Nucleotide sequence of the 5' terminus of satellite tobacco necrosis virus ribonucleic acid

Bacterio-opsin mRNA in wild-type and bacterio-opsin-deficient Halobacterium halobium strains

We have examined transcripts corresponding to the Halobacterium halobium bacterio-opsin (BO) gene in wild-type and in BO-deficient mutant strains containing the insertion elements ISH1 or ISH2 in the BO gene. BO mRNA from the wild-type strain was purified by hybrid selection using single-stranded cDNA. Labeling by vaccinia virus capping enzyme and [alpha-(32)P]GTP showed that it contains the 5'-terminal nucleotide of the primary transcript. Sequence analysis showed that transcription begins only two nucleotides upstream of the initiator codon for BO. Two species of BO mRNA were found; the major species has a ragged 3' terminus approximately 45 nucleotides downstream from the terminator codon for BO, while the minor species is about 170 nucleotides longer at the 3' end. Analysis of the transcripts in several BO gene mutant strains by RNA gel-transfer hybridization showed that (i) mutants with ISH1 insertions within the NH(2)-terminal coding region of the gene contain no detectable transcripts, (ii) mutants with ISH2 near the middle of the coding region of the gene contain multiple incomplete transcripts, and (iii) a mutant that is partially BO deficient due to an insertion of ISH2 100 base pairs upstream of the site of initiation of transcription contains a decreased level of BO mRNA.

Satellite RNAs and Satellite Viruses of Plants

The view that satellite RNAs (satRNAs) and satellite viruses are purely molecular parasites of their cognate helper viruses has changed. The molecular mechanisms underlying the synergistic and/or antagonistic interactions among satRNAs/satellite viruses, helper viruses, and host plants are beginning to be comprehended. This review aims to summarize the recent achievements in basic and practical research, with special emphasis on the involvement of RNA silencing mechanisms in the pathogenicity, population dynamics, and, possibly, the origin(s) of these subviral agents. With further research following current trends, the comprehensive understanding of satRNAs and satellite viruses could lead to new insights into the trilateral interactions among host plants, viruses, and satellites.

A novel interaction of Cap-binding protein complexes eukaryotic initiation factor (eIF) 4F and eIF(iso)4F with a region in the 3'-untranslated region of satellite tobacco necrosis virus

Satellite tobacco necrosis virus (STNV) RNA is naturally uncapped at its 5' end and lacks polyadenylation at its 3' end. Despite lacking these two hallmarks of eukaryotic mRNAs, STNV-1 RNA is translated very efficiently. A approximately 130-nucleotide translational enhancer (TED), located 3' to the termination codon, is necessary for efficient cap-independent translation of STNV-1 RNA. The STNV-1 TED RNA fragment binds to the eukaryotic cap-binding complexes, initiation factor (eIF) 4F and eIF(iso)4F, as measured by nitrocellulose binding and fluorescence titration. STNV-1 TED is a potent inhibitor of in vitro translation when added in trans. This inhibition is reversed by the addition of eIF4F or eIF(iso)4F, and the subunits of eIF4F and eIF(iso)4F cross-link to STNV-1 TED, providing additional evidence that these factors interact directly with STNV-1 TED. Deletion mutagenesis of the STNV-1 TED indicates that a minimal region of approximately 100 nucleotides is necessary to promote cap-independent translation primarily through interaction with the cap binding subunits (eIF4E or eIF(iso)4E) of eIF4F or eIF(iso)4F.

Viral and cellular mRNA capping: past and prospects

This chapter focuses on the history of the discovery of cap and an update of research on viral and cellular-messenger RNA (mRNA) capping. Cap structures of the type m⁷ GpppN(m)pN(m)p are present at the 5′ ends of nearly all eukaryotic cellular and viral mRNAs. A cap is added to cellular mRNA precursors and to the transcripts of viruses that replicate in the nucleus during the initial phases of transcription and before other processing events, including internal N⁶A methylation, 3′-poly (A) addition, and exon splicing. Despite the variations on the methylation theme, the important biological consequences of a cap structure appear to correlate with the N⁷-methyl on the 5′-terminal G and the two pyrophosphoryl bonds that connect m⁷G in a 5′–5′ configuration to the first nucleotide of mRNA. In addition to elucidating the biochemical mechanisms of capping and the downstream effects of this 5′- modification on gene expression, the advent of gene cloning has made available an ever-increasing amount of information on the proteins responsible for producing caps and the functional effects of other cap-related interactions. Genetic approaches have demonstrated the lethal consequences of cap failure in yeasts, and complementation studies have shown the evolutionary functional conservation of capping from unicellular to metazoan organisms.

Structure and stability of mRNA synthesized by vaccinia virus-encoded bacteriophage T7 RNA polymerase in mammalian cells. Importance of the 5' untranslated leader

We have analyzed the structure and stability of RNA synthesized by bacteriophage T7 RNA polymerase in mammalian cells. The T7 polymerase, expressed by a recombinant vaccinia virus, transcribed the Escherichia coli lacZ gene flanked by T7 promoter and terminator signals. The lacZ gene cassette was introduced into infected cells within either a transfected plasmid or a second recombinant vaccinia virus. The T7-lacZ transcripts, which had a half-life of approximately 75 minutes, represented approximately 30% of total cytoplasmic RNA after a 24 hour period. The latter estimation indicated a disparity between the levels of lacZ RNA and beta-galactosidase synthesis. Analysis of the T7 transcripts indicated that they were initiated correctly but that only 5 to 10% contained terminal cap structures, providing an explanation for the low translatability of the RNA. Since the 5' end of the T7 transcripts can form a stem-loop structure that might interfere with capping by vaccinia virus RNA guanylyltransferase, as well as ribosome binding and scanning, a similar vector lacking such sequences was constructed. In vitro experiments demonstrated that T7 RNA polymerase transcribed both templates with similar efficiency and that the RNA lacking the potential to form the stem-loop was capped more rapidly by the purified vaccinia virus enzyme. Nevertheless, when the stem-loop was removed, beta-galactosidase was not expressed in infected cells; moreover, no T7 transcripts could be detected, suggesting that the RNA was not made or more likely was degraded during or shortly after synthesis. There is previous evidence that vaccinia virus RNA guanylyltransferase is associated with the viral transcription complex, thereby allowing RNA synthesis and capping to occur concurrently. We suggest that a lack of coupling between the vaccinia viral RNA guanylyltransferase and bacteriophage T7 RNA polymerase delays capping of T7 transcripts and that, under these conditions, the 5'-terminal double-stranded stem is required to stabilize the nascent RNA against degradation. Although deletion of the 3' palindromic sequence specifying T7 transcriptional termination from the expression cassette resulted in RNA of more heterogeneous lengths, neither the apparent turnover rate nor translation of the RNAs was diminished appreciably.

The sequence of foot-and-mouth disease virus RNA to the 5' side of the poly(C) tract

The nucleotide sequence of foot-and-mouth disease virus (FMDV) RNA to the 5' side of the poly(C) tract (S fragment) has been determined for representatives of the A and O serotypes of the virus. The two S fragments differ in length by five nucleotides (nt), with 367 nt for O1 compared with 362 nt for A10, due to a number of insertions/deletions. However, the two sequences show 86% homology. There are no conserved open reading frames (ORFs). Secondary structure predictions reveal a high degree of potential base-pairing, such that the entire S fragment sequence can be folded into a hairpin structure.

The nature of the interaction of eukaryotic initiation factor 2 with double-stranded RNA

In addition to binding messenger RNA molecules at specific sequences, eukaryotic initiation factor 2 (eIF-2) also binds to double-stranded RNA (dsRNA). The dsRNA is a powerful inhibitor of initiation of eukaryotic translation, causing the inactivation of eIF-2, but in the presence of certain mRNA templates, dsRNA fails to establish inhibition. Such mRNA templates bind to eIF-2 with higher affinity than does dsRNA, while globin mRNA, a template sensitive to inhibition, binds with lower affinity. Here, the nature of the interaction between dsRNA and eIF-2 was studied by examining both the binding of eIF-2 to Penicillium chrysogenum dsRNA molecules carrying 32P label at their 5' ends, and the ability of eIF-2 to protect such label against pancreatic ribonuclease digestion. The results reveal binding sites for eIF-2 at the 5' ends, as well as throughout internal regions of the dsRNA molecule. At least 15 molecules of eIF-2 can be accommodated on a 3000-base molecule of P. chrysogenum dsRNA. eIF-2 protects a 105-base-pair 5'-terminal fragment in dsRNA against digestion, but exhibits no noticeable preference for the 5' ends. By contrast, eIF-2 fails to protect label at the 5' ends of denatured dsRNA molecules, even though it binds to them at internal sites more avidly than to native dsRNA. Binding of eIF-2 to dsRNA is not restricted to specific sequences: eIF-2 binds with equal affinity to the synthetic dsRNA sequence, poly(rI . rC). The data support the interpretation that eIF-2 recognizes the A conformation in dsRNA rather than sequence. Apparently binding of eIF-2 at sites spaced 200 base pairs apart prevents relaxation of the intervening length of the double helix, thereby stabilizing the dsRNA molecule against ribonuclease attack. These results show that, even though dsRNA and mRNA compete in their binding to eIF-2, the structural features recognized by eIF-2 in these RNA species are distinct.

Nucleotide sequence and secondary structure of VSV leader RNA and homologous DNA involved in inhibition of DNA-dependent transcription

We have analyzed the nucleotide sequences and secondary structure required for the transcriptional inhibitory activity of the plus-strand leader RNA of vesicular stomatitis virus (VSV) in a reconstituted HeLa cell transcription system using the adenovirus-2 late promoter (LP) and virus-associated (VA) genes as templates. The New Jersey serotype (VSVNJ) leader and the leader of the Indiana serotype (VSVInd) both contain cleavage sites for the double-strand-specific ribonuclease V1, and these sites are consistent with the presence of a predicted AU-rich stem-loop structure. Studies in which the secondary structure was perturbed with the intercalating agent proflavin suggested that a stem-loop structure enhances the efficiency of transcription inhibition in the VSVNJ leader. Experiments using leader RNA fragments, a VSVInd cDNA derived from the 3' end of the genome, and synthetic oligodeoxynucleotide homologous to regions of the VSV leader indicated that the AU(AT)-rich center region of the VSV leader molecule is sufficient to inhibit DNA-dependent transcription directed by both polymerase II and III, but flanking nucleotide sequences are important for more efficient inhibition of transcription.

Efficient translation of the coat protein cistron of tobacco mosaic virus in a cell-free system from Escherichia coli

Translation of tobacco mosaic virus (TMV) RNA in a cell-free system derived from Escherichia coli (MRE 600) reveals several discrete polypeptides in the Mr range of 10,000-50,000. The major product is a polypeptide of Mr 17,500 which comigrates with authentic TMV coat protein on sodium dodecyl sulphate/polyacrylamide gel electrophoresis. Structural investigations by peptide-mapping techniques and differential radiolabelling confirm that the major product is TMV coat protein with an N-terminal methionine. The major polypeptide product can be assembled in vitro into virus-like ribonucleoprotein particles. The structural and evolutionary implications of this observation, and the values of TMV in elucidating eukaryotic mRNA interactions with the prokaryotic protein-synthesizing machinery, are discussed.

Probing the function of the eucaryotic 5' cap structure by using a monoclonal antibody directed against cap-binding proteins

A monoclonal antibody directed against cap-binding proteins was used to elucidate the possible mechanism by which cap-binding proteins function in initiation of eucaryotic translation. The monoclonal antibody preparation employed in this study exhibited a marked differential effect in inhibiting the translation of folded, capped eucaryotic-mRNAs to a far greater extent than naturally uncapped mRNAs or native capped mRNAs that do not possess extensive 5' end secondary structure. These findings were consistent with the effects of the antibody on initiation complex formation with three different types of reovirus mRNA: native reovirus mRNA; inosine-substituted reovirus mRNA, which has a relaxed secondary structure; and bromouridine-substituted reovirus mRNA, in which base pairing is enhanced relative to regular reovirus mRNA. The extent that translation initiation complex formation was inhibited by the monoclonal antibody directly correlated to the degree of secondary structure present in the mRNA. Binding of bromouridine-substituted reovirus mRNA to ribosomes was inhibited to the greatest extent, while binding of inosine-substituted reovirus mRNA was not inhibited at all in the reticulocyte lysate system or was slightly inhibited in a wheat-germ system. These results support the hypothesis that cap-binding proteins are involved in unwinding of the 5' terminal, secondary structure of many eucaryotic mRNAs, thus facilitating the attachment of ribosomes to mRNA.

The bacteriorhodopsin gene

The bacteriorhodopsin gene has been identified in a 5.3-kilobase restriction endonuclease fragment isolated from Halobacterium halobium DNA, using a cloned cDNA fragment as the probe. Of the 1229 nucleotides whose sequence was determined in the genomic fragment, 786 correspond to the structural gene of bacteriorhodopsin, 360 are upstream from the initiator methionine codon, and 83 are downstream from the COOH terminus. The bacteriorhodopsin gene codes for a precursor sequence of 13 amino acids at the NH2 terminus, 248 amino acids that are present in the mature protein and an additional aspartic acid at the COOH terminus. This determination of the DNA sequence for an archaebacterial gene reveals that the standard genetic code is used; however, there is a marked preference for either G or C in the third codon position. The gene does not contain any intervening sequences and no prokaryotic promoter can be identified in the region immediately upstream from the structural gene. The bacteriorhodopsin mRNA contains at the 5' terminus only three nucleotides beyond the initiating AUG codon and this terminus can form a hairpin structure. Immediately downstream from this structure there is a sequence complementary to the 3' terminus of H. halobium 16S rRNA.

Specific binding of eukaryotic initiation factor 2 to satellite tobacco necrosis virus RNA at a 5'-terminal sequence comprising the ribosome binding site

The mRNA-binding property of eukaryotic initiation factor 2 (eIF-2) was examined by studying its interaction with satellite tobacco necrosis virus (STNV) RNA carrying a (32)P-labeled 5' end. The RNA molecules bound by limiting amounts of eIF-2 were isolated and digested with pancreatic and T1 RNases. Digestion patterns showed that the labeled STNV RNA preparation offered to eIF-2 was heterogeneous, containing more than 30 different 5' ends; by contrast, the RNA selected by eIF-2 possessed predominantly one 5' end, pApGpUp..., the 5'-terminal sequence of intact STNV RNA. Binding analysis of individual 5'-terminal fragments generated from isolated, intact, STNV RNA by partial digestion with T1 RNase showed that eIF-2 does not bind detectably to the 32-nucleotide fragment ending with the initiation codon AUG or to shorter ones, but it does bind the 44-nucleotide fragment that contains the ribosome binding site. In addition to the structural features localized at the 5' end of STNV RNA, eIF-2 appears to recognize a conformation found only in larger molecules, because intact RNA and large 5-'-terminal fragments are bound preferentially over smaller ones. However, binding of short 5'-terminal STNV RNA fragments to eIF-2 is specific, as judged by competition with STNV and ribosomal RNA. Finally, binding of eIF-2 to intact STNV RNA leads to a conformational change in the RNA that greatly facilitates cleavage by T1 and P1 RNases at sites in the vicinity of the initiation region. These results show that eIF-2 interacts specifically with the 5'-terminal region of STNV RNA that contains the ribosome binding site and causes local unfolding of the RNA structure.

Stable hairpin structure within the 5'-terminal 85 nucleotides of poliovirus RNA

The primary sequence of a 5'-terminal fragment of poliovirus type 1 RNA, generated by digestion with RNase III, has been determined. This sequence reveals the presence of a stable hairpin structure beginning nine nucleotides from the terminally linked protein VPg. The sequence does not contain (i) the initiation codons AUG or GUG or (ii) the putative ribosome-binding sequence complementary to the 3' end of eucaryotic ribosomal 18S RNA. The stem-and-loop structure identified can be drawn in either plus or minus RNA strands. It is unclear to which strand functional significance (if any) can be assigned. It is possible that the hairpin structure is involved in ribosomal recognition and translation or in RNA synthesis by interacting with replicase molecules.

Do eukaryotic mRNA 5' noncoding sequences base-pair with the 18 S ribosomal RNA 3' terminus?

Protein synthesis initiation on prokaryotic mRNAs involves base-pairing of a site preceding the initiation codon with the 3' terminal sequence of 16 S rRNA. It has been suggested that a similar situation may prevail in eukaryotic mRNAs. This suggestion is not based on experiments, but on observation of complementarities between mRNA 5' noncoding sequences and a conserved sequence near the 18 S rRNA 3' terminus. The hypothesis can be evaluated by comparing the number of potential binding sites found in the 5' noncoding sequences with the number of such sites expected to occur by chance. A method for computing this number is presented. The 5' noncoding sequences contain more binding sites than expected for a random RNA chain, but the same is true for 3' noncoding sequences. The effect can be traced to a clustering of purines and pyrimidines, common to noncoding sequences. In conclusion, a close inspection of the available mRNA sequences does not reveal any indication of a specific base-pairing ability between their 5' noncoding segments and the 18 S rRNA 3' terminus.