Two ribosome binding sites from the gene 0.3 messenger RNA of bacteriophage T7

Ribosome-protected regions have been isolated and analyzed from the bacteriophage T7 gene 0.3 mRNA labeled in vivo. Two discrete sites which are nearly equally protected by ribosomes are obtained from what was previously assumed to be a monocistronic message. Use of appropriate T7 deletion mutant RNAs has allowed mapping of both ribosome-recognized regions. Site a is positioned very close to the 5′ terminus of the mRNA and is apparently the initiator region for the major gene 0.3 protein, which acts to overcome the host DNA restriction system. Site b is located within several hundred nucleotides of the 3′ end of the RNA and probably initiates synthesis of a small polypeptide of unknown function. Both ribosome binding sites exhibit features common to other initiator regions from Escherichia coli and bacteriophage mRNAs. The proximity of site a to the RNase III cleavage site at the left end of gene 0.3 may explain why processing by RNase III is required for efficient translation of the major gene 0.3 protein.     

The blocked and methylated 5′ terminal sequence of a specific cellular messenger: the mRNA for silk fibroin of bombyx mori

The messenger RNA for silk fibroin, labeled with ³²PO₄ and methyl-³H L-methionine, was purified to near homogeneity from the posterior silk gland of the silkworm Bombyx mori, and the sequence of a methylated, RNAase T2-resistant structure was determined. This sequence is similar structurally to 5′ terminal blocked and methylated sequences found on the total populations of polyadenylated eucaryotic cellular and certain viral mRNAs. The RNAase T2-resistant oligomer from fibroin mRNA was cleaved by nuclease P1 into three components: a blocked and methylated sequence containing three phosphates; a 2′-0-methyl UMP residue (pU^m), and an unmethylated CMP (pC). The blocked and methylated sequence comigrated in three chromatographic systems with the blocked and methylated terminus of silkworm cytoplasmic polyhedrosis virus mRNA, which has the structure m⁷GpppA^m. The fibroin mRNA cap was cleaved by nucleotide pyrophosphatase to yield 7-methyl GMP (pm⁷G) and 2′-0-methyl AMP (pA^m). This sequence also appeared to be terminally located, with the m⁷G joined by a 5′-5′ pyrophosphate linkage to the A^m. It was concluded that the 5′ terminal sequence of fibroin mRNA molecules is m⁷G(5′)ppp(5′)A^mpU^mpCp. The regulation of expression of the highly specialized gene for fibroin is discussed in light of this finding.     

Chemical synthesis and properties of modified oligonucleotides containing 5′-amino-5′-deoxy-5′-hydroxymethylthymidine residues

In this study, we designed 5′-amino-5′-deoxy-5′-hydroxymethylthymidine as a new oligonucleotide modification with an amino group directly attached to the 5′-carbon atom. We successfully synthesized two isomers of 5′-amino-5′-deoxy-5′-hydroxymethylthymidine via dihydroxylation of the 5′-vinyl group incorporated into 5′-deoxy-5′-C-methenylthymidine derivative. Moreover, it was found that the nuclease resistance, binding selectivity to single-stranded RNA, and triplex-forming ability of an oligonucleotide containing _RT residues of the new compound were higher than those of the unmodified oligonucleotide.     

Transcription of polyoma virus DNA after interaction with nuclear proteins in vitro.

Analysis of the nucleotide sequence at the 5′-triphosphate termini of RNA chains synthesized by T7 RNA polymerase from T7 DNA template indicates that nearly all RNA chains synthesized in this polymerase reaction contain the sequence, pppGpGp. In addition, studies carried out on T7 DNA-dependent ³²PP_i exchange into ribonucleoside triphosphates suggest that immediately following the guanine residues at the 5′-end of RNA formed in the T7 RNA polymerase reaction, there is one or more adenine residues. These results indicate a high degree of specificity of initiation of RNA synthesis by T7 RNA polymerase.     

Structural organization of ribonucleoproteins containing small nuclear RNAs from HeLa cells. Proteins interact closely with a similar structural domain of U1, U2, U4 and U5 small nuclear RNAs

U₁ snRNP² isolated from HeLa cells and purified by centrifugation in cesium chloride contains a set of proteins that may be resolved into four/five polypeptides by gel electrophoresis. When this particle was submitted to extensive digestion with micrococcal nuclease, RNA fragments of about 25 nucleotides in length were obtained. Sequence analyses showed that these highly protected fragments were derived from the same region of the U₁ molecule, spanning positions 119 to 143. At low concentrations of nuclease, a longer fragment, from nucleotide 119 to the 3′ OH end, was also detected. U₁ core-resistant snRNP, isolated by high performance liquid chromatography, still contains all the protein components of the intact particle.When a less drastically purified U₁ snRNP containing, beside the four/five polypeptides remaining after centrifugation in cesium chloride, a set of at least three polypeptides of larger size, was digested with the nuclease, no other protected RNA fragment was detected.When a mixture of U₁, U₂, U₄, U₅ and U₆ snRNPs, which contains the same four/five polypeptides as U₁ snRNP, was treated with micrococcal nuclease, protected fragments of snRNAs U₂, U₄ and U₅ were found in addition to those derived from U₁. No fragment derived from U₆ was found.In all cases, the region of snRNA shielded from nuclease attack corresponds to a distinctive feature of the molecule. It is a single-stranded region, comprising the sequence A(U)_nG with n ≥ 3, bordered by two double-stranded stems. One of these stems includes the 3′ terminus of the RNA, except in the case of U₂, where there are two stems instead of one on the 3′ side of the single-stranded stretch. Although a comparable structural domain exists also in U₆ snRNA, it does not contain the sequence A(U)_nG which correlates well with the fact that no U₆ snRNA fragment seems to resist micrococcal nuclease digestion.     

The tRNA-like structure of turnip yellow mosaic virus RNA: structural organization of the last 159 nucleotides from the 3' OH terminus

The secondary structure of the isolated tRNA-like sequence (n=159) present at the 3' OH terminus of turnip yellow mosaic virus RNA has been established from partial nuclease digestion with S1 nuclease and T1, CL₃, and Naja oxiana RNases. The fragment folds into a 6-armed structure with two main domains. The first domain, of loose structure and nearest the 5' OH terminus, is composed of one large arm which extends into the coat protein cistron. The second, more compact domain, is composed of the five other arms and most probably contains the structure recognized by valyl-tRNA synthetase. In this domain three successive arms strikingly resemble the T[unk], anticodon, and D arms found in tRNA. Near the amino-acid accepting terminus, however, there is a new stem and loop region not found in standard tRNA. This secondary structure is compatible with a L-shaped three-dimensional organization in which the corner of the L and the anticodon-containing limb are similar to, and the amino-acid accepting region different from, that in tRNA. Ethylnitrosourea accessibility studies have shown similar tertiary structure features in the T[unk] loop of tRNA^Val and in the homologous region of the viral RNA.     

An electron microscopic analysis of pathways for bacteriophage T4 DNA recombination

The interaction between ribosomes of Bacillus stearothermophilus and the RNA genomes of R17 and Qβ bacteriophage has been studied. Whereas Escherichia coli ribosomes can initiate the synthesis of all three RNA phage-specific proteins in vitro, ribosomes of B. stearothermophilus were previously shown to recognize only the A (or maturation) protein initiation site of f2 or R17 RNA. Under these same conditions, a Qβ region is bound and protected from nuclease digestion. Qβ RNA, however, does not direct the synthesis of any formylmethionyl dipeptide in the presence of B. stearothermophilus ribosomes, nor does the binding of either this Qβ region or the R17 A protein initiation site to these ribosomes show the same fMet-tRNA requirement for recognition of initiator regions as that previously established with E. coli ribosomes. Analysis of a 38-nucleotide sequence in the protected Qβ region reveals no AUG or GUG initiator codon. These observations suggest that messenger RNA may be recognized and bound by B. stearothermophilus ribosomes quite independently of polypeptide chain initiation.Binding experiments using R17 RNA and mixtures of components from B. stearothermophilus and E. coli ribosomes confirm the conclusion drawn by Lodish (1970a) that specificity in the selection of authentic phage initiator regions by the two species resides in the ribosomal subunit(s). However, anomalous attachment of B. stearothermophilus ribosomes to R17 RNA, which is observed upon lowering the incubation temperature of the binding reaction, is clearly a property of the initiation factor fraction. The results are discussed with respect to current ideas on the role of ribosomes and initiation factors in determining the specificity of polypeptide chain initiation.     

Effect of a Purified Initiation Factor F3 (B) on the Selection of Ribosomal Binding Sites on Phage MS2 RNA

STUDIES with T4 mRNA showed that initiation factor F2 (C) promotes the attachment of ribosomes to mRNA¹. On the 30S ribosomal subunit this effect is independent of the function of F2 in the binding of formylmethionyl tRNA², whereas formation of a 70S-mRNA complex depends on the binding of fMet-tRNA³. Template competition experiments⁴ showed that, with F2 (C), the ribosome seems to have the same affinity for synthetic polynucleotides as for natural mRNA. Addition of initiation factor F3 (B), however, leads to preferential binding of ribosomes to the natural mRNA. This suggests⁴ that while factor F2 (C) binds the ribosome to any site on the mRNA, the function of factor F3 (B) is to recognize some specific signal in natural mRNA corresponding, perhaps, to the beginning of a cistron. Fractionation of initiation factor F3 (B) into several species differing in their specificity for different mRNA templates⁵ gave further support to the hypothesis that this protein can select binding sites. An excellent system to demonstrate this effect of F3 (B) would be the binding of ribosomes to RNA from E. coli RNA bacteriophages, since Steitz⁶ has analysed and determined the nucleotide sequence of the three binding sites corresponding to the three cistrons of R17 mRNA. Experiments were thus undertaken to study the effect of a purified fraction of F3 (B) on the binding of ribosomes to the different sites of such a phage RNA.     

The origin of multiple polypeptides of molecular weight below 110 000 encoded by tobacco mosaic virus RNA in the messenger-dependent rabbit reticulocyte lysate

Multiple polypeptides encoded by tobacco mosaic virus (TMV) RNA in the messenger-dependent rabbit reticulocyte lysate are not attributable to contaminating 3′-coterminal RNA fragments, multiple leaky termination codons or endonuclease activity opening-up legitimate or spurious internal initiation sites. Quantitative analysis of polypeptides encoded over a range of added RNA concentrations from 0.09 μg·ml^?1 to 180 μg·ml^?1 compared wi preparation, or with RNA extracted from the alkali-stable fraction of TMV suggest that apart from four legitimate virus-coded products of apparent M_r approx. 165 000, 110 000, 30 000 and 17 500 all other polypeptides arise from the overlapping 5′-proximal cistrons either by (i) site-selective endonucleolytic cleavage, (ii) sense codon misreading, or (iii) specific regions of secondary structure on TMV RNA which impede ribosome translocation.     

A new type of IRES within gag coding region recruits three initiation complexes on HIV-2 genomic RNA

Genomic RNA of primate lentiviruses serves both as an mRNA that encodes Gag and Gag-Pol polyproteins and as a propagated genome. Translation of this RNA is initiated by standard cap dependant mechanism or by internal entry of the ribosome. Two regions of the genomic RNA are able to attract initiation complexes, the 5′ untranslated region and the gag coding region itself. Relying on probing data and a phylogenetic study, we have modelled the secondary structure of HIV-1, HIV-2 and SIV_Mac coding region. This approach brings to light conserved secondary-structure elements that were shown by mutations to be required for internal entry of the ribosome. No structural homologies with other described viral or cellular IRES can be identified and lentiviral IRESes show many peculiar properties. Most notably, the IRES present in HIV-2 gag coding region is endowed with the unique ability to recruit up to three initiation complexes on a single RNA molecule. The structural and functional properties of gag coding sequence define a new type of IRES. Although its precise role is unknown, the conservation of the IRES among fast evolving lentiviruses suggests an important physiological role.