Maturation of the large subunit (HYCE) of Escherichia coli hydrogenase 3 requires nickel incorporation followed by C-terminal processing at Arg537

Purification of the large subunit, HYCE, of Escherichia coli hydrogenase 3 revealed that it is a nickel-containing polypeptide, which is subject to C-terminal proteolytic processing. This processing reaction could be performed in vitro with partially purified components, yielding a low-molecular mass C-terminal peptide which was resolved in a Tricine/SDS/polyacrylamide gel. N-terminal sequencing of this peptide revealed that proteolytic cleavage occurred at the C-terminal side of the arginine residue at position 537, which corresponds to the histidine residue in the highly conserved motif, DPCXXCXXH, of other (NiFe) hydrogenases thought to be involved in active site nickel coordination. Nickel-containing HYCE precursor for in vitro processing, was partially purified from strain HD708 (delta hycH) in the presence of the reducing agent dithiothreitol. Using 2-mercaptoethanol instead of dithiothreitol provided pure precursor, which was, however, no longer susceptible to in vitro processing; it proved to be devoid of nickel indicating that nickel incorporation into the HYCE precursor is a prerequisite for processing. This conclusion was supported by the finding that HYCE precursor from strain HD708 (delta hycH) chromatographed with radioactivity from 83Ni incorporated in vivo and could be processed in vitro, whereas HYCE precursor from strain BEF314 (delta hypB-E) lacking the nickel insertion system appeared to be devoid of nickel and was not sensitive to in vitro processing.

Recognition of the mRNA selenocysteine insertion sequence by the specialized translational elongation factor SELB

In Escherichia coli the unusual amino acid selenocysteine is incorporated cotranslationally at an in-frame UGA codon. Incorporation of selenocysteine relies, in part, on the interaction between a specialized elongation factor, the SELB protein, and a cis-acting element within the mRNA. Boundary and toeprint experiments illustrate that the SELB-GTP-Sec-tRNA(Sec) ternary complex binds to the selenoprotein encoding mRNAs fdhF and fdnG, serving to increase the concentration of SELB and Sec-tRNA(Sec) on these mRNAs in vivo. Moreover, toeprint experiments indicate that SELB recognizes the ribosome-bound message and that, upon binding, SELB may protrude out of the ribosomal-mRNA track so as to approach the large ribosomal subunit. The results place the mRNA-bound SELB-GTP-Sec-tRNA(Sec) ternary complex at the selenocysteine codon (as expected) and suggest a mechanism to explain the specificity of selenocysteine insertion. Cis-acting mRNA regulatory elements can tether protein factors to the translation complex during protein synthesis.

[Monosymptomatic familial Mediterranean fever as the cause of fever of unknown origin]

A previously healthy 2 year old female child developed fever of unknown origin recurring in monthly cycles. The periodic fever attacks, family history and ethnologic criteria were in agreement with familial mediterranean fever, although further more major symptoms were missing. It was highly unusual to find repeatedly raised levels of angiotensin I converting enzyme, a finding previously not described in literature. Excluding any other differential diagnosis by intensive investigations, together with a positive metaraminol provocation test, the diagnosis of a rare, monosymptomatic variant of familial mediterranean fever was proposed. Amyloidosis was excluded by rectal biopsy. Monosymptomatic familial mediterranean fever is very seldom. We suggest to measure routinely angiotensin I converting enzyme for further evaluation of our findings.

Molecular basis of the exclusive low-temperature synthesis of an enzyme in E. coli: penicillin acylase

The enzyme penicillin acylase is synthesized by Escherichia coli only at growth temperatures below 30 degrees C. The biochemical basis of this strict temperature-dependent formation of an enzyme was investigated. When the gene (pac) was under the control of the lacUV5 promoter it showed the same temperature-dependent expression as the chromosomally encoded gene transcribed from its own promoter. This indicates that translation of the pac mRNA rather than transcription of the gene is temperature-dependent. This conclusion could be further confirmed by Northern hybridisation and by analysis of pac-lacZ transcriptional fusions. TnphoA insertion mutagenesis and experiments in which the promoter and 5' sequence encoding the signal peptide of the pac gene was exchanged with those of the cyclodextrin glycosyltransferase gene from Klebsiella oxytoca localised the region of pac mRNA responsible for the temperature-sensitive translation to the 5'-untranslated region and/or the signal peptide. Extension of the 5 nucleotide long spacer separating the Shine-Dalgarno motif from the AUG initiation codon by one or three nucleotides lead to partial or full synthesis of penicillin acylase precursor at 40 degrees C, respectively. The precursor of penicillin acylase formed at 40 degrees C by the mutant variants or when placed under the control of a heterologous upstream region was associated with the membrane but could not be translocated. Taken together these data suggest that transport and translation of the penicillin acylase precursor are coupled and that the short Shine-Dalgarno-AUG distance interferes with a competent interaction between the translation initiation complex and the export system at high temperature. Moreover, evidence was also provided which indicates a direct effect of temperature on the conformation of the precursor and it is proposed that the lack of translation at high temperatures has been selected to prevent the accumulation of transport-incompetent protein locked in the membrane.

Solution structure of selenocysteine-inserting tRNA(Sec) from Escherichia coli. Comparison with canonical tRNA(Ser)

Selenocysteine-inserting tRNAs (or tRNA(Sec)) are structurally untypical tRNAs that are charged by seryl-tRNA synthetase before being recognized by the selenocysteine synthase that converts serine into selenocysteine. tRNA(Sec) from Escherichia coli contains 95 nucleotides and is the longest tRNA known to date, in contrast to canonical tRNA(Ser), 88 nucleotides-long. We have studied its solution conformation by chemical and enzymatic probing. Global structural features were obtained by cobra venom and S1 nuclease mapping, as well as by probing with Pb2+. Accessibilities of phosphate groups were measured by ethylnitrosourea probing. Information about positions in bases involved in Watson-Crick pairing, in stacking or in tertiary interactions were obtained by chemical probing with dimethylsulfate, diethylpyrocarbonate, kethoxal and carbodiimide. On the basis of these chemical data, a three-dimensional model was constructed by computer modeling and compared to that of canonical tRNA(Ser). tRNA(Sec) resembles tRNA(Ser) at the level of its T-arm and anticodon-arm conformations, as well as at the joining of the D- and T-loops by a tertiary Watson-Crick G19-C56 interaction. Its extra-long variable arm is a double-stranded structure closed by a four nucleotide loop that is linked to the body of the tRNA in a way different from that found in tRNA(Ser). As anticipated from the peculiar features of the sequence in the D-loop and at the junction of amino acid and D-arms, tRNA(Sec) possesses a novel but restricted set of tertiary interactions in the core of its three-dimensional structure: a G8-A21-U14 triple pair and a novel interaction between C16 of the D-loop and C59 of the T-loop. A third triple interaction involving C15-G20a-G48 is suggested but some experimental evidence for it is still lacking. It is furthermore concluded that the D-arm has six base-pairs instead of three, as in canonical class II tRNA(Ser), with the D-loop containing only four nucleotides. Finally, the amino acid accepting arm forms a stack of eight Watson-Crick base-pairs (instead of 7 in other tRNAs). The biological relevance of this model with regard to interaction with seryl-tRNA synthetase and enzymes from the selenocysteine metabolism is discussed.

Interaction of translation factor SELB with the formate dehydrogenase H selenopolypeptide mRNA

The SELB protein from Escherichia coli is a specialized elongation factor required for the UGA-directed insertion of the amino acid selenocysteine into selenopolypeptides. Discrimination of the UGA codon requires the presence of a recognition element within the mRNA, which is located at the 3' side of the UGA codon; a hairpin structure can be formed within this mRNA region. By gel shift assays, a specific interaction between SELB and the mRNA recognition element could be demonstrated. Footprinting experiments, using nucleases or iodine as cleaving agents, showed that SELB binds to the loop region of the hairpin structure. In the presence of selenocysteinyl-tRNA, SELB formed a complex with the charged tRNA and the mRNA. The results indicate that targeted insertion of selenocysteine is accomplished by the binding of the SELB protein to this mRNA recognition element, resulting in the formation of a selenocysteinyl-tRNA.SELB complex at the mRNA in the immediate neighborhood of the UGA codon.

The product of the hypB gene, which is required for nickel incorporation into hydrogenases, is a novel guanine nucleotide-binding protein

The products of the hyp operon genes are essential for the formation of catalytically active hydrogenases in Escherichia coli. At least one of these auxiliary proteins, HYPB, appears to be involved in nickel liganding to the hydrogenase apoprotein, since mutations in hypB can be phenotypically suppressed by high nickel concentrations in the medium (R. Waugh and D. H. Boxer, Biochimie 68:157-166, 1986). To approach the identification of the specific function of HYPB, we overexpressed the hypB gene and purified and characterized the gene product. HYPB is a homodimer of 31.6-kDa subunits, and it binds guanine nucleotides, with a Kd for GDP of 1.2 microM. The protein displays a low level of GTPase activity, with a kcat of 0.17 min-1. The apparent Km for GTP, as measured in the GTP hydrolysis reaction, was determined to be 4 microM. A chromatography system was established to measure nickel insertion into hydrogenase 3 from E. coli and to determine the effects of lesions in hypB. Nickel appears to be associated only with the processed large subunit of hydrogenase 3 in the wild type, and hypB mutants accumulate the precursor form of this subunit, which is devoid of nickel. The results are discussed in terms of a model in which HYPB is involved in nickel donation to the hydrogenase apoprotein and in which GTP hydrolysis is thought to reverse the interaction between either HYPB or another nickel-binding protein and the hydrogenase apoprotein after the nickel has been released.

Structure of selenocysteine synthase from Escherichia coli and location of tRNA in the seryl-tRNA(sec)-enzyme complex

Selenocysteine synthase of Escherichia coli catalyses the biosynthesis of selenocysteine in the form of the aminoacyl-tRNA complex, the reaction intermediate being aminoacrylyl-tRNA(sec) covalently bound to the prosthetic group of the enzyme. Selenocysteine synthase and the specific aminoacrylyl-tRNA(sec)-enzyme complex as well as the isolated seryl-tRNA(sec) were investigated in the electron microscope and analysed by means of image processing to a resolution of 2 nm in projection. The stoichiometric composition of the selenocysteine synthase molecule was elucidated by scanning transmission electron microscopic mass determination. The enzyme has a fivefold symmetric structure and consists of 10 monomers arranged in two rings. The tRNA is bound near the margin of the dimeric subunits. Principal component analysis of the tRNA-enzyme complexes revealed that the selenocysteine synthase appears to bind only one seryl-tRNA(sec) per dimer, which is consistent with the result of biochemical binding studies.

The hyp operon gene products are required for the maturation of catalytically active hydrogenase isoenzymes in Escherichia coli

The hyp operon of Escherichia coli comprises several genes which are required for the synthesis of all three hydrogenase isoenzymes. Deletions were introduced into each of the hypA-E genes, transferred to the chromosome and the resulting mutants were analysed for hydrogenase 1, 2 and 3 activity. The products of three of the genes, hypB, hypD and hypE were found to be essential for the synthesis of all three hydrogenase isoenzymes. A defect in hypB, as previously observed, could be complemented by high nickel concentrations in the medium, whereas the effects of mutants in the other genes could not. Lesions in hypA prevented development of hydrogenase 3 activity, did not influence the level of hydrogenase 1 but led to a considerable increase in hydrogenase 2 activity although the amount of hydrogenase 2 protein was not drastically altered. Lesions in hypC, on the other hand, led to a reduction of hydrogenase 1 activity and abolished hydrogenase 3 activity. HYPA and HYPC, besides being required for hydrogenase 3 formation, therefore may have a function in modulating the activities of the three isoenzymes with respect to each other and adjusting their levels to the requirement imposed by the physiological situation. Mutations in all five hyp genes prevented the apparent processing of the large subunits of all three hydrogenase isoenzymes. It is concluded that the products of the hypA-E genes play a role in nickel incorporation into hydrogenase apoprotein and/or processing of the constituent subunits of this enzyme. The importance of their roles is also reflected in their phylogenetic conservation in distantly related organisms.

Coding from a distance: dissection of the mRNA determinants required for the incorporation of selenocysteine into protein

Incorporation of selenocysteine into proteins is directed by specifically 'programmed' UGA codons. The determinants for recognition of the selenocysteine codon have been investigated by analysing the effect of mutations in fdhF, the gene for formate dehydrogenase H of Escherichia coli, on selenocysteine incorporation. It was found that selenocysteine was also encoded when the UGA codon was replaced by UAA and UAG, provided a proper codon-anticodon interaction was possible with tRNA(Sec). This indicates that none of the three termination codons can function as efficient translational stop signals in that particular mRNA position. The discrimination of the selenocysteine 'sense' codon from a regular stop codon has previously been shown to be dependent on an RNA secondary structure immediately 3' of the UGA codon in the fdhF mRNA. It is demonstrated here that the correct folding of this structure as well as the existence of primary sequence elements located within the loop portion at an appropriate distance to the UGA codon are absolutely required. A recognition sequence can be defined which mediates specific translation of a particular codon inside an mRNA with selenocysteine and a model is proposed in which translation factor SELB interacts with this recognition sequence, thus forming a quaternary complex at the mRNA together with GTP and selenocysteyl-tRNA(Sec).

Prokaryotic polyprotein precursors

Polyproteins have been found only recently in prokaryotes. The four known examples of single bacterial genes encoding precursors that are posttranslationally processed into two mature proteins are addressed here with respect to (i) their genomic arrangement, (ii) the sites of proteolytic processing, (iii) the relevant proteases, (iv) their maturation pathway, and (v) the function of the mature proteins. How these polyproteins may have evolved is also discussed.

Selenoprotein synthesis in E. coli. Purification and characterisation of the enzyme catalysing selenium activation

The product of the selD gene from Escherichia coli catalyses the formation of an activated selenium compound which is required for the synthesis of Sec-tRNA (Sec, selenocysteine) from Ser-tRNA and for the formation of the unusual nucleoside 5-methylaminomethyl-2-selenouridine in several tRNA species. selD was overexpressed in a T7 promoter/polymerase system and purified to apparent homogeneity. Purified SELD protein is a monomer of 37 kDa in its native state and catalyses a selenium-dependent ATP-cleavage reaction delivering AMP and releasing the beta-phosphate as orthophosphate. The gamma-phosphate group of ATP was not liberated in a form able to form a complex with molybdate. It was precluded that any putative covalent or non-covalent ligand of SELD not removed during purification participated in the reaction. In a double-labelling experiment employing [75Se]selenite plus dithiothreitol and [gamma-32P]ATP the 75Se and 32P radioactivities co-chromatographed on a poly(ethyleneimine)-cellulose column. No radioactivity originating from ATP eluted in this position when [alpha-32P]ATP or [beta-32P]ATP or [14C]ATP were offered as substrates. The results support the speculation that the product of SELD is a phosphoselenoate with the phosphate moiety derived phosphoselenoate from the gamma-phosphate group of ATP. The alpha,beta cleavage of ATP is also supported by the finding that neither adenosine 5'-[alpha,beta-methylene]triphosphate nor adenosine 5'-[beta,gamma-methylene]triphosphate served as substrates in the reaction.

Mutational analysis of the operon (hyc) determining hydrogenase 3 formation in Escherichia coli

In-frame deletions were introduced into each of the eight genes of the hyc operon coding for products required for the formation of the formate hydrogenlyase (FHL) system. The deletions were transferred to the chromosome and the resulting mutants were analysed for development of formate dehydrogenase H and hydrogenase 1, 2 and 3 activity. It was found that hycA, the promoter-proximal gene, is a regulatory gene and that it codes for a product counteracting transcriptional activation by FhlA. Deletions within the hycB to hycH genes specifically affected formate dehydrogenase H activity or hydrogenase 3 activity, or both. None of the mutations affected hydrogenase 1 or 2 activity. A model is proposed for the functional interaction of the different hyc operon gene products in the formate hydrogenlyase complex, which is based on the results of the mutational analysis, on the determination of the subcellular localization of the FdhF, HycE, HycF and HycG polypeptides and on the similarity of hyc gene product sequences with those from other hydrogenase systems. HycH, the product of the most promoter-distal gene, does not seem to form part of the functional FHL complex but rather is required for the conversion of a precursor form of the large subunit of hydrogenase 3 into the mature form.

Pediatric and perinatal reference intervals for immunoglobulin light chains kappa and lambda

In routine analysis for immunoglobulin light chains in pediatric diagnostics, the age-related reference intervals for serum kappa (kappa) and lambda (lambda) light chains were evaluated in 1543 healthy subjects (newborns to age 16 years, including 168 premature infants). Light-chain analysis was performed by rate nephelometry. IgG, IgA, and IgM were measured simultaneously, and heavy- and light-chain differences were calculated for control purposes. Results for IgG, IgA, and IgM generally agreed with reference intervals reported in the literature. kappa showed age-related changes comparable with changes in IgG concentrations, whereas lambda showed moderate fluctuations. The kappa/lambda ratio showed an almost linear increase with age, starting with 0.97 at four months and reaching the highest value of 2.21 at 15 years (mean values). Preterm infants presented with markedly low serum concentrations of IgG and corresponding light chains but with adult-type kappa/lambda ratios because of the maternal-origin IgG.

Pediatric and Perinatal Reference Intervals for Immunoglobulin Light Chains Kappa and Lambda

In routine analysis for immunoglobulin light chains in pediatric diagnostics, the age-related reference intervals for serum kappa (kappa) and lambda (lambda) light chains were evaluated in 1543 healthy subjects (newborns to age 16 years, including 168 premature infants). Light-chain analysis was performed by rate nephelometry. IgG, IgA, and IgM were measured simultaneously, and heavy- and light-chain differences were calculated for control purposes. Results for IgG, IgA, and IgM generally agreed with reference intervals reported in the literature. kappa showed age-related changes comparable with changes in IgG concentrations, whereas lambda showed moderate fluctuations. The kappa/lambda ratio showed an almost linear increase with age, starting with 0.97 at four months and reaching the highest value of 2.21 at 15 years (mean values). Preterm infants presented with markedly low serum concentrations of IgG and corresponding light chains but with adult-type kappa/lambda ratios because of the maternal-origin IgG.

Targeted insertion of selenocysteine into the alpha subunit of formate dehydrogenase from Methanobacterium formicicum

Selenocysteine incorporation into proteins is directed by an opal (UGA) codon and requires the existence of a stem-loop structure in the mRNA flanking the UGA at its 3' side. To analyze the sequence and secondary-structure requirements for UGA decoding, we have introduced mutations into the fdhA gene from Methanobacterium formicicum, which codes for the alpha subunit of the F420-reducing formate dehydrogenase. The M. formicicum enzyme contains a cysteine residue at the position where the Escherichia coli formate dehydrogenase H carries a selenocysteine moiety. The codon (UGC) for this cysteine residue was changed into a UGA codon, and mutations were successively introduced at the 5' and 3' sides to generate a stable secondary structure of the mRNA and to approximate the sequence of the predicted E. coli fdhF mRNA hairpin structure. It was found that introduction of the UGA and generation of a stable putative stem-loop structure were not sufficient for decoding with selenocysteine. Efficient selenocysteine incorporation, however, was obtained when the loop and the immediately adjacent portion of the putative stem had a sequence identical to that present in the E. coli fdhF mRNA structure.

Influences of the cellular and humoral immune system in bronchoalveolar lavage on lung function in pulmonary sarcoidosis

We investigated the changes in the cellular and humoral immune system in bronchoalveolar lavage (BAL) performed in 22 patients with pulmonary sarcoidosis and in 14 normal control subjects and their interactions with lung function parameters. Lymphocytosis, the increase in OKT4+ lymphocytes and OKT4+OKDR+ lymphocytes correlated with the increase in immunoglobulins, especially IgG, IgA and kappa chain assembled immunoglobulins. The transferrin levels obtained in BAL were found to be higher in patients with sarcoidosis, and they correlated with the cellular and, more closely, with other humoral findings. A negative correlation existed between the ventilatory parameters and the cell count and humoral findings. In addition, we found a negative correlation between the diffusing capacity for carbon monoxide and other cellular findings, which was most pronounced with reference to lymphocytes, OKT4+ lymphocytes and the OKT4+/OKT8+ ratio. These results underscore the role of OKT4+ lymphocytes, activated OKT4+OKDR+ lymphocytes and transferrin in the increase in immunoglobulins, mainly kappa chain isotypes. Because of the relationship between these changes and ventilatory parameters, and the diffusing capacity, the above results also reveal the clinical relevance of our findings.

The function of selenocysteine synthase and SELB in the synthesis and incorporation of selenocysteine

The selAB operon codes for the proteins selenocysteine synthase and SELB which catalyse the synthesis and cotranslational insertion of selenocysteine into protein. This communication deals with the biochemical characterisation of these proteins and in particular with their specific interaction with the selenocysteine-incorporating tRNA(Sec). Selenocysteine synthase catalyses the synthesis of selenocysteyl-tRNA(Sec) from seryl-tRNA(Sec) in a pyridoxal phosphate-dependent reaction mechanism. The enzyme specifically recognizes the tRNA(Sec) molecule; a cooperative interaction between the tRNA binding site and the catalytically active pyridoxal phosphate site is suggested. SELB is an EF-Tu-like protein which specifically complexes selenocysteyl-tRNA(Sec). Interaction with the selenol group of the side chain of the aminoacylated residue is a prerequisite for the formation of a stable SELB.tRNA complex. Mechanistically, this provides the biochemical basis for the exclusive selection of selenocysteyl-tRNA(Sec) in the decoding step of a selenocysteine-specific UGA triplet.

Selenoprotein synthesis: an expansion of the genetic code

A number of enzymes employ the unusual amino acid selenocysteine as part of their active site because of its high chemical reactivity. Selenocysteine is incorporated into these proteins co-translationally: biosynthesis occurs on a specific tRNA and insertion into a growing polypeptide is directed by a UGA codon in the mRNA. In E. coli, this requires a specific translation factor. Selenocysteine thus represents a unique expansion of the genetic code.

Mechanism of regulation of the formate-hydrogenlyase pathway by oxygen, nitrate, and pH: definition of the formate regulon

The products of a minimum of 15 genes are required for the synthesis of an active formate-hydrogenlyase (FHL) system in Escherichia coli. All are co-ordinately regulated in response to variations in the oxygen and nitrate concentration and the pH of the culture medium. Formate is obligately required for transcriptional activation of these genes. Analysis of the transcription of one of these genes, hycB linked to the lacZ reporter gene, revealed that oxygen and nitrate repression of transcription could be relieved completely, or partially in the case of nitrate, either by the addition of formate to the medium or by increasing the copy number of the gene encoding the transcriptional activator (fhlA) of this regulon. These studies uncovered a further level of regulation in which the transcription of hycB was reduced in cells grown on glucose. This effect was most clearly seen in aerobically grown cells when formate was added externally. Addition of cAMP overcame this glucose repression, which could be shown to be mediated by the cAMP receptor protein. These results would be consistent with the transport of formate being regulated by catabolite repression. Moreover, the repression of transcription through high pH also could be partially overcome by addition of increasing concentrations of formate to the medium, again being consistent with regulation at the level of formate import and export. Taken together, all these observations indicate that it is the intracellular level of formate that determines the transcription of the genes of the formate regulon by FhlA. This represents a novel positive feedback mechanism in which the activator of a regulon induces its own synthesis in response to increases in the concentration of the catabolic substrate, and this in turn is governed by the relative affinities of FhlA and the three formate dehydrogenase isoenzymes for formate.

[Biology and biochemistry of selenium]

The importance of selenium as an essential trace element has progressively emerged during the last years due to the analysis of selenium deficiency diseases and to the identification and characterization of a number of selenoenzymes. Selenium is incorporated in the catalytic site of the enzymes as an integral selenocysteine residue. The pathway of selenocysteine biosynthesis and incorporation has been elucidated recently for Escherichia coli. This article presents an overview on these subjects and describes the mechanisms which confer selenocysteine specificity in the framework of protein biosynthesis. In addition, some considerations concerning the phylogeny of selenocysteine incorporation are presented and a model for the evolution of the selenocysteine pathway is proposed.

Effects of ozone on the respiratory health, allergic sensitization, and cellular immune system in children

To investigate the lasting effects of high ozone concentrations under environmental conditions, we examined the respiratory health, pulmonary function, bronchial hyperresponsiveness to methacholine, allergic sensitization, and lymphocyte subpopulations of 10- to 14-yr-old children. A total of 218 children recruited from an area with high ozone concentrations (Group A) were tested against 281 children coming from an area with low ozone concentrations (Group B). As to subjective complaints, categorized as "usually cough with or without phlegm," "breathlessness," and "susceptibility to chest colds," there was no difference between the two groups. The lung function parameters were similar, but in Group A subjects' bronchial hyperresponsiveness occurred more frequently and was found to be more severe than in Group B (29.4 versus 19.9%, p less than 0.02; PD20 2,100 +/- 87 versus 2,350 +/- 58 micrograms, p less than 0.05). In both groups the number of children who had been suffering from allergic diseases and sensitization to aeroallergens, found by means of the skin test, was the same. Comparison of the total IgE levels showed no difference at all between the two groups. As far as the white blood cells are concerned, the total and differential cell count was the same, whereas lymphocyte subpopulations showed readily recognizable changes.(ABSTRACT TRUNCATED AT 250 WORDS)

The length of the aminoacyl-acceptor stem of the selenocysteine-specific tRNA(Sec) of Escherichia coli is the determinant for binding to elongation factors SELB or Tu

Mutations in selC, which reduce the 8-base pair aminoacyl-acceptor helix to the canonical 7-base pair length (tRNA(Sec)(delAc] or which replace the extra arm of tRNA(Sec) by that of a serine acceptor tRNA species (tRNA(Sec)(ExS), block the function in selenoprotein synthesis in vivo (Baron, C., Heider, J., and Böck, A. (1990) Nucleic Acids Res. 18, 6761-6766). tRNA(Sec), tRNA(Sec)(delAc), and tRNA(Sec)(ExS) were purified and analyzed for their interaction with purified seryl-tRNA synthetase, selenocysteine synthase and translation factors SELB and EF-Tu. It was found that seryl-tRNA synthetase displays 10-fold impaired Km and Kcat values for tRNA(Sec) in comparison to tRNA(Ser), decreasing the overall charging efficiency (Kcat/Km) of tRNA(Sec) to 1% of that characteristic for tRNA(Ser). tRNA(Sec)(ExS) was a less efficient substrate for the enzyme (Kcat/Km 0.2% of the tRNA(Ser) value) whereas the tRNA(Ser)(delAc) variant was charged with an approximately 2-3-fold improved rate compared to wild-type tRNA(Sec). Both mutant tRNA variants, when charged with L-serine, were able to interact with selenocysteine synthase to give rise to selenocysteyl-tRNA with tRNA(Sec)(ExS) being as efficient as wild-type tRNA(Sec). Seryl-tRNA(Sec)(delAc), on the other hand, was selenylated very slowly. Reduction of the length of the aminoacyl-acceptor stem to 7 base pairs prevented the interaction with translation factor SELB but allowed binding to EF-Tu, irrespective of whether tRNA(Sec)(delAc) was charged with serine or selenocysteine. The aminoacyl-acceptor helix of tRNA(Sec), therefore, is a major determinant directing binding to SELB and precluding interaction with EF-Tu.