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Events during eucaryotic rRNA transcription initiation and elongation: conversion from the closed to the open promoter complex requires nucleotide substrates.

Chemical footprinting and topological analysis were carried out on the Acanthamoeba castellanii rRNA transcription initiation factor (TIF) and RNA polymerase I complexes with DNA during transcription initiation and elongation. The results show that the binding of TIF and polymerase to the promoter does not alter the supercoiling of the DNA template and the template does not become sensitive to modification by diethylpyrocarbonate, which can identify melted DNA regions. Thus, in contrast to bacterial RNA polymerase, the eucaryotic RNA polymerase I-promoter complex is in a closed configuration preceding addition of nucleotides in vitro. Initiation and 3'-O-methyl CTP-limited translocation by RNA polymerase I results in separation of the polymerase-TIF footprints, leaving the TIF footprint unaltered. In contrast, initiation and translocation result in a significant change in the conformation of the polymerase-DNA complex, culminating in an unwound DNA region of at least 10 base pairs.

Acanthamoeba↗

Selective inhibition of precursor incorporation into ribosomal RNA in gamma-irradiated Tetrahymena pyriformis.

Sublethal doses of gamma radiation are known to inhibit total RNA synthesis in the ciliate protozoan Tetrahymena. To determine if the synthesis of a particular class of RNA is preferentially inhibited, pulse-labeled RNA was isolated from normal exponentially growing cells, irradiated cells, and cells in which total RNA synthesis had recovered to the pre-irradiation level. The RNAs were analyzed by SDS-polyacrylamide gel electrophoresis and oligo(dT)-cellulose column chromatography. Inhibition of RNA synthesis primarily involves ribosomal RNA. However, radiation does not cause a delay in the processing of precursor rRNA or a preferential loss of either of the mature rRNAs. Following irradiation, poly(A)-containing RNA [poly(A+)RNA] is synthesized at a rate up to three times greater than the control rate. The elevated poly(A+)RNA synthesis occurs during the period of depressed rRNA synthesis and even after rRNA synthesis has recovered to its pre-irradiation rate. While the sizes of the total cellular ribonucleoside triphosphate pools are depressed in the irradiated cells, these pools probably do not represent the actual compartments containing the precursors for RNA synthesis, and the observed changes cannot explain the modifications in macromolecular synthesis in irradiated Tetrahymena.

Gamma Rays↗

Methods used for the isolation, enumeration, characterisation and identification of Enterococcus spp. 2. Pheno- and genotypic criteria.

This paper reviews the methodology applied for the identification and characterisation of enterococci and covers phenotypic, genotypic and phylogenetic techniques. Although conventional phenotypic typing schemes are useful for rapid and simple identification of enterococcal species for routine applications, other methods like standardised sodium dodecylsulfate polyacrylamide gel electrophoresis (SDS-PAGE), multilocus enzyme electrophoresis (MLEE), antimicrobial susceptibility testing, serotyping, pyrolysis mass spectrometry (pyMS) and vibrational spectroscopic methods allow a more in-depth characterisation of enterococci. Many of the recently described enterococcal species exhibit deviations from hitherto so-called classical enterococci with regard to their phenotypical properties. Therefore, genotypic methods have to be used to clarify their possible assignment to the genus Enterococcus. In this review, special emphasis is given on recently developed polymerase chain reaction (PCR)-based typing methods such as random amplified polymorphic DNA (RAPD), amplified fragment length polymorphism (AFLP), specific and random amplification (SARA) and modifications of PCR-ribotyping as well as pulsed-field gel electrophoresis (PFGE) and partial sequence analysis. The use of PCR and probes for genus and species identification of enterococci is also considered like the application of sequence data of conserved DNA regions (e.g., ribosomal ribonucleic acid (rRNA) genes) in the case of species identification.

Bacterial Typing Techniques↗

Interaction of Cibacron blue F3GA and polynucleotides with ricin A-chain, 60 S ribosomal subunit-inactivating protein.

Cibacron blue F3GA, a sulfonated polyaromatic blue dye, inhibited the ability of ricin A-chain to inactivate ribosomes. Difference-spectroscopic study revealed that the dye bound to the A-chain (Kd = 0.72 microM), producing a difference spectrum with a single maximum at 688 nm and two minima at 585 and 628 nm. Such a significant difference spectrum was not observed in the presence of ricin B-chain or intact ricin, neither of which can inactivate ribosomes. Modification of arginine residues in the A-chain with phenylglyoxal showed a correlation between the loss of inhibitory activity on protein synthesis and the loss of difference absorbance produced by the dye-A-chain interaction. Both losses occurred significantly at an early stage of the modification. Furthermore, the dye protected the A-chain against a loss of its inhibitory activity resulting from the modification of arginine residues. These results suggest that the same arginine residues participate both in the interaction with the dye and in the inactivation of ribosomes. Based on these data, the dye appears to interact with the active site of the A-chain. Addition of several polynucleotides, namely rRNA, tRNA, poly(U) and DNA, to the dye-A-chain complex resulted in a marked displacement of the dye, whereas mono- and dinucleotides had little or no effect on the dye-A-chain interaction. These findings indicate the possible existence of a polynucleotide binding site in the active site of the A-chain. A combination of these and other results suggests that the A-chain recognizes and acts on some part of RNA of the 60 S ribosomal subunit.

Animals↗

Aminoglycosides modified by resistance enzymes display diminished binding to the bacterial ribosomal aminoacyl-tRNA site.

Understanding the basic principles that govern RNA binding by aminoglycosides is important for the design of new generations of antibiotics that do not suffer from the known mechanisms of drug resistance. With this goal in mind, we examined the binding of kanamycin A and four derivatives (the products of enzymic turnovers of kanamycin A by aminoglycoside-modifying enzymes) to a 27 nucleotide RNA representing the bacterial ribosomal A site. Modification of kanamycin A functional groups that have been directly implicated in the maintenance of specific interactions with RNA led to a decrease in affinity for the target RNA. Overall, the products of reactions catalyzed by aminoglycoside resistance enzymes exhibit diminished binding to the A site of bacterial 16S rRNA, which correlates well with a loss of antibacterial ability in resistant organisms that harbor these enzymes.

Anti-Bacterial Agents↗

Detection of potentially novel bacterial components of the human skin microbiota using culture-independent molecular profiling.

Although the micro-organisms forming the cutaneous microbiota are considered to play important roles in the modification and prevention of skin diseases, a comprehensive analysis of their composition has not yet been carried out because of difficulties in determining yet-to-be-cultured micro-organisms in the samples. Swab-scrubbed forehead skin samples of five healthy volunteers were analysed by profiling 16S rRNA genes, as well as by conventional culture methods, to provide a profile of the cutaneous microbiota that included yet-to-be-cultured bacteria from normal human skin. Cluster analyses of the 16S rRNA gene sequences indicated a marked increase in diversity compared with that derived from the culture methods. Nineteen previously recognized species and 13 novel phylotypes were obtained from the analysis of 416 clones. In addition to well-known bacteria such as Staphylococcus epidermidis and Propionibacterium acnes, phylotype A, the 16S rRNA gene of which is 97 % similar to that of Methylophilus methylotrophus, was detected in three of the five samples, in one of which it was the predominant clone. Culture-independent genetic profiling of 16S rRNA genes for detecting human cutaneous microbiota has allowed us to detect potentially novel components of the cutaneous microbiota in humans.

Bacteria↗

[Affinity modification of 40S ribosomal subparticles from human placenta with mRNA analogs--AUGUnC oligoribonucleotide analogs with an alkylating group at the 3'-end].

Using 2',3'-O-[4-N-(2-chloroethyl)-N-methylamino]benzylidene derivatives of AUGUn[32P]pC (mRNA analogues), affinity labelling of human placenta 40S ribosomal subunits has been investigated in model initiation complexes obtained in the presence of the ternary complex eIF-2.GTP.Met-tRNA(fMet). The regions of 18S rRNA labelled with these mRNA analogues were identified. The main targets of 18S rRNA alkylation by the derivative of AUG[32P]pC were located within positions 1610-1747 and 1748-1869. The site of covalent attachment of AUGU3[32P]pC derivative to 18S rRNA was found within positions 593-673. Taking into account the data on labelling of human placenta ribosomes with the same derivatives of oligourydilates obtained previously, the conclusion was made that the arrangement of the codon U3 in the mRNA-binding centre of the initiation complex 40S.AUGU3[32P]pC derivative.eIF-2.GTP.Met-TPHK(fMet) differs from the arrangement of the same codon at the A-site of the complex imitating the pretranslocation state of ribosomes.

Affinity Labels↗

Signals determining translational start-site recognition in eukaryotes and their role in prediction of genetic reading frames.

A special methionyl-tRNA (RNAi) is universally required to initiate translation. The conversation of this reactant throughout evolution, as well as its unusual decoding properties, suggested an alternate mechanism for tRNA-mRNA interactions at initiation. We have reported that the sequence of bases neighboring the start codons of many eubacterial genes are complementary not only to the 16S rRNA 3' end and to the anticodon of tRNAi, but, also, have the potential to base-pair the D, T or extended anticodon loops of this tRNAi. The coding properties of tRNAi and mutations that affect translation suggest that these signals may function. This hypothesis explains the observation that unusual triplets can start prokaryotic and mitochondrial genes and predicts the occurrence of other reading frames. Furthermore, it suggests a unifying model of chain initiation based on RNA-RNA contacts and displacements. Here we examine the start domain of 290 eukaryotic genes for their ability to base-pair the tRNAi loops and the 18S rRNA. We observe that both methionine start, and methionine coding regions have the potential to pair with the 18S rRNA, but that the nucleotide distribution about start codons strongly favoured such pairings over that near internal AUGs. The 5' extended anticodon of tRNAi is methylated, and was not represented in the mRNA with high frequency. However, the tetramer AUGg did occur with high frequency in the start domain. A modification of the tRNAi T loop also decreases its base-pairing potential. Interestingly, complementarity to the T loop did not occur with high frequency in the start sites. The early coding region, 10 to 34 nucleotides 3' to the initiator AUG, is complementary to the tRNAi D loop in many cases, while no such affinity is found near internal AUGs. The nucleotides around initiator AUGs were heavily biassed toward the sequence gccaccAUGgcg. No such tendency was noted around internal AUGs. Although the role of this sequence bias is unclear, the sequence gccaccAUGg has been shown by Kozak to promote initiation. Another distinguishing feature was a C-rich tract 7 to 34 nucleotides 5' to the initiator AUGs. Ability to pair with more than eight bases of the start consensus sequence, matching of 6 or 7 nucleotides to the D loop on the 3' side, an C-richness on the 5' side were used as criteria for distinguishing start AUGs.(ABSTRACT TRUNCATED AT 400 WORDS)

Animals↗

Bactericidal antisense effects of peptide-PNA conjugates.

Antisense peptide nucleic acids (PNAs) can specifically inhibit Escherichia coli gene expression and growth and hold promise as anti-infective agents and as tools for microbial functional genomics. Here we demonstrate that chemical modification improves the potency of standard PNAs. We show that 9- to 12-mer PNAs, especially when attached to the cell wall/membrane-active peptide KFFKFFKFFK, provide improvements in antisense potency in E. coli amounting to two orders of magnitude while retaining target specificity. Peptide-PNA conjugates targeted to ribosomal RNA (rRNA) and to messenger RNA (mRNA) encoding the essential fatty acid biosynthesis protein Acp prevented cell growth. The anti-acpP PNA at 2 microM concentration cured HeLa cell cultures noninvasively infected with E. coli K12 without any apparent toxicity to the human cells. These results indicate that peptides can be used to carry antisense PNA agents into bacteria. Such peptide-PNA conjugates open exciting possibilities for anti-infective drug development and provide new tools for microbial genetics.

Amino Acid Sequence↗

RNA methylation and control of eukaryotic RNA biosynthesis. Effects of cycloleucine, a specific inhibitor of methylation, on ribosomal RNA maturation.

The role of RNA methylation in the control of ribosome production in mammalian cells has been reinvestigated through a study of the effects in vivo of cycloleucine, a specific and reversible inhibitor of nucleic acid methylations. No close coupling is observed between methylation and transcription. During an extensive blocking of methylation, the synthesis of preribosomal 45-S RNA continues, although at a slightly reduced rate. Transcription and methylation can be temporarily uncoupled in vivo without impairing significantly the efficiency of the subsequent maturation of the transcript which takes place when the methylation is resumed. At the post-transcriptional level, two main observations are made. First, the cleavage pattern of ribosomal RNA is not qualitatively modified by the drug treatment. Preribosomal 45-S RNA which is synthesized during an extensive blocking of methylations (95%) is cleaved in vivo in a stepwise fashion into molecules resembling the species of the normal processing. Undermethylation does not result in the appearance of new size products and no extensive or rapid degredation of rRNA precursors can be detected. Second, the global efficiency of the process of ribosomal RNA maturation is severely affected, in quantitative terms; the undermethylation partially inhibits several stages in the maturation pathway and does not block selectively at a definite step of processing, as previously reported by others. Two main modifications are observed. (a) In the nucleus, the life times of the various undermethylated intermediates of ribosomal RNA maturation are markedly increased and a significant accumulation of these forms is detected by long-term labelling studies. (b) The rate of appearance of ribosomal 28-S RNA into the cytoplasm is severely affected (85-90% inhibition), in much higher proportions than the formation of its immediate precursor, nuclear 28-S RNA, thus suggesting a particular sensitivity to a lack of methylation of the last stage of preribosome maturation (possibly the release to cytoplasm). Finally, if a normal level of methylation does not appear to be stringently required for the completion of ribosomal RNA maturation, our results indicate that the degree of RNA methylation can modulate the general efficiency of the maturation processess.

Amino Acids↗

Proteolytic and chemical modification of colicin E3 activity.

Proteolyses of colicin E3 by both trypsin and subtilisin yield fragments of various molecular weights. On the basis of sodium dodecyl sulfate gel electrophoresis, tryptic cleavage yields peptides of molecular weight about 42 000 and 18 000, while the comparable pieces in a subtilisin digest have apparent weights of about 36 000 and 24 000. The digests lose almost all of their in vivo cell killing activity but the in vitro activity leading to ribosomal inactivation is augmented. Trypsin-treated colicin E3 shows a 20-30-fold increase in its ability to release the 52 nucleotide fragment from the 16S ribosomal ribonucleic acid (rRNA) and this activity is associated with the smaller fragment. Subtilisin-treated colicin E3 is only about two to three fold more active than the native protein in vitro, and the peptides obtained upon cleavage cannot be separated by gel filtration or polyacrylamide gel electrophoresis without sodium dodecyl sulfate. However, in the presence of 0.1% sodium dodecyl sulfate, subtilisin-treated E3 shows a 20-30-fold augmentation in in vitro activity which is again associated with the smaller fragment extracted from the sodium dodecyl sulfate gel. Amino terminal end-group studies showed that the two larger fragments and intact E3 have the same N-terminal residue, valine. These fragments presumably originate from the amino end of the native protein. The smaller tryptic fragment has an N-terminal alanine, while the smaller subtilisin piece has an N-terminal leucine. In addition, modification of a single carbosyl group in intact colicin E3 abolishes more than 90% of the in vivo activity with a simultaneous increase in in vitro activity. This carboxyl group is located in the larger fragments obtained in both trypsin and subtilisin cleavage. Binding of E3 to sensitive cells is drastically reduced or eliniated by this chemical modification and by both of the limited proteolytic cleavages.

Amino Acids↗

The mechanism of action of ricin and related toxic lectins on eukaryotic ribosomes. The site and the characteristics of the modification in 28 S ribosomal RNA caused by the toxins.

Ricin is a potent cytotoxic protein derived from the higher plant Ricinus communis that inactivates eukaryotic ribosomes. In this paper we have studied the mechanism of action of ricin A-chain on rat liver ribosomes in vitro. Our findings indicate that the toxin inactivates the ribosomes by modifying both or either of two nucleoside residues, G4323 and A4324, in 28 S rRNA. These nucleotides are located close to the alpha-sarcin cleavage site and become resistant to all ribonucleases tested. The examination of the lability of phosphodiester bonds of these nucleotides to both mild alkaline digestion and aniline treatment at acidic pH suggests that the base of A4324 is removed by the toxin. This unique activity of ricin A-chain was also observed when naked 28 S rRNA is used as a substrate, indicating that the toxin directly acts on the RNA. Similar activity on 28 S rRNA is also exhibited by abrin and modeccin, ricin-related toxins, suggesting a general mechanistic pathway for ribosome inactivation by lectin toxins.

Animals↗

Structure-mapping of the hairpin ribozyme. Magnesium-dependent folding and evidence for tertiary interactions within the ribozyme-substrate complex.

We have used chemical modification analysis to probe the solution structure of the hairpin ribozyme. The modifying reagents dimethylsulfate, 1-cyclohexyl-N'-[2-(N-methylmorpholino) ethyl-carbodiimide-p-toluenesulfonate, kethoxal, diethylpyrocarbonate and (2,12-dimethyl-3,7,11,17- tetraazabicyclo [11.3.1]heptadeca-1(17),2,11,13,15-pentaenato) nickel(II) perchlorate were used to probe functional groups that participate in Watson-Crick and non-canonical base-pairs. Our results confirm the existence of four short helices (3 to 6 bp) within the ribozyme-substrate complex, and demonstrate that one intramolecular helix (helix 4) is comprised of three base-pairs rather than the previously suggested five. In the absence of magnesium, the ribozyme is observed to fold into its secondary structure. Upon addition of magnesium, a striking difference in chemical modification is observed, particularly at sites within the ribozyme's large internal loop (loop B) that are essential for catalytic function (bases 21 to 26). Moreover, magnesium-dependent folding clearly destabilizes an A-U base-pair in a region where a proposed bend is required to juxtapose the catalytically essential loops A and B. Upon addition of substrate, no changes are observed in the structure of helix 3, loop B or helix 4. However, strong protection of bases in the substrate-binding domain is observed, including those located across internal loop A. The modification data are consistent with the formation of a previously proposed tertiary structure motif within loop B that includes non-canonical G-A, A-U and A-A base-pairs, and that is identical with those identified by NMR analysis of loop E of 5 S rRNA and the sarcin/ricin loop of 28 S rRNA. Our results indicate that the hairpin ribozyme adopts a stable magnesium-dependent tertiary structure to which the substrate binds without inducing major conformational changes, and that substrate recognition is likely to involve non-canonical base-pairs between the ribozyme and substrate sequences adjacent to the cleavage site.

Base Sequence↗

[Study of the mRNA-binding region of ribosomes at different steps of translation. II. Affinity modification of Escherichia coli ribosomes by benzylidene derivative of AUGU6 in the 70S initiation complex].

2',3'-O-(4-[N-(2-chloroethyl)-N-methylamino]) benzylidene derivative of AUGU6 was used for identification of the proteins in the region of the mRNA-binding centre of E. coli ribosomes. This derivative alkylated ribosomes (preferentially 30S ribosomal) with high efficiency within the 70S initiation complex. In both 30S and 50S ribosomal subunits proteins and rRNA were modified. Specificity of the alkylation of ribosomal proteins and rRNA with the reagent was proved by the inhibitory action of AUGU6. Using the method of two-dimensional electrophoresis in polyacrylamide gel the proteins S4, S12, S13, S14, S15, S18, S19 and S20/L26 which are labelled by the analog of mRNA were identified.

Alkylation↗

Molecular profiling of microbial communities associated with seeds of Beta vulgaris subsp. Vulgaris (sugar beet).

The composition of microbial communities on and within seeds may effect their storage and field performance, whether they are indigenous or applied as biocontrol agents. In this study, we have explored the usefulness of profiling small subunit ribosomal (SSR) gene fragments for studying the microflora associated with seeds. DNA was amplified by the polymerase chain reaction (PCR) and the amplicons separated using denaturing gradient gel electrophoresis (DGGE). Primers targeting eukaryotic SSRs were used to investigate fungal communities, and primers targeting bacterial SSRs were employed to study the eubacterial microflora. As a case study, we attempted to profile the fungi and bacteria associated with seeds of Beta vulgaris (sugar beet) to permit an insight into the varying field performance of several well-characterised commercial seed lots. Serious interference with the microbial signals was observed from the plant's own nuclear 18S rRNA genes and chloroplast 16S rRNA genes using standard PCR conditions and DNA extracted from whole seeds as template. Hot-start and touchdown PCR made no appreciable improvement to these signals. Seed imbibition and dissection into operculum and fruit wall and true seed prior to DNA extraction improved signal recovery in the fruit fraction. With primer modification, bacteria and fungi were detected in an excess of plant DNA of 100:1 and 10:1, respectively. With this method, microbial communities on seeds could be profiled, however, it is likely that targeted depletion of plant rDNA targets will be a necessary extra step before this approach can be used to screen seeds routinely.

Beta vulgaris↗

Suicide polymerase endonuclease restriction, a novel technique for enhancing PCR amplification of minor DNA templates.

PCR-based molecular analyses can be hindered by the presence of unwanted or dominant DNA templates that reduce or eliminate detection of alternate templates. We describe here a reaction in which such templates can be exclusively digested by endonuclease restriction, leaving all other DNAs unmodified. After such a modification, the digested template is no longer available for PCR amplification, while nontarget DNAs remain intact and can be amplified. We demonstrate the application of this method and use denaturing gradient gel electrophoresis to ascertain the removal of target DNA templates and the subsequent enhanced amplification of nondigested DNAs. Specifically, plastid 16S rRNA genes were exclusively digested from environmental DNA extracted from plant roots. In addition, pure culture and environmental DNA extracts were spiked with various amounts of genomic DNA extracted from Streptomyces spp., and selective restriction of the Streptomyces 16S rRNA genes via the suicide polymerase endonuclease restriction PCR method was employed to remove the amended DNA.

Arachis↗

Cloning of the mistletoe lectin gene and characterization of the recombinant A-chain.

Mistletoe lectin I (MLI) is the major active constituent of mistletoe extracts, which are widely used for adjuvant tumour therapy. The 66-kDa heterodimeric disulphide-linked glycoprotein is classified as type II ribosome-inactivating protein (RIP) due to the rRNA-cleaving enzyme activity of the A-subunit, also referred to as toxic entity. MLI and the close relative ricin both belong to the family of the two-chain plant type II RIP proteins. Isolation of the glycosylated proteins from plant material yield inhomogeneous material probably due to post-translational modifications. The aim of this study was to prepare pure and homogeneous protein as a prerequisite for structural and mechanistic studies in order to gain insight into the mode of action of this cytotoxic plant protein on tumour and immune cells. Of particular interest was to explain whether the differences in toxicity of ML and ricin are the result of variations of their enzymatic activities. By investigating the sequence homologies between the active sites of different RIPs we were able to deduce a set of primers which were suitable for specific amplification of the mistletoe lectin gene. Applying this PCR strategy the full-length 1923 nucleotide DNA sequence coding for the prepro-protein was obtained showing the existence of a single intron-free gene. In order to elucidate the molecular basis for the observed differences in cytotoxicity within the family of RIP the enzymatic A-subunit was expressed in a heterologous system. Expression of the A-chain in E. coli BL21/pT7 resulted in production of insoluble inclusion bodies constituting 20-30% of total protein. Refolding led to a pure and homogeneous protein species with an apparent molecular mass of 27 kDa and a pI value of 6.4. The ribosome-inactivating activity of the unglycosylated recombinant A-chain (IC50 20.5 pM) protein was in the same range as that of the glycosylated plant-derived ML A-chain (IC50 3.7 pM), which was very similar to that of ricin A-chain (IC50 4.9 pM). Thus, the higher cytotoxicity of ricin cannot be accountable for differences in the enzymatic activities of the type II RIP A-chains.

Amino Acid Sequence↗

Dynamics of ribosomal RNA structure.

The structural dynamics of ribosomal 5S RNAs have been investigated by probing single strandedness through enzymatic cleavage and chemical modification. This comparative study includes 5S rRNAs from E. coli, B. stearothermophilus, T. thermophilus, H. cutirubrum, spinach chloroplast, spinach cytomplasm, and Artemia salina. The structural studies support a unique tertiary interaction in eubacterial 5S rRNAs, involving nucleotides around positions 43 and 75. In addition long range structural effects are demonstrated in E. coli 5S rRNA due to the conversion of C to U at position 92.

Base Sequence↗