Interaction of E. coli Ffh/4.5S ribonucleoprotein and FtsY mimics that of mammalian signal recognition particle and its receptor

The mechanism of protein translocation across the endoplasmic reticulum membrane of eukaryotic cells and the plasma membrane of prokaryotic cells are thought to be evolutionarily related. Protein targeting to the eukaryotic translocation apparatus is mediated by the signal recognition particle (SRP), a cytosolic ribonucleoprotein, and the SRP receptor, an endoplasmic reticulum membrane protein. During targeting, the 54K SRP subunit (M(r) 54,000; SRP54), a GTP-binding protein, binds to signal sequences and then interacts with the alpha-subunit of the SRP receptor (SR alpha), another GTP-binding protein. Two proteins from Escherichia coli, Ffh and FTsY, structurally resemble SRP54 and SR alpha. Like SRP54, Ffh is a subunit of a cytosolic ribonucleoprotein that also contains the E. coli 4.5S RNA. Although there is genetic and biochemical evidence that the E. coli Ffh/4.5S ribonucleoprotein has an SRP-like function, there is no evidence for an SR alpha-like role for FtsY. Here we show that the Ffh/4.5S ribonucleoprotein binds tightly to FtsY in a GTP-dependent manner. This interaction results in the stimulation of GTP hydrolysis which can be inhibited by synthetic signal peptides. These properties mimic those of mammalian SRP and its receptor, suggesting that the E. coli Ffh/4.5S ribonucleoprotein and FtsY have functions in protein targeting that are similar to those of their mammalian counterparts.

Role of the histone amino termini in facilitated binding of a transcription factor, GAL4-AH, to nucleosome cores

Facilitated, "cooperative" binding of GAL4-AH to nucleosomal DNA occurred in response to inhibition from the core histone amino termini. The binding of GAL4-AH (which contains the DNA-binding and dimerization domains of GAL4) to nucleosome cores containing multiple binding sites initiated at the end of a nucleosome core and proceeded in a cooperative manner until all sites were occupied. However, following tryptic removal of the core histone amino termini, GAL4-AH binding appeared to be noncooperative, similar to binding naked DNA. Binding of GAL4-AH to nucleosomes bearing a single GAL4 site at different positions indicated that inhibition of GAL4 binding was largely mediated by the histone amino termini and primarily occurred at sites well within the core and not near the end. When the histone amino termini were intact, binding of GAL4-AH to sites near the center of a nucleosome core was greatly enhanced by the presence of additional GAL4 dimers bound to more-accessible positions. These data illustrate that the binding of a factor to more-accessible sites, near the end of a nucleosome, allows facilitated binding of additional factors to the center of the nucleosome, thereby overcoming repression from the core histone amino termini. This mechanism may contribute to the binding of multiple factors to complex promoter and enhancer elements in cellular chromatin.

The unfolded-protein-response pathway in yeast

The accumulation of unfolded proteins in the endoplasmic reticulum (ER) triggers the increased production of several ER-resident proteins. This signalling pathway exists in organisms as divergent as mammals and yeast, and is the only known example of an intracellular signalling system that links the ER and the nucleus. Recently, a transmembrane kinase similar in structure to growth-factor receptor kinases has been identified as a key component of the unfolded-protein-response pathway in yeast.

Pretreatment with methylprednisolone in coronary artery bypass grafting influences the levels of histamine and tryptase in serum but not in bronchoalveolar lavage fluid

1. The presence of histamine and tryptase in serum during and after coronary artery bypass grafting may be an indication of the induction of inflammation. 2. One group of patients received no glucocorticoids and a second group received methylprednisolone before extracorporeal circulation. In the steroid group no effects were seen on the basal levels of histamine (2.84 +/- 0.12 ng/ml) and tryptase (0.50 +/- 0.05 ng/ml) during and after surgery. In the other group two peak levels of histamine were observed: one at 10 min after starting extracorporeal circulation (4.19 +/- 1.79 ng/ml) and another at 4 h after surgery (8.26 +/- 4.85 ng/ml). In this group tryptase was only elevated during the period of extracorporeal circulation (1.54 +/- 0.16 ng/ml). 3. There were no differences between the two groups in complement activation. C3a levels rose to 170 +/- 8% and 180 +/- 10% of the initial value in the steroid and non-steroid group, respectively. 4. It was concluded that during surgery mast cells were activated, but since tryptase levels decreased in the post-operative period, the second increase in the histamine level can be explained by activation of basophils or by an unknown mechanism for the release of histamine but not tryptase by mast cells. 5. In the bronchoalveolar lavage fluid the levels of histamine and tryptase showed no differences between the two groups of patients, but histamine was enhanced compared with normal levels.

[Digital data collection and analysis in clinical electrophysiology. A PC program for electrophysiology]

BACKGROUND

The number of data acquired in electrophysiology and the need for immediate analysis and interpretation justified the use of computerized signal acquisition and -analysis. Since such techniques are well known and established in computerized perimetry, the user of EOG, ERG, and VEP will profit from such an approach.

METHOD

A computer program for signal acquisition and -analysis of clinical electrophysiology is described. The software is written in BASIC for IBM compatible PC/XT/AT computers. The program supports all electrophysiological examinations (EOG, ERG, VEP). It contains a subroutine for automated EOG recording and -analysis, allows measurements of amplitude and latency of standardized Ganzfeld-ERG and pattern-ERG. Options for recording and analysis of further stimuli are prepared. The possibility of determination of area under the curve is implemented. In the VEP subroutine evaluation of amplitude, latency, area under the curve, and Fast-Fourier-Transformation--necessary for steady-state-VEP--is installed.

CONCLUSION

The program demonstrates that with the now available low cost Personal Computer technology it is possible to build up individual software systems for clinical electrophysiology supporting and optimizing all recording, analysing, and administrative procedures.

A mitochondrial protease with two catalytic subunits of nonoverlapping specificities

The mitochondrial inner membrane protease is required for the maturation of mitochondrial proteins that are delivered to the intermembrane space. In the yeast Saccharomyces cerevisiae, this protease is now shown to be a complex that contains two catalytic subunits, Imp2p and the previously identified Imp1p. Primary structure similarity indicates that Imp1p and Imp2p are related to each other and to the family of eubacterial and eukaryotic signal peptidases. Imp1p and Imp2p have separate, nonoverlapping substrate specificities. In addition to its catalyzing the cleavage of intermembrane space sorting signals, Imp2p is required for the stable and functional expression of Imp1p. Thus, inner membrane protease, and by analogy eukaryotic multisubunit signal peptidases, may have acquired multiple catalytic subunits by gene duplication to broaden their range of substrate specificity.

GTP binding and hydrolysis by the signal recognition particle during initiation of protein translocation

The signal recognition particle (SRP) consists of one RNA and six protein subunits. The N-terminal domain of the 54K subunit contains a putative GTP-binding site, whereas the C-terminal domain binds signal sequences and SRP RNA. Binding of SRP to the signal sequence as it emerges from the ribosome creates a cytosolic targeting complex containing the nascent polypeptide chain, the translating ribosome, and SRP. This complex is directed to the endoplasmic reticulum membrane as a result of its interaction with the SRP receptor, a membrane protein composed of two subunits, SR alpha and SR beta, each of which also contains a GTP-binding domain. In the presence of GTP, SRP receptor binding to SRP causes the latter to dissociate from both the signal sequence and the ribosome. GTP is then hydrolysed so that SRP can be released from the SRP receptor and returned to the cytosol. Here we show that the 54K subunit (M(r) 54,000) of SRP (SRP54) is a GTP-binding protein stabilized in a nucleotide-free state by signal sequences, and that the SRP receptor both increases the affinity of SRP54 for GTP and activates its GTPase. We propose that nucleotide-mediated conformational changes in SRP54 regulate the release of signal sequences and the docking of ribosomes at the endoplasmic reticulum.

Interleukin-8 production in patients undergoing cardiopulmonary bypass. The influence of pretreatment with methylprednisolone

Pulmonary dysfunction caused by pulmonary neutrophil sequestration is a frequent postoperative complication in patients undergoing cardiopulmonary bypass (CPB) surgery. It is yet unclear whether treatment with corticosteroids in vivo in these patients can prevent complement-mediated neutrophil activation and sequestration in the lungs. Therefore, we conducted a prospective study in order to investigate whether methylprednisolone (MP) pretreatment (30 mg/kg) could influence the appearance of IL-8 (a recently discovered cytokine with potent neutrophil-chemotactic activity) in the peripheral circulation. We also studied the effects of MP pretreatment on the inflammatory parameters in the bronchoalveolar lavage (BAL) fluid 4 h postoperatively. Although peripheral neutropenia and the rise in IL-8 serum levels was less pronounced in MP-treated than in non-steroid-treated patients, there was no significant difference in albumin, total protein, concentrations of IL-8 and C3a, and the number of neutrophils in the BAL fluid between the two groups. However, when cultured in vitro, alveolar macrophages from patients treated with MP released significantly lower IL-8, both in basal conditions and after stimulation with lipopolysaccharide. Our results show that MP does not prevent (IL-8-mediated) pulmonary neutrophil infiltration after CPB, although it might affect certain aspects of the microvascular lung injury.

Transcriptional induction of genes encoding endoplasmic reticulum resident proteins requires a transmembrane protein kinase

The transcription of genes encoding soluble proteins that reside in the endoplasmic reticulum (ER) is induced when unfolded proteins accumulate in the ER. Thus, an intracellular signal transduction pathway must exist that mediates communication between the ER lumen and the nucleus. We have identified a gene in S. cerevisiae, IRE1, that is required for this pathway: ire1- mutants cannot activate transcription of KAR2 and PDI1, which encode the ER resident proteins BiP and protein disulfide isomerase. Moreover, IRE1 is essential for cell viability under stress conditions that cause unfolded proteins to accumulate in the ER. IRE1 encodes a transmembrane serine/threonine kinase that we propose transmits the unfolded protein signal across the ER or inner nuclear membrane. IRE1 is also required for inositol prototrophy, suggesting that the induction of ER resident proteins is coupled to the biogenesis of new ER membrane.

Functional substitution of the signal recognition particle 54-kDa subunit by its Escherichia coli homolog

The 54-kDa subunit of the mammalian signal recognition particle (SRP54) binds to the signal sequences of nascent secretory and transmembrane proteins and facilitates their cotranslational targeting to the membrane translocation apparatus in the endoplasmic reticulum (ER). A 48-kDa Escherichia coli protein that shares extensive sequence similarity with SRP54 was identified in homology searches. Recent genetic experiments by Phillips and Silhavy [Phillips, G. J. & Silhavy, T. J. (1992) Nature (London) 359, 744-746] have shown that depletion of this protein, designated Ffh (fifty-four homolog), leads to a significant secretory defect in vivo. We demonstrate here that Ffh is structurally and functionally related to SRP54 by virtue of its ability to mimic closely its mammalian counterpart in several established biochemical assays, thereby suggesting that it plays a direct role in protein export. Ffh assembled efficiently with mammalian SRP components into a chimeric ribonucleoprotein ["SRP(Ffh)"] and bound at the site normally occupied by SRP54. Like SRP54, the Ffh moiety of the chimeric particle specifically recognized the signal sequence of preprolactin in a photocrosslinking assay. Moreover, Ffh could also act in concert with other SRP components to arrest elongation of preprolactin upon recognition of the signal sequence. In all of these assays, Ffh had approximately the same specific activity as SRP54. In contrast, SRP(Ffh) did not promote the translocation of preprolactin across the membrane of microsomal vesicles, suggesting that Ffh cannot mediate an interaction with a membrane component that is required for the translocation of nascent chains.

Discrete nascent chain lengths are required for the insertion of presecretory proteins into microsomal membranes

Ribosomes synthesizing nascent secretory proteins are targeted to the membrane by the signal recognition particle (SRP), a small ribonucleoprotein that binds to the signal peptide as it emerges from the ribosome. SRP arrests further elongation, causing ribosomes to stack behind the arrested ribosome. Upon interaction of SRP with its receptor on the ER membrane, the translation arrest is released and the ribosome becomes bound to the ER membrane. We have examined the distribution of unattached and membrane-bound ribosomes during the translation of mRNAs encoding two secretory proteins, bovine preprolactin and rat preproinsulin I. We find that the enhancement of ribosome stacking that occurs when SRP arrests translation of these proteins is relaxed in the presence of microsomal membranes. We also demonstrate that two previously described populations of membrane-associated ribosomes, distinguished by their sensitivity to high salt or EDTA extraction, correspond to ribosomes that have synthesized differing lengths of the nascent polypeptide. This analysis has revealed that nascent chain insertion into the membrane begins at distinct points for different presecretory proteins.

A physiological role of Mn2+ in the regulation of cytosolic phosphoenolpyruvate carboxykinase from rat liver is unlikely

A cytosolic cell-free system prepared from rat liver was used to study the effect of bivalent cations on the activity of the gluconeogenic enzyme phosphoenolpyruvate carboxykinase (PEPCK). Steady-state concentrations of oxaloacetate in the range 5-50 microM were generated from increasing concentrations of malate+fumarate (10:1); 2 mM ITP and 3 mM Mg2+ were added as cofactors. Micromolar concentrations of Mn2+, Fe2+ and, to a lesser extent, of Zn2+ and Co2+ were shown to stimulate PEPCK activity. Vmax. (mumol/min per g of liver) increased from 0.67 to 1.68 on addition of 5 microM Fe2+ and to 2.34 with 2 microM Mn2+, whereas no significant effect on the Km for oxaloacetate was observed. The apparent K(a) values (total) were 0.62 microM for Mn2+, 1.48 microM for Zn2+, 1.92 microM for Co2+ and 3.37 microM for Fe2+, being 2-8-fold lower than the corresponding published values. Variations of the free Mn2+ concentration were obtained (a) by increasing the Mn2+ concentration (i.e. activation curve) and (b) by simultaneous addition of Mn2+ and increasing concentrations of the chelating agent EGTA (i.e. inactivation curve). Different results were obtained for the activation and inactivation curves. The inactivation curve showed that PEPCK activity was almost unaffected by variations of the free Mn2+ concentration over the range 0.05-0.15 microM. Under comparable experimental conditions, rat liver arginase (another Mn(2+)-dependent enzyme) was completely inactivated. From kinetic evidence, the existence of two distinct molecular forms of cytosolic rat liver PEPCK with different Mn2+ affinities is postulated. Considering the high affinity of PEPCK for Mn2+ and its relative insensitivity to changes in the free Mn2+ concentration, it seems rather unlikely that changes in the free cation concentration play a major role in regulating PEPCK activity in vivo.

Stimulation of rat liver glycogen synthesis by the adenosine kinase inhibitor 5-iodotubercidin

The adenosine kinase inhibitor 5-iodotubercidin (Itu) was found to have the following effects on glycogen metabolism in hepatocytes of fasted rats. (1) Itu strongly stimulated glycogen synthesis from different substrates (glucose, lactate plus pyruvate, dihydroxyacetone, glycerol and fructose). In cells incubated with these substrates, the well-known stimulating effect of amino acids and that of Itu was more than additive. (2) In parallel with the increase in glycogen deposition, there was an increase in synthase a and a decrease in phosphorylase a concentrations after administration of Itu. Synthase a was increased by Itu and amino acids in an additive manner, whereas the observed activation of phosphorylase after addition of amino acids was antagonized by Itu. (3) In contrast with amino acids, Itu increased neither the cell volume nor the aspartate and glutamate concentrations. (4) Itu enhanced the levels of cyclic AMP. The stimulation of glycogen deposition in the presence of Itu persisted when the cyclic AMP concentration was further increased by adenosine or 2-chloroadenosine. (5) Itu decreased the concentration of ATP, but its effects on glycogen synthesis, synthase a and phosphorylase a concentrations persisted when the ATP catabolism was prevented by adenosine. (6) The effect of Itu on glycogen synthesis was not the result of inhibition of adenosine kinase, since 5'-amino-5'-deoxyadenosine, another inhibitor of this enzyme, had no effect on glycogen deposition.

GTPase domain of the 54-kD subunit of the mammalian signal recognition particle is required for protein translocation but not for signal sequence binding

The 54-kD subunit of the signal recognition particle (SRP54) binds to signal sequences of nascent secretory and transmembrane proteins. SRP54 consists of two separable domains, a 33-kD amino-terminal domain that contains a GTP-binding site (SRP54G) and a 22-kD carboxy-terminal domain (SRP54M) containing binding sites for both the signal sequence and SRP RNA. To examine the function of the two domains in more detail, we have purified SRP54M and used it to assemble a partial SRP that lacks the amino-terminal domain of SRP54 [SRP(-54G)]. This particle recognized signal sequences in two independent assays, albeit less efficiently than intact SRP. Analysis of the signal sequence binding activity of free SRP54 and SRP54M supports the conclusion that SRP54M binds signal sequences with lower affinity than the intact protein. In contrast, when SRP(-54G) was assayed for its ability to promote the translocation of preprolactin across microsomal membranes, it was completely inactive, apparently because it was unable to interact normally with the SRP receptor. These results imply that SRP54G plays an essential role in SRP-mediated targeting of nascent chain-ribosome complexes to the ER membrane and also influences signal sequence recognition, possibly by promoting a tighter association between signal sequences and SRP54M.

Non-Hodgkin lymphoma in Gabon and its relation to HTLV-I

A case-control study was performed in Libreville, Gabon, to determine whether a relationship can be established between the relatively high proportion of non-Hodgkin lymphoma (NHL) among all cancer cases and the high seroprevalence rate of HTLV-I observed, and to discover whether cases of adult T-leukemia/lymphoma (ATLL) related to HTLV-I exist in Gabon. From November 1987 to April 1989, a total of 32 patients with NHL were recruited; 6 were infants with Burkitt's lymphoma and 26 were adults with NHL. Each patient was matched with 2 asymptomatic controls for age, sex and ethnic group. HTLV-I serology was done by ELISA and Western blot. Comparison of the groups was done by chi-square analysis. None of the 6 infants with Burkitt's lymphoma and none of their controls had antibodies to HTLV-I. Of the 26 patients with NHL, 7 (26.9%) had HTLV-I antibodies. Among the 52 controls, the HTLV-I rate was 13.4% (n = 7). There was no difference between cases and controls (Fisher's exact test, p = 0.16). Among the 26 NHL, 4 cases fitted the criteria of ATLL and were HTLV-I-positive; 3 others who were positive for HTLV-I were a woman with lymphoblastic gastric NHL and 2 old men with an unclassified lymphoma. From the results of this limited series it is not possible to state that there is an association between NHL and HTLV-I infection. Nevertheless, cases of ATLL related to HTLV-I are reported from this area. Based on the HTLV-I seroprevalence rates reported in Gabon, the estimated incidence rate of ATL among seropositive people in Gabon appears much lower than in Japan. Different explanations can be proposed, but under-diagnosis of ATLL is probably one of the main factors.

The anticodon triplet is not sufficient to confer methionine acceptance to a transfer RNA

Previous work suggested that the presence of the anticodon CAU alone was enough to confer methionine acceptance to a tRNA. Conversions of Escherichia coli nonmethionine tRNAs to a methionine-accepting species were obtained by substitutions reconstructing the whole methionine anticodon loop together with preservation (or introduction) of the acceptor stem base A73. We show here that the CAU triplet alone is unable to confer methionine acceptance when transplanted into a yeast aspartic tRNA. Both non-anticodon bases of the anticodon loop of yeast tRNA(Met) and A73 are required in addition to CAU for methionine acceptance. The importance of these non-anticodon bases in other CAU-containing tRNA frameworks was also established. These specific non-anticodon base interactions make a substantial thermodynamic contribution to the methionine acceptance of a transfer RNA.

Signal recognition particle receptor is important for cell growth and protein secretion in Saccharomyces cerevisiae

In mammalian cells, the signal recognition particle (SRP) receptor is required for the targeting of nascent secretory proteins to the endoplasmic reticulum (ER) membrane. We have identified the Saccharomyces cerevisiae homologue of the alpha-subunit of the SRP receptor (SR alpha) and characterized its function in vivo. S. cerevisiae SR alpha is a 69-kDa peripheral membrane protein that is 32% identical (54% chemically similar) to its mammalian homologue and, like mammalian SR alpha, is predicted to contain a GTP binding domain. Yeast cells that contain the SR alpha gene (SRP101) under control of the GAL1 promoter show impaired translocation of soluble and membrane proteins across the ER membrane after depletion of SR alpha. The degree of the translocation defect varies for different proteins. The defects are similar to those observed in SRP deficient cells. Disruption of the SRP101 gene results in an approximately sixfold reduction in the growth rate of the cells. Disruption of the gene encoding SRP RNA (SCR1) or both SCR1 and SRP101 resulted in an indistinguishable growth phenotype, indicating that SRP receptor and SRP function in the same pathway. Taken together, these results suggest that the components and the mechanism of the SRP-dependent protein targeting pathway are evolutionarily conserved yet not essential for cell growth. Surprisingly, cells that are grown for a prolonged time in the absence of SRP or SRP receptor no longer show pronounced protein translocation defects. This adaptation is a physiological process and is not due to the accumulation of a suppressor mutation. The degree of this adaptation is strain dependent.

Protein targeting to and translocation across the membrane of the endoplasmic reticulum

Several approaches are currently being taken to elucidate the mechanisms and the molecular components responsible for protein targeting to and translocation across the membrane of the endoplasmic reticulum. Two experimental systems dominate the field: a biochemical system derived from mammalian exocrine pancreas, and a combined genetic and biochemical system employing the yeast, Saccharomyces cerevisiae. Results obtained in each of these systems have contributed novel, mostly non-overlapping information. Recently, much effort in the field has been dedicated to identifying membrane proteins that comprise the translocon. Membrane proteins involved in translocation have been identified both in the mammalian system, using a combination of crosslinking and reconstitution approaches, and in S. cerevisiae, by selecting for mutants in the translocation pathway. None of the membrane proteins isolated, however, appears to be homologous between the two experimental systems. In the case of the signal recognition particle, the two systems have converged, which has led to a better understanding of how proteins are targeted to the endoplasmic reticulum membrane.

Fluorescence-detected assembly of the signal recognition particle: binding of the two SRP protein heterodimers to SRP RNA is noncooperative

Protein-RNA and protein-protein interactions involved in the assembly of the signal recognition particle (SRP) were examined using fluorescence spectroscopy. Fluorescein was covalently attached to the 3'-terminal ribose of SRP RNA following periodate oxidation, and the resulting SRP RNA-Fl was reconstituted into a fluorescent SRP species that was functional in promoting translocation of secretory proteins across the membrane of the endoplasmic reticulum. Each of the two protein heterodimers purified from SRP elicited a substantial change in fluorescein emission upon association with the modified RNA. The binding of SRP9/14 to singly-labeled SRP RNA-Fl increased fluorescein emission intensity by 41% at pH 7.5 and decreased its anisotropy from 0.18 to 0.16. The binding of SRP68/72 increased the fluorescein anisotropy from 0.18 to 0.23 but did not alter the emission intensity of SRP RNA-Fl. These fluorescence changes did not result from a direct interaction between the dye and protein because the fluorescein remained accessible to both iodide ions and fluorescein-specific antibodies in the complexes. The spectral changes were elicited by specific SRP RNA-protein interactions, since (i) the SRP9/14- and SRP68/72-dependent changes were unique, (ii) an excess of unlabeled SRP RNA, but not of tRNA, blocked the fluorescence changes, and (iii) no emission changes were observed when SRP RNA-Fl was titrated with other RNA-binding proteins. Each heterodimer bound tightly to the RNA, since the Kd values determined spectroscopically and at equilibrium for the SRP9/14 and the SRP68/72 complexes with SRP RNA-Fl were less than 0.1 and 7 +/- 3 nM, respectively. The binding affinity of SRP68/72 for SRP RNA-Fl was unaffected by the presence of SRP9/14, and hence the binding of the heterodimers to SRP RNA is noncooperative in the absence of SRP54 and SRP19. The SRP protein heterodimers therefore associate randomly and independently with SRP RNA to form domains in the particle that are distinct both structurally and functionally. Any cooperativity in SRP assembly would have to be mediated by SRP54 and/or SRP19.

Binding of the yeast tRNA(Met) anticodon by the cognate methionyl-tRNA synthetase involves at least two independent peptide regions

As for Escherichia coli methionine tRNAs, the anticodon triplet of yeast tRNA(Met) plays an important role in the recognition by the yeast methionyl-tRNA synthetase (MetRS), indicating that this determinant for methionine identity is conserved in yeast. Efficient aminoacylation of the E. coli tRNA(Met) transcript by the heterologous yeast methionine enzyme also suggests conservation of the protein determinants that interact with the CAU anticodon sequence. We have analysed by site-directed mutagenesis the peptide region 655 to 663 of the yeast MetRS that is equivalent to the anticodon binding region of the E. coli methionine enzyme. Only one change, converting Leu658 into Ala significantly reduced tRNA aminoacylation. Semi-conservative substitutions of L658 allow a correlation to be drawn between side-chain volume of the hydrophobic residue at this site and activity. The analysis of the L658A mutant shows that Km is mainly affected. This suggests that the peptide region 655 to 663 contributes partially to the binding of the anticodon, since separate mutational analysis of the anticodon bases shows that kcat is the most critical parameter in the recognition of tRNA(Met) by the yeast synthetase. We have analysed the role of peptide region (583-GNLVNR-588) that is spatially close to the region 655 to 663. Replacements of residues N584 and R588 reduces significantly the kcat of aminoacylation. The peptide region 583-GNLVNR-588 is highly conserved in all MetRS so far sequenced. We therefore propose that the hydrogen donor/acceptor amino acid residues within this region are the most critical protein determinants for the positive selection of the methionine tRNAs.