the tRNA molecule plays a passive role in the recognition process. In both Gln and Asp systems, tRNA and proteins make additional specific interactions at the level of the anticodon loop. The key features of anticodon-protein interactions are (1) large conformational changes of the anticodon loop and (2) the idiosyncratic character of the recognition process at this level. Model building analyses allow application of the observations made in the glutaminyl and aspartic acid systems to other members of their classes. Such model binding for the methionine system showed that a binding mode of tRNAMeh similar to that observed in the Gin system was compatible with all known experimental observations in solution3L Similarly, the aspartic acid system recognition model should apply to all dimeric c¢2 synthetases of class I!. Of course, such a model cannot provide details of the recognition pattern since conformational changes may play an important role.

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Evolutionaryimplications

thetase-tRNA recognition process (from no interaction at all to intricate networks of highly specific H-bonds) could be an efficient way to accommodate the parallel evolution of the two sequential steps of the translation of the genetic information. The absence of a direct link between the anticodon loop and the site of aminoacylation suggests that the search for a simple stereochemical correlation between the threeletter genetic code and the amino acid or the synthetase (associated with the idea of a second genetic code) is hopeless.

571-576 12 Webster, T. et al. (1984) Science 226,

In this new binary classification, the 1315-1317 only clear functional correlation lies in 13 Delarue, M. and Moras, D. in Nucleic Acids and the catalysed reaction itself. The major Molecular Biology (Vol. 6) (Eckstein, F. and Lilley, D., eds), Springer-Verlag (in press) difference is the first site of amino14 Hountondji, C., Dessen, P. and Blanquet, S. acylation, 2' OH or 3' OH of the terminal (1986) Biochimie 68, 1071-1078 adenosine for class I and class II 15 Burbaum, J. J., Starzyk, R. M. and Scliimmel, P. (1990) Proteins 7, 99-111 enzymes respectively. This is indicative 16 Mechulam, Y. et al. (1991) J. Mol. Biol, 217, of different relative positioning of the 465-474 protein to its ligands. The two sub17 Hou, Y-M. et al. (1991) Proc. Natl Acad. Sci. strates in the aminoacylation step, the USA 88, 976-980 18 Eriani, G., Dirheimer, G. and Gangloff, J. (1991) adenylate and the terminal adenosine, Nucleic Acids Res. 19, 265-269 are approached from different sides. 19 Cusack, S., Haertlein, M. and Leberman, R. Due to the asymmetric nature of these (1991) Nucleic Acids Res. 19, 3489-3498 20 Nagel, G. M. and Doolittle, R. F. (1991) Proc. molecules, the recognition function is Natl Acad. Sci. USA 88, 8121-8125 performed by very different molecular 21 Gi6g~, R. et al. (1974) Eur. J. Biochem. 45, patterns. These mechanisms strongly Acknowledgements 351-362 I am particularly indebted to A. 22 Freist, W., Sternbach, H. and Cramer, F. (1981) suggest a primordial separation imHoppe-Seyler's Z. Physiol. Chem. 362, posed by the asymmetric nature of the Poterszman for Fig. 1 and to M. Ruff for 1247-1254 RNA substrate and are further evidence Figs 2--4. 23 Sprintzl, M. and Cramer, F. (1975) Proc. Natl for RNA as the first 'biological' molAcad. Sci. USA 72, 3049-3053 ecules on earth 32. References 24 Brick, P., Bhat, T. N. and Blow, D. M. (1988) I Schimmet, P. (1987) Annu. Rev. Biochem. 56, J. Mol. Biol. 208, 83-98 The subclasses, as defined from 125-158 25 Brunie, S., Zelwer, C. and Risler, J-L. (1990) sequence homologies, assemble amino 2 Freist, W. (1989) Biochemistry 28, 6787-6795 J. Mol. Biol. 216, 411-424 acids whose side chains have similar 3 Fersht, A. R. (1977) Biochemistry 16, 26 Rould, M. A., Perona, J. J. and Steitz, T. A. 1025-1030 (1991) Nature 352, 213-218 physicochemical properties (polarity, 4 Burbaum, J. J. and Schimmel, P. (1991) J. Biol. 27 Fersht, A. R. (1987) Biochemistry 26, hydrophobicity, molecular weight). Chem. 266, 16965-16968 8031-8037 These similarities could explain the 5 Rould, M. A., Perona, J., S611,D. and Steitz, 28 Jasin, M., Regan, L. and Schimmel, P. (1983) larger homologies observed by the T. A. (1989) Science 246, 1135-1142 Cell 36, 1089-1095 6 Cusack, S. et al. (1990) Nature 347, 249-255 29 Rossman, M. G., Moras, D. and Olsen, K. W. selection of more closely related sol7 Ruff, M. et al. (1991) Science 252, 1682-1689 (1974) Nature 250, 194-199 utions for recognition and specificity, 8 Eriani, G. et al. (1990) Nature 347,203-206 30 Eriani, G., Dirheimer, G. and Gangloff, J. (1990) i.e. hydrophobic pockets and proof9 Schimmel, P. (1991) Trends Biochem. Sci. 16, Nucleic Acids Res. 18, 7109-7117 1-2 31 Perona, J. J. et al. (1991) Proc. Natl Acad. ScL reading mechanisms for hydrophobic USA 88, 2903-2907 amino acids. Within each subclass the 10 Barker, D. G. and Winter, G. (1982) FEBS Lett. 145, 191-193 32 Rich, A. (1962) in Horizons in Biochemistry diversity of the structure and location 11 Blow, D. M. et al. (1983) J. Mol. Biol. 171, (Kashe, M. and Pullman, B., eds), Academic Press of the extra domains suggests that the amino acid group separation occurred before the final and more idiosyncratic tRNA selection. This last step would be obtained by addition and/or insertion of domains of variable size to the central core. The final refinement aimed for Additional copies of the November 1991 Special Issue on transcription tRNA selection would give the 20 differcan be obtained at a cost of £ 8 . 5 0 / $ 1 5 . 0 0 each, with discounts available ent systems which can vary significantly from one species to another in on orders of ten or more copies. Payment must. accompany your order, order to further protect the specificity and may be made by cheque (made out to Elsevier) or by credit cards. We of recognition. In this general scheme accept AmEx, Visa and Access/Mastercara/Eurocard. the peculiar situation of PheRS remains an open question. Send copies of this order form to: Elsevier Sciences Publishers Ltd, This divergent evolution model leads Crown House, Linton Road, Barking, Essex, UK I G l l 8JU to the suggestion of a genetic code (Prices valid until 3 1 December 1 9 9 2 ) emerging from the concomitant evolution of its expression and its translation, tRNA being the essential link. Name ....................................................................................................... The meaning of the code would result Address ......................................................................................................................... from a mutual adaptation rather than a sequential selection based on stereochemical parameters. The~ sophisticated simplicity of the tRNA molecule Credit card number ......................................................................................................................... permits system adaptability while keepIssuing Bank ......................................................................................................................... ing parameters essential for the fidelity Expiry date ......................................................................................................................... of translation and for the deciphering of separated mRNA. The variable role Signature ............................. : .......................................................................................... played by the anticodon in the syn-

TIBS Transcription Special Issue

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