Bacteriophage coat protein as repressor

Cryo-EM reveals infection steps of single-stranded RNA bacteriophages

Single-stranded RNA bacteriophages (ssRNA phages) are small spherical RNA viruses that infect bacteria with retractile pili. The single positive-sense genomic RNA of ssRNA phages, which is protected by a capsid shell, is delivered into the host via the retraction of the host pili. Structures involved in ssRNA phage infection cycle are essential for understanding the underlying mechanisms that can be used to engineer them for therapeutic applications. This review summarizes the recent breakthroughs in high-resolution structural studies of two ssRNA phages, MS2 and Qβ, and their interaction with the host, E. coli, by cryo-electron microscopy (cryo-EM). These studies revealed new cryo-EM structures, which provide insights into how MS2 and Qβ package the RNA, lyse E. coli, and adsorb to the receptor F-pili, responsible for conjugation. Methodologies described here can be expanded to study other ssRNA phages that target pathogenic bacteria.

RNA Phage Biology in a Metagenomic Era

The number of novel bacteriophage sequences has expanded significantly as a result of many metagenomic studies of phage populations in diverse environments. Most of these novel sequences bear little or no homology to existing databases (referred to as the "viral dark matter"). Also, these sequences are primarily derived from DNA-encoded bacteriophages (phages) with few RNA phages included. Despite the rapid advancements in high-throughput sequencing, few studies enrich for RNA viruses, i.e., target viral rather than cellular fraction and/or RNA rather than DNA via a reverse transcriptase step, in an attempt to capture the RNA viruses present in a microbial communities. It is timely to compile existing and relevant information about RNA phages to provide an insight into many of their important biological features, which should aid in sequence-based discovery and in their subsequent annotation. Without comprehensive studies, the biological significance of RNA phages has been largely ignored. Future bacteriophage studies should be adapted to ensure they are properly represented in phageomic studies.

Pathogen-derived resistance to viral infection using a negative regulatory molecule

The principle of pathogen-derived resistance (the use of pathogen-derived genes to interfere with the pathogenic process and thereby confer disease resistance to the host) has been put forward as a broadly applicable conceptual tool for use in the genetic engineering of resistance to pathogens and parasites. It was previously predicted that four mechanisms of pathogen-derived resistance could be established using the bacteriophage QB and its host, Escherichia coli, as a model system. This paper demonstrates and helps ellucidate the first of these mechanisms by using a viral regulatory protein, the QB coat protein, to block viral replication. The QB coat protein gene was transferred to susceptible E. coli. Expression of this gene had no obvious detrimental effect on the host. Low-level, constitutive expression of the coat protein conditions very high levels of resistance to QB infection. The resulting resistance is not associated with RNA interference or loss of pili as attachment sites, and does not appear to be associated with premature encapsidation. This low-level expression of the QB coat protein also produces an intermediate level of resistance to the closely related phage SP, but fails to protect against the unrelated phage f2. Thus the resistance does not result from a generalized antiviral host response induced by the presence of the coat protein. We conclude that the QB coat protein blocks viral infection, as was predicted, due to its action as a negative regulatory molecule. The use of negative regulatory molecules may provide an effective mechanism for use in the genetic engineering of pathogen-derived resistance.

Mechanism of translational coupling between coat protein and replicase genes of RNA bacteriophage MS2

We have analyzed the molecular mechanism that makes translation of the MS2 replicase cistron dependent on the translation of the upstream coat cistron. Deletion mapping on cloned cDNA of the phage shows that the ribosomal binding site of the replicase cistron is masked by a long distance basepairing to an internal coat cistron region. Removal of the internal coat cistron region leads to uncoupled replicase synthesis. Our results confirm the model as originally proposed by Min Jou et al. (1). Activation of the replicase start is sensitive to the frequency of upstream translation, but never reaches the level of uncoupled replicase synthesis.

Characterization of Op3, a lysis-defective mutant of bacteriophage f2

We have isolated a conditional lethal mutant of bacteriophage 12 which makes plaques only on E. coli strains carrying a UGA suppressor. It grows normally in nonsuppressing hosts but does not lyse such strains. The mutation complements with amber mutations in each of the three known phage cistrons. These observations lead us to postulate the existence of a fourth gene in the RNA phage.

The synthetase gene of the RNA phages R17, MS2 and f2 has a single UAG terminator codon

Translation of the RNA from the wild-type bacteriophages R17, MS2, and f2 in bacterial cell-free extracts containing an amber suppressor yields 30-40% of the synthetase with an approximate molecular weight of 63 500, slightly larger than the major synthetase product (63 000 daltons). The occurrence of the 63 500 dalton in vitro product is dependent on the presence of an amber suppressor, and we predict that it is due to read-through of a UAG termination codon at the end of the synthetase gene. Previous results of Capecchi and Klein (Nature, 226, 1029-1033, 1070) showed that antibodies to both release factors RF1 and RF2 are required to block release of synthetase, suggesting that synthetase is released at a UAA codon. If the interpretations of both experiments are correct, the termination and release may not be synonomous and may be spatially separated. In addition there is the unexplained fact that 7% of the synthetase made in vitro in both su+ and su- extracts with either R17, MS2 or f2 as template has an apparent molecular weight of 66 000.

Specificity of formation of complexes between coat protein and bacteriophage f2 RNA

The specificity of formation of phage f2 RNA-protein complexes was studied. Complex I contains up to 8 mol of coat protein per 1 mol of RNA. Its formation proceeds equally well in medium (i) without magnesium ions, (ii) containing magnesium ions, (iii) containing 4 mM EDTA, and (iv) at temperatures from 0 to 45 C. Complex II contains up to 200 mol of coat protein per 1 mol of RNA. Its formation is inhibited by the presence of magnesium ions in medium. Formaldehyde- or methoxyamine-treated f2 RNA in which only exposed bases were modified showed a normal pattern of complex II formation, whereas formation of complex I was inhibited or abolished. We conclude that complex I formation involves the interaction between coat protein and specific region of exposed bases in RNA. A possible site of attachment of coat protein is discussed.

Nucleotide sequence at the binding site for coat protein on RNA of bacteriophage R17

The binding of a few molecules [1-6] of RNA bacteriophage coat protein to 1 molecule of RNA represses in vitro translation of the RNA synthetase cistron. Digestion of the complex, R17 coat protein-R17 RNA, by T1 RNase yields an RNA fragment bound to the coat protein. The nucleotide sequence of this fragment (59 residues) reveals that it contains the punctuation signal between the coat protein and RNA synthetase cistrons, suggesting that this is the site on the RNA where the coat protein acts as a translational repressor.

Assay for secondary structure in ribonucleic acid

A chromatographic procedure is described which can discriminate among single-stranded ribonucleic acid (RNA) molecules in solution on the basis of the extent of their secondary structure. The assay is effected through chromatography at different temperatures with columns of cellulose CF-11. When Sindbis virus RNA is chromatographed in this system, the ratio of the amounts of RNA eluting in the single-stranded peak to those eluting in the double-stranded peak increases at higher temperatures, presumably a measure of the relative amounts of Sindbis virus RNA secondary structure at different temperatures. With this assay, Sindbis virus RNA, phage f2 RNA, and polyuridylate have been found to have different amounts of secondary structure.

Characterization of the initial peptide of Q-beta RNA polymerase and control of its synthesis

Bacteriophage Qbeta RNA directs the cell-free synthesis of two fMet-containing dipeptides prior to the first round of ribosomal translocation. One of these, fMet-Ala, corresponds to the initial segment of the Qbeta coat protein. In the present work we take advantage of the fact that translation of the Qbeta RNA polymerase cistron is repressed by its coat protein to correlate the other peptide, fMet-Ser, with the Qbeta RNA polymerase gene. Our experiments show that repression affects translation of the first two codons of the polymerase cistron, thereby isolating the effect of Qbeta coat repressor. Further studies, using single rounds of translocation of the Qbeta mRNA and codon-specific tRNAs, allow us to predict the first three amino acids of the Qbeta RNA polymerase protein, fMet-Ser-Lys, and to suggest that the initiation region of the Qbeta RNA polymerase cistron has the sequence ...AUG.UC[unk].AA[unk]...

Messenger characteristics of nascent bacteriophage RNA

The proteins initiated in vitro by nascent bacteriophage f2 RNA strands attached to isolated replicating structures have been analyzed. The observations confirm that coat protein is the major product initiated and completed. Nascent strands direct the initiation of viral maturation protein in amounts similar to the maximum levels observed in vivo; this synthesis is independent of translation of the coat protein gene. However, only a fraction of these maturation protein molecules initiated in vitro is completed. Nascent RNA molecules also direct the initiation of appreciable amounts of viral RNA polymerase protein, very little of which is completed. Certain constraints on the in vitro translation of the polymerase gene from single-stranded RNA appear to be relaxed in the nascent strands, as indicated by the reduced effect of a polar amber mutation in the coat cistron upon polymerase protein initiation from nascent RNA.

Self-assembly of Q-beta and MS2 phage particles: possible function of initiation complexes

Four kinds of particles were reconstituted with RNA and protein from the genetically unrelated bacteriophages Qbeta and MS2, namely, two homologous and two heterologous, with respect to RNA and protein. However, once Qbeta RNA (or MS2 RNA) reacted with a few molecules of either Qbeta or MS2 protein to form a nucleoprotein complex (initiation complex), it formed a phagelike particle only with subsequent addition of the same protein.