Prohead RNA of bacteriophage phi 29: size, stoichiometry and biological activity

Structure and Mechanical Stabilities of the Three-Way Junction Motifs in Prohead RNA

The core structure of phi29 prohead RNA (pRNA) is composed of three major helices organized into three-way junction pRNA (3WJ-pRNA) and has stout structural rigidity along the coaxial helices. Prohead RNAs of the other Bacillus subtilis bacteriophages such as GA1 and SF5 share similar secondary structure and function with phi29; whether these pRNAs have similar mechanical rigidity remains to be elucidated. In this study, we constructed the tertiary structures of GA1 and SF5 3WJ-pRNAs by comparative modeling. Both GA1 and SF5 3WJ-pRNAs adopt a similar structure, in which three helices are organized as the three-way junction and two of the three helices are stacked coaxially. Moreover, detailed structural features of GA1 and SF5 3WJ-pRNAs are also similar to those of phi29 3WJ-pRNA: all of the bases of the coaxial helices are paired, and all of the adenines in the junction region are paired, which eliminates the interference of A-minor tertiary interactions. Hence, the coaxial helices tightly join to each other, and the major groove between them is very narrow. Two Mg²⁺ ions can thus fit into this major groove and form double Mg clamps. A steered molecular dynamics simulation was used to study the mechanical properties of these 3WJ-pRNAs. Both GA1 and SF5 3WJ-pRNAs show strong resistance to applied force in the direction of their coaxial helices. Such mechanical stability can be attributed to the Mg clamps.

ATP/ADP modulates gp16-pRNA conformational change in the Phi29 DNA packaging motor

Packaging of phage phi29 genome requires the ATPase gp16 and prohead RNA (pRNA). The highly conserved pRNA forms the interface between the connector complex and gp16. Understanding how pRNA interacts with gp16 under packaging conditions can shed light on the molecular mechanism of the packaging motor. Here, we present 3D models of the pRNA–gp16 complex and its conformation change in response to ATP or ADP binding. Using a combination of crystallography, small angle X-ray scattering and chemical probing, we find that the pRNA and gp16 forms a ‘Z’-shaped complex, with gp16 specifically binds to pRNA domain II. The whole complex closes in the presence of ATP, and pRNA domain II rotates open as ATP hydrolyzes, before resetting after ADP is released. Our results suggest that pRNA domain II actively participates in the packaging process.

Prohead RNA: a noncoding viral RNA of novel structure and function

Prohead RNA (pRNA) is an essential component of the powerful Φ29-like bacteriophage DNA packaging motor. However, the specific role of this unique RNA in the Φ29 packaging motor remains unknown. This review examines pRNA as a noncoding RNA of novel structure and function. In order to highlight the reasons for exploring the structure and function of pRNA, we (1) provide an overview of Φ29-like bacteriophage and the Φ29 DNA packaging motor, including putative motor mechanisms and structures of its component parts; (2) discuss pRNA structure and possible roles for pRNA in the Φ29 packaging motor; (3) summarize pRNA self-assembly; and (4) describe the prospective therapeutic applications of pRNA. Many questions remain to be answered in order to connect what is currently known about pRNA structure to its novel function in the Φ29 packaging motor. The knowledge gained from studying the structure, function, and sequence variation in pRNA will help develop tools to better navigate the conformational landscapes of RNA. WIREs RNA 2016, 7:428-437. doi: 10.1002/wrna.1330 For further resources related to this article, please visit the WIREs website.

An RNA Domain Imparts Specificity and Selectivity to a Viral DNA Packaging Motor

During assembly, double-stranded DNA viruses, including bacteriophages and herpesviruses, utilize a powerful molecular motor to package their genomic DNA into a preformed viral capsid. An integral component of the packaging motor in the Bacillus subtilis bacteriophage ϕ29 is a viral genome-encoded pentameric ring of RNA (prohead RNA [pRNA]). pRNA is a 174-base transcript comprised of two domains, domains I and II. Early studies initially isolated a 120-base form (domain I only) that retains high biological activity in vitro; hence, no function could be assigned to domain II. Here we define a role for this domain in the packaging process. DNA packaging using restriction digests of ϕ29 DNA showed that motors with the 174-base pRNA supported the correct polarity of DNA packaging, selectively packaging the DNA left end. In contrast, motors containing the 120-base pRNA had compromised specificity, packaging both left- and right-end fragments. The presence of domain II also provides selectivity in competition assays with genomes from related phages. Furthermore, motors with the 174-base pRNA were restrictive, in that they packaged only one DNA fragment into the head, whereas motors with the 120-base pRNA packaged several fragments into the head, indicating multiple initiation events. These results show that domain II imparts specificity and stringency to the motor during the packaging initiation events that precede DNA translocation. Heteromeric rings of pRNA demonstrated that one or two copies of domain II were sufficient to impart this selectivity/stringency. Although ϕ29 differs from other double-stranded DNA phages in having an RNA motor component, the function provided by pRNA is carried on the motor protein components in other phages.

IMPORTANCE

During virus assembly, genome packaging involves the delivery of newly synthesized viral nucleic acid into a protein shell. In the double-stranded DNA phages and herpesviruses, this is accomplished by a powerful molecular motor that translocates the viral DNA into a preformed viral shell. A key event in DNA packaging is recognition of the viral DNA among other nucleic acids in the host cell. Commonly, a DNA-binding protein mediates the interaction of viral DNA with the motor/head shell. Here we show that for the bacteriophage ϕ29, this essential step of genome recognition is mediated by a viral genome-encoded RNA rather than a protein. A domain of the prohead RNA (pRNA) imparts specificity and stringency to the motor by ensuring the correct orientation of DNA packaging and restricting initiation to a single event. Since this assembly step is unique to the virus, DNA packaging is a novel target for the development of antiviral drugs.

Structure of the RNA claw of the DNA packaging motor of bacteriophage Φ29

Bacteriophage DNA packaging motors translocate their genomic DNA into viral heads, compacting it to near-crystalline density. The Bacillus subtilis phage ϕ29 has a unique ring of RNA (pRNA) that is an essential component of its motor, serving as a scaffold for the packaging ATPase. Previously, deletion of a three-base bulge (18-CCA-20) in the pRNA A-helix was shown to abolish packaging activity. Here, we solved the structure of this crucial bulge by nuclear magnetic resonance (NMR) using a 27mer RNA fragment containing the bulge (27b). The bulge actually involves five nucleotides (17-UCCA-20 and A100), as U17 and A100 are not base paired as predicted. Mutational analysis showed these newly identified bulge residues are important for DNA packaging. The bulge introduces a 33–35° bend in the helical axis, and inter-helical motion around this bend appears to be restricted. A model of the functional 120b pRNA was generated using a 27b NMR structure and the crystal structure of the 66b prohead-binding domain. Fitting this model into a cryo-EM map generated a pentameric pRNA structure; five helices projecting from the pRNA ring resemble an RNA claw. Biochemical analysis suggested that this shape is important for coordinated motor action required for DNA translocation.

Current advances in Phi29 pRNA biology and its application in drug delivery

Bacteriophage 29 (Phi29) packaging RNA (pRNA) is one of the key components in the viral DNA-packaging motor. It contains two functional domains facilitating the translocation of DNA into the viral capsid by interacting with other elements in the motor and promoting adenosine triphosphates hydrolysis. Through the connection between interlocking loops in adjacent pRNA monomers, pRNA functions in the form of multimer ring in the motor. Previous studies have addressed the unique structure and conformation of pRNA. However, there are different DNA-packaging models proposed for the viral genome transportation mechanism. The DNA-packaging ability and the unique features of pRNA have been attracting efforts to study its potential applications in nanotechnology. The pRNA has been proved to be a promising tool for delivering nucleic acid-based therapeutic molecules by covalent linkage with ribozymes, small interfering RNAs, aptamers, and artificial microRNAs. The flexibility in constructing dimers, trimers, and hexamers enables the assembly of polyvalent nanoparticles to carry drug molecules for therapeutic purposes, cell ligands for target delivery, image detector for drug entry monitoring, and endosome disrupter for drug release. Besides these fascinating pharmacological advantages, pRNA-based drug delivery has also been demonstrated to prolong the drug half life with minimal induction of immune response and toxicity.

Role of the CCA bulge of prohead RNA of bacteriophage ø29 in DNA packaging

The oligomeric ring of prohead RNA (pRNA) is an essential component of the ATP-driven DNA packaging motor of bacteriophage ø29. The A-helix of pRNA binds the DNA translocating ATPase gp16 (gene product 16) and the CCA bulge in this helix is essential for DNA packaging in vitro. Mutation of the bulge by base substitution or deletion showed that the size of the bulge, rather than its sequence, is primary in DNA packaging activity. Proheads reconstituted with CCA bulge mutant pRNAs bound the packaging ATPase gp16 and the packaging substrate DNA-gp3, although DNA translocation was not detected with several mutants. Prohead/bulge-mutant pRNA complexes with low packaging activity had a higher rate of ATP hydrolysis per base pair of DNA packaged than proheads with wild-type pRNA. Cryoelectron microscopy three-dimensional reconstruction of proheads reconstituted with a CCA deletion pRNA showed that the protruding pRNA spokes of the motor occupy a different position relative to the head when compared to particles with wild-type pRNA. Therefore, the CCA bulge seems to dictate the orientation of the pRNA spokes. The conformational changes observed for this mutant pRNA may affect gp16 conformation and/or subsequent ATPase-DNA interaction and, consequently, explain the decreased packaging activity observed for CCA mutants.

DNA packaging motor assembly intermediate of bacteriophage phi29

Unraveling the structure and assembly of the DNA packaging ATPases of the tailed double-stranded DNA bacteriophages is integral to understanding the mechanism of DNA translocation. Here, the bacteriophage phi29 packaging ATPase gene product 16 (gp16) was overexpressed in soluble form in Bacillus subtilis (pSAC), purified to near homogeneity, and assembled to the phi29 precursor capsid (prohead) to produce a packaging motor intermediate that was fully active in in vitro DNA packaging. The formation of higher oligomers of the gp16 from monomers was concentration dependent and was characterized by analytical ultracentrifugation, gel filtration, and electron microscopy. The binding of multiple copies of gp16 to the prohead was dependent on the presence of an oligomer of 174- or 120-base prohead RNA (pRNA) fixed to the head-tail connector at the unique portal vertex of the prohead. The use of mutant pRNAs demonstrated that gp16 bound specifically to the A-helix of pRNA, and ribonuclease footprinting of gp16 on pRNA showed that gp16 protected the CC residues of the CCA bulge (residues 18-20) of the A-helix. The binding of gp16 to the prohead/pRNA to constitute the complete and active packaging motor was confirmed by cryo-electron microscopy three-dimensional reconstruction of the prohead/pRNA/gp16 complex. The complex was capable of supercoiling DNA-gp3 as observed previously for gp16 alone; therefore, the binding of gp16 to the prohead, rather than first to DNA-gp3, represents an alternative packaging motor assembly pathway.

Analysis of intermolecular base pair formation of prohead RNA of the phage phi29 DNA packaging motor using NMR spectroscopy

The bacteriophage ø29 DNA packaging motor that assembles on the precursor capsid (prohead) contains an essential 174-nt structural RNA (pRNA) that forms multimers. To determine the structural features of the CE- and D-loops believed to be involved in multimerization of pRNA, 35- and 19-nt RNA molecules containing the CE-loop or the D-loop, respectively, were produced and shown to form a heterodimer in a Mg²⁺-dependent manner, similar to that with full-length pRNA. It has been hypothesized that four intermolecular base pairs are formed between pRNA molecules. Our NMR study of the heterodimer, for the first time, proved directly the existence of two intermolecular Watson–Crick G–C base pairs. The two potential intermolecular A–U base pairs were not observed. In addition, flexibility of the D-loop was found to be important since a Watson–Crick base pair introduced at the base of the D-loop disrupted the formation of the intermolecular G–C hydrogen bonds, and therefore affected heterodimerization. Introduction of this mutation into the biologically active 120-nt pRNA (U80C mutant) resulted in no detectable dimerization at ambient temperature as shown by native gel and sedimentation velocity analyses. Interestingly, this pRNA bound to prohead and packaged DNA as well as the wild-type 120-nt pRNA.

Three-dimensional architecture of the bacteriophage phi29 packaged genome and elucidation of its packaging process

The goal of the work reported here is to understand the precise molecular mechanism of the process of DNA packaging in dsDNA bacteriophages. Cryo-EM was used to directly visualize the architecture of the DNA inside the capsid and thus to measure fundamental physical parameters such as inter-strand distances, local curvatures, and the degree of order. We obtained cryo-EM images of bacteriophage that had packaged defined fragments of the genome as well as particles that had partially completed the packaging process. The resulting comparison of structures observed at intermediate and final stages shows that there is no unique, deterministic DNA packaging pathway. Monte Carlo simulations of the packaging process provide insights on the forces involved and the resultant structures.

Determinants of bacteriophage phi29 head morphology

Bacteriophage phi29 requires scaffolding protein to assemble the 450 x 540 A prolate prohead with T = 3 symmetry end caps. In infections with a temperature-sensitive mutant scaffolding protein, capsids assemble predominantly into 370 A diameter isometric particles with T = 3 symmetry that lack a head-tail connector. However, a few larger, 430 A diameter, particles are also assembled. Cryo-electron microscopy shows that these larger particles are icosahedral with T = 4 symmetry. The prolate prohead, as well as the two isometric capsids with T = 3 and T = 4 symmetry, are composed of similar pentamers and differently skewed hexamers. The skewing of the hexamers in the equatorial region of proheads and in the T = 4 isometric particles reflects their different environments. One of the functions of the scaffolding protein, present in the prohead, may be to stabilize skewed hexamers during assembly.

Conservation of the Capsid Structure in Tailed dsDNA Bacteriophages: the Pseudoatomic Structure of ϕ29

Bacteriophage phi29 is one of the smallest and simplest known dsDNA phages, making it amenable to structural investigations. The three-dimensional structure of a fiberless, isometric variant has been determined to 7.9 A resolution by cryo-electron microscopy (cryo-EM), allowing the identification of alpha helices and beta sheets. Their arrangement indicates that the folds of the phi29 and bacteriophage HK97 capsid proteins are similar except for an additional immunoglobulin-like domain of the phi29 protein. An atomic model that incorporates these two domains fits well into the cryo-EM density of the T = 3, fiberless isometric phi29 particle, and cryo-EM structures of fibered isometric and fiberless prolate prohead phi29 particles at resolutions of 8.7 A and 12.7 A, respectively. Thus, phi29 joins the growing number of phages that utilize the HK97 capsid structure, suggesting that this protein fold may be as prevalent in capsids of dsDNA phages as the jelly roll fold is in eukaryotic viruses.

Bacterial virus phi29 pRNA as a hammerhead ribozyme escort to destroy hepatitis B virus

The DNA-packaging pRNA of bacterial virus phi29, which forms dimers and then hexamers, contains two independent tightly self-folded domains. Circularly permuted pRNAs were constructed without impacting pRNA folding. Connecting the pRNA 5'/3' ends with variable sequences did not disturb its folding and function. These unique features, which help prevent two common problems - exonuclease degradation and misfolding in the cell, make pRNA an ideal vector to carry therapeutic RNAs. A pRNA-based vector was designed to carry hammerhead ribozymes that cleave the hepatitis B virus (HBV) polyA signal. The chimeric HBV-targeting ribozyme was connected to the pRNA 5'/3' ends as circularly permuted pRNA. Two cis-cleaving ribozymes were used to flank and process the chimeric ribozyme. The hammerhead ribozyme including its two arms for HBV targeting was able to fold correctly while escorted by the pRNA. The chimeric ribozyme cleaved the polyA signal of HBV mRNA in vitro almost completely. Cell culture studies showed that the chimeric ribozyme was able to enhance the inhibition of HBV replication when compared with the ribozyme not escorted by pRNA, as demonstrated by Northern blot and e-antigen assays. pRNA could also carry another hammerhead ribozyme to cleave other RNA substrate. These findings suggest that pRNA can be used as a vector for imparting stability to ribozymes, antisense, and other therapeutic RNA molecules in vivo.

Bacteriophage phi29 scaffolding protein gp7 before and after prohead assembly

Three-dimensional structures of the double-stranded DNA bacteriophage phi29 scaffolding protein (gp7) before and after prohead assembly have been determined at resolutions of 2.2 and 2.8 A, respectively. Both structures are dimers that resemble arrows, with a four-helix bundle composing the arrowhead and a coiled coil forming the tail. The structural resemblance of gp7 to the yeast transcription factor GCN4 suggests a DNA-binding function that was confirmed by native gel electrophoresis. DNA binding to gp7 may have a role in mediating the structural transition from prohead to mature virus and scaffold release. A cryo-EM analysis indicates that gp7 is arranged inside the capsid as a series of concentric shells. The position of the higher density features in these shells correlates with the positions of hexamers in the equatorial region of the capsid, suggesting that gp7 may regulate formation of the prolate head through interactions with these hexamers.

Structure and function of phi29 hexameric RNA that drives the viral DNA packaging motor: review

One notable feature of linear dsDNA viruses is that, during replication, their lengthy genome is squeezed with remarkable velocity into a preformed procapsid and packed into near crystalline density. A molecular motor using ATP as energy accomplishes this energetically unfavorable motion tack. In bacterial virus phi29, an RNA (pRNA) molecule is a vital component of this motor. This 120-base RNA has many novel and distinctive features. It contains strong secondary structure, is tightly folded, and unusually stable. Upon interaction with ion and proteins, it has a knack to adapt numerous conformations to perform versatile function. It can be easily manipulated to form stable homologous monomers, dimers, trimers and hexamers. As a result, many unknown properties of RNA have been and will be unfolded by the study of this extraordinary molecule. This article reviews the structure and function of this pRNA and focuses on novel methods and unique approaches that lead to the illumination of its structure and function.

Implication of the prohead RNA in phage phi29 DNA packaging

RNA is an important component of many biological processes, including DNA encapsidation of bacteriophage phi29 of Bacillus subtilis. Interestingly, the prohead RNA is involved in this encapsidation, and was found in monomer, dimer, pentamer and hexamer conformations. This article presents and debates current knowledge about the prohead RNA structures, mechanisms, and roles in DNA encapsidation. A new dimer structure is presented, and its specific role in DNA encapsidation is discussed.

Three-dimensional interaction of Phi29 pRNA dimer probed by chemical modification interference, cryo-AFM, and cross-linking

Six pRNAs (p for packaging) of bacterial virus phi29 form a hexamer complex that is an essential component of the viral DNA translocating motor. Dimers, the building block of pRNA hexamer, assemble in the order of dimer --> tetramer --> hexamer. The two-dimensional structure of the pRNA monomer has been investigated extensively; however, the three-dimensional structure concerning the distance constraints of the three stems and loops are unknown. In this report, we probed the three-dimensional structure of pRNA monomer and dimer by photo affinity cross-linking with azidophenacyl. Bases 75-81 of the left stem were found to be oriented toward the head loop and proximate to bases 26-31 in a parallel orientation. Chemical modification interference indicates the involvement of bases 45-71 and 82-91 in dimer formation. Dimer was formed via hand-in-hand contact, a novel RNA dimerization that in some aspects is similar to the kissing loops of the human immunodeficiency virus. The covalently linked dimers were found to be biologically active. Both the native dimer and the covalently linked dimer were found by cryo-atomic force microscopy to be similar in global conformation and size.

The structural protein E of the archaeal virus phiCh1: evidence for processing in Natrialba magadii during virus maturation

phiCh1 is a lysogenic virus for the haloalkalophilic archaeon Natrialba magadii. The virus morphology resembles other members of Myoviridae infecting Halobacterium species. The gene of the major capsid protein E of virus phiCh1 was cloned and the DNA sequence was determined. Gene E was mapped to a 3.2-kbp ClaI fragment, localized to the 5'-end of the phiCh1 genome. The complete nucleotide sequence of this region was determined and the identity of gene E was confirmed by comparing the experimentally determined N-terminal amino acid sequence of the purified protein to the translated DNA sequence of its open reading frame. We present evidence that the gene E product is proteolytically cleaved between Lys(16) and Asn(17) to yield the 305 residue polypeptides found in the mature viral capsid. Processing of the protein itself during virus development was determined by 2D gel electrophoresis using protein E-specific antibodies. Sequence similarity studies revealed an 80% identity to capsid protein Hp32 of phiH, infecting Halobacterium salinarum. RT-PCR analysis as well as Western blot studies revealed gene E as a late gene. Transcripts and proteins could be detected shortly before onset of lysis of the lysogenic strain N. magadii L11.

A dimer as a building block in assembling RNA. A hexamer that gears bacterial virus phi29 DNA-translocating machinery

Six RNA (pRNA) molecules form a hexamer, via hand-in-hand interaction, to gear bacterial virus phi29 DNA translocation machinery. Here we report the pathway and the conditions for the hexamer formation. Stable pRNA dimers and trimers were assembled in solution, isolated from native gels, and separated by sedimentation, providing a model system for the study of RNA dimers and trimers in a protein-free environment. Cryo-atomic force microscopy revealed that monomers displayed a check mark outline, dimers exhibited an elongated shape, and trimers formed a triangle. Dimerization of pRNA was promoted by a variety of cations including spermidine, whereas procapsid binding and DNA packaging required specific divalent cations, including Mg(2+), Ca(2+), and Mn(2+). Both the tandem and fused pRNA dimers with complementary loops designed to form even-numbered rings were active in DNA packaging, whereas those without complementary loops were inactive. We conclude that dimers are the building blocks of the hexamer, and the pathway of building a hexamer is: dimer --> tetramer --> hexamer. The Hill coefficient of 2.5 suggests that there are three binding sites with cooperative binding on the surface of the procapsid. The two interacting loops played a key role in recruiting the incoming dimer, whereas the procapsid served as the foundation for hexamer assembly.

Sequence requirement for hand-in-hand interaction in formation of RNA dimers and hexamers to gear phi29 DNA translocation motor

Translocation of DNA or RNA is a ubiquitous phenomenon. One intricate translocation process is viral DNA packaging. During maturation, the lengthy genome of dsDNA viruses is translocated with remarkable velocity into a limited space within the procapsid. We have revealed that phi29 DNA packaging is accomplished by a mechanism similar to driving a bolt with a hex nut, which consists of six DNA-packaging pRNAs. Four bases in each of the two pRNA loops are involved in RNA/RNA interactions to form a hexagonal complex that gears the DNA translocating machine. Without considering the tertiary interaction, in some cases only two G/C pairs between the interacting loops could provide certain pRNAs with activity. When all four bases were paired, at least one G/C pair was required for DNA packaging. The maximum number of base pairings between the two loops to allow pRNA to retain wild-type activity was five, whereas the minimum number was five for one loop and three for the other. The findings were supported by phylogenetic analysis of seven pRNAs from different phages. A 75-base RNA segment, bases 23-97, was able to form dimer, to interlock into the hexamer, to compete with full-length pRNA for procapsid binding, and therefore to inhibit phi29 assembly in vitro. Our result suggests that segment 23-97 is a self-folded, independent domain involved in procapsid binding and RNA/RNA interaction in dimer and hexamer formation, whereas bases 1-22 and 98-120 are involved in DNA translocation but dispensable for RNA/RNA interaction. Therefore, this 75-base RNA could be a model for structural studies in RNA dimerization.

Assembly of a tailed bacterial virus and its genome release studied in three dimensions

We present the first three-dimensional reconstruction of a prolate, tailed phage, and its empty prohead precursor by cryo-electron microscopy. The head-tail connector, the central component of the DNA packaging machine, is visualized for the first time in situ within the Bacillus subtilis dsDNA phage phi29. The connector, with 12- or 13-fold symmetry, appears to fit loosely into a pentameric vertex of the head, a symmetry mismatch that may be required to rotate the connector to package DNA. The prolate head of phi29 has 10 hexameric units in its cylindrical equatorial region, and 11 pentameric and 20 hexameric units comprise icosahedral end-caps with T=3 quasi-symmetry. Reconstruction of an emptied phage particle shows that the connector and neck/tail assembly undergo significant conformational changes upon ejection of DNA.

Inter-RNA interaction of phage phi29 pRNA to form a hexameric complex for viral DNA transportation

Ds-DNA viruses package their DNA into a preformed protein shell (procapsid) during maturation. Bacteriophage phi29 requires an RNA (pRNA) to package its genomic DNA into the procapsid. We report here that the pRNA upper and lower loops are involved in RNA/RNA interactions. Mutation in only one loop results in inactive pRNAs. However, mixing of two, three and six inactive mutant pRNAs restores DNA packaging activity as long as an interlocking hexameric ring can be predicted to form by base pairing of the mutated loops in separate RNA molecules. The stoichiometry of pRNA for the packaging of one viral DNA genome is six. Homogeneous pRNA purified from a single band in denaturing gels showed six bands when rerun in native gels. These results suggest that six pRNAs form a hexameric ring by the intermolecular interaction of two RNA loops to serve as part of the DNA transportation machinery.

In vitro selection of bacteriophage phi29 prohead RNA aptamers for prohead binding

Prohead RNA (pRNA) of the Bacillus subtilis bacteriophage phi29 is needed for in vitro packaging of DNA-gene product 3 (DNA-gp3). Residues 22-84 of the 174-base pRNA bind the portal vertex of the prohead, the site of DNA packaging. To define the nucleotides of pRNA needed for prohead binding and DNA-gp3 packaging and to seek biologically active variants of pRNA, segments of pRNA were randomized to obtain vast repertoires of RNA molecules. RNA aptamers, ligands best suited for prohead binding, were obtained by multiple rounds of in vitro selection. Evolution of pRNA aptamers was followed by a competition binding assay and nucleotide sequencing, and mutants were tested for DNA-gp3 packaging. Aptamers selected following randomization of the E stem and loop and a part of the C-E loop that were active in DNA-gp3 packaging were invariably wild-type. DNA-gp3 packaging activity also required nucleotides G82 and G83 that form base pairs intermolecularly with C47 and C48 to produce a novel hexameric oligomer of pRNA. The only mutant aptamers that retained full DNA-gp3 packaging activity showed changes of the U residues at positions 81, 84, and 85 of the D loop. Thus, the in vitro selections essentially recapitulated the natural evolution of pRNA.

Sequential action of six virus-encoded DNA-packaging RNAs during phage phi29 genomic DNA translocation

A 120-base pRNA encoded by bacteriophage b29 has a novel and essential role in genomic DNA packaging. Six DNA-packaging RNAs (pRNAs) were bound to the sixfold symmetrical portal vertex of procapsids during the DNA translocation process and left the procapsid after the DNA-packaging reaction was completed, suggesting that the pRNA participated in the translocation of genomic DNA into procapsids. To further investigate the mechanism of DNA packaging, it is crucial to determine whether these six pRNA molecules work as an integrated entity or each pRNA acts as a functional individual. If pRNAs work individually, then do they work in sequence with communication or in random order without interaction? Results from compensation and complementation analysis did not support the integrated model. Computation of the probability of combination between wild-type and mutant pRNAs and experimental data of competitive inhibition excluded the random model while favoring the proposal that the six pRNAs functioned sequentially. Sequential action of the pRNA also explains why the pRNA is so sensitive to mutation, since the effect of a pRNA mutation will be amplified by 6 orders of magnitude after six consecutive steps, resulting in the observed complete loss of DNA-packaging activity caused by small alterations. When any one of the six pRNAs was replaced with an inactive one, complete blockage of DNA packaging resulted, strongly supporting the speculation that individual pRNAs, presumably together with other components such as the packaging ATPase gp16, take turns mediating successive steps of packaging. Although the data provided here could not exclude the integrated model completely, there is no evidence so far to argue against the model of sequential action.

The bacteriophage phi29 packaging proteins supercoil the DNA ends

Bacteriophage phi29 DNA with covalently bound terminal protein (DNA-gp3) and its left and right-end restriction fragments (L and R-DNA-gp3) sedimented faster in sucrose density gradients than their proteinase K-treated counterparts, and the faster sedimentation was both gp3 and Mg2+-dependent. Addition of gp16, the phi29 DNA packaging ATPase, further increased the sedimentation rates of both intact DNA-gp3 and L and R-DNA-gp3 fragments. Thus, DNAs with gp3 were more compact than gp3-free DNA, and gp16 further condensed the DNA-gp3 forms. [35S]gp16 cosedimented with the fast-sedimenting DNA-gp3 fragments, and the putative L-DNA-gp3-gp16 complexes were packaged preferentially into proheads in the defined in vitro system. Lariats of DNA-gp3 and L and R-DNA-gp3 observed by electron microscopy rationalized the sedimentation results, and lariats with multiple loops or coils increased tenfold in a preparation of L-DNA-gp3-gp16 complexes. The rapid sedimentation and the structure of the DNA-gp3-gp16 complexes were consistent with supercoiling of lariat loops, and treatment with topoisomerase I shifted fast-sedimenting complexes toward the uncoiled lariat position in sucrose density gradients. DNA-gp3 has a maturation pathway in which the packaging proteins gp3 and gp16 supercoil the DNA ends, probably as a prerequisite for efficient interaction with the prohead.

Approaches to determine stoichiometry of viral assembly components

Due to the rapidity of biological reactions, it is difficult to isolate intermediates or to determine the stoichiometry of participants in intermediate reactions. Instead of determining the absolute amount of each component, this study involved the use of relative parameters, such as dilution factors, percentages probabilities, and slopes of titration curves, that can be more accurately quantified to determine the stoichiometry of components involved in bacteriophage phi29 assembly. This work takes advantage of the sensitive in vitro phage phi29 assembly system, in which 10(8) infectious virions per ml without background can be assembled from eight purified components. It provides a convenient assay for quantification of the stoichiometry of packaging components, including the viral procapsid, genomic DNA, DNA-packaging pRNA, and other structural proteins and enzymes. The presence of a procapsid binding domain and another essential functional domain within the pRNA makes it an ideal component for constructing lethal mutants for competitive procapsid binding. Two methods were used for stoichiometry determination. Method 1 was to determine the combination probability of mutant and wild-type pRNAs bound to procapsids. The probability of procapsids that possess a certain amount of mutant and a certain amount of wild-type pRNA, both with an equal binding affinity, was predicted with the binomial equation [EQUATION IN TEXT] where Z is the total number of pRNAs per procapsid, M is the number of mutant pRNAs bound to one procapsid, and (ZM) is equal to [FORMULA IN TEXT]. With various ratios of mutant to wild-type pRNA in in vitro viral assembly, the percent mutant pRNA versus the yield of virions was plotted and compared to a series of predicted curves to find a best fit. It was determined that five or six copies of pRNA were required for one DNA-packaging event, while only one mutant pRNA per procapsid was sufficient to block packaging. Method 2 involved the comparison of slopes of curves of dilution factors versus the yield of virions. Components with known stoichiometries served as standard controls. The larger the stoichiometry of the component, the more dramatic the influence of the dilution factor on the reaction. A slope of 1 indicates that one copy of the component is involved in the assembly of one virion. A slope larger than 1 would indicate multiple-copy involvement. By this method, the stoichiometry of gp11 in phi29 particles was determined to be approximately 12. These approaches are useful for the determination of the stoichiometry of functional units involved in viral assembly, be they single molecules or oligomers. However, these approaches are not suitable for the determination of exact copy numbers of individual molecules involved if the functional unit is composed of multiple subunits prior to assembly.

Magnesium-induced conformational change of packaging RNA for procapsid recognition and binding during phage phi29 DNA encapsidation

Bacteriophage phi29 is typical of double-stranded DNA viruses in that its genome is packaged into a preformed procapsid during maturation. An intriguing feature of phi29 assembly is that a virus-encoded RNA (pRNA) is required for the packaging of its genomic DNA. Psoralen cross-linking, primer extension, and T1 RNase partial digestion revealed that pRNA had at least two conformations; one was able to bind procapsids, and the other was not. In the presence of Mg2+, one stretch of pRNA, consisting of bases 31 to 35, was confirmed to be proximal to base 69, as revealed by its efficient cross-linking by psoralen. Two cross-linking sites in the helical region were identified. Mg2+ induced a conformational change of pRNA that exposes the portal protein binding site by promoting the refolding of two strands of the procapsid binding region, resulting in the formation of pRNA-procapsid complexes. The procapsid binding region in this binding-competent conformation could not be cross-linked with psoralen. When the two strands of the procapsid binding region were fastened by cross-linking, pRNA could neither bind procapsids nor package phi29 DNA. A pRNA conformational change was also discernible by comparison of migration rates in native EDTA and Mg2+ polyacrylamide gel electrophoresis and was revealed by T1 RNase probing. The Mg2+ concentration required for the detection of a change in pRNA cross-linking patterns was 1 mM, which was the same as that required for pRNA-procapsid complex formation and DNA packaging and was also close to that in normal host cells.

Complete inhibition of virion assembly in vivo with mutant procapsid RNA essential for phage phi 29 DNA packaging

A highly efficient method for the inhibition of bacteriophage phi 29 assembly was developed with the use of mutant forms of the viral procapsid (or packaging) RNA (pRNA) indispensable for phi 29 DNA packaging. Phage phi 29 assembly was severely reduced in vitro in the presence of mutant pRNA and completely blocked in vivo when the host cell expressed mutant pRNA. Addition of 45% mutant pRNA resulted in a reduction of infectious virion production by 4 orders of magnitude, indicating that factors involved in viral assembly can be targets for efficient and specific antiviral treatment. The mechanism leading to the high efficiency of inhibition was attributed to two pivotal features. First, the pRNA contains two separate, essential functional domains, one for procapsid binding and the other for a DNA-packaging role other than procapsid binding. Mutation of the DNA-packaging domain resulted in a pRNA with no DNA-packaging activity but intact procapsid binding competence. Second, multiple copies of the pRNA were involved in the packaging of one genome. This higher-order dependence of pRNA in viral replication concomitantly resulted in its higher-order inhibitory effect. This finding suggested that the collective DNA-packaging activity of multiple copies of pRNA could be disrupted by the incorporation of perhaps an individual mutant pRNA into the group. Although this mutant pRNA could not be used for the inhibition of the replication of other viruses directly, the principle of using molecules with two functional domains and multiple-copy involvement as targets for antiviral agents could be applied to certain viral structural proteins, enzymes, and other factors or RNAs involved in the viral life cycle. This principle also implies a strategy for gene therapy, intracellular immunization, or construction of transgenic plants resistant to viral infection.

Sequential interactions of structural proteins in phage phi 29 procapsid assembly

The mechanism of viral capsid assembly is an intriguing problem because of its fundamental importance to research on synthetic viral particle vaccines, gene delivery systems, antiviral drugs, chimeric viruses displaying antigens or ligands, and the study of macromolecular interactions. The genes coding for the scaffolding (gp7), capsid (gp8), and portal vertex (gp10) proteins of the procapsid of bacteriophage phi 29 of Bacillus subtilis were expressed in Escherichia coli individually or in combination to study the mechanism of phi 29 procapsid assembly. When expressed alone, gp7 existed as a soluble monomer, gp8 aggregated into inclusion bodies, and gp10 formed the portal vertex. Circular dichroisin spectrum analysis indicated that gp7 is mainly composed of alpha helices. When two of the proteins were coexpressed, gp7 and gp8 assembled into procapsid-like particles with variable sizes and shapes, gp7 and gp10 formed unstable complexes, and gp8 and gp10 did not interact. These results suggested that gp7 served as a bridge for gp8 and gp10. When gp7, gp8, and gp10 were coexpressed, active procapsids were produced. Complementation of extracts containing one or two structural components could not produce active procapsids, indicating that no stable intermediates were formed. A dimeric gp7 concatemer promoted the solubility of gp8 but was inactive in the assembly of procapsid or procapsid-like particles. Mutation at the C terminus of gp7 prevented it from interacting with gp8, indicating that this part of gp7 may be important for interaction with gp8. Coexpression of the portal protein (gp20) of phage T4 with phi 29 gp7 and gp8 revealed the lack of interaction between T4 gp20 and phi 29 gp7 and/or gp8. Perturbing the ratio of the three structural proteins by duplicating one or another gene did not reduce the yield of potentially infectious particles. Changing of the order of gene arrangement in plasmids did not affect the formation of active procapsids significantly. These results indicate that phi 29 procapsid assembly deviated from the single-assembly pathway and that coexistence of all three components with a threshold concentration was required for procapsid assembly. The trimolecular interaction was so rapid that no true intermediates could be isolated. This finding is in accord with the result of capsid assembly obtained by the equilibrium model proposed by A. Zlotnick (J. Mol. Biol. 241:59-67, 1994).

Bacillus subtilis mutants defective in bacteriophage phi 29 head assembly

Virus assembly mutants of asporogenous Bacillus subtilis defective in bacteriophage phi 29 head assembly were detected by the use of antibodies that reacted strongly with the free dodecameric phi 29 portal vertex composed of gene product 10 (gp10) but weakly with the portal vertex assembled into proheads or phage. Phage adsorption and the synthesis of phage proteins, DNA-gene product 3, and prohead RNA were normal in these mutants, but prohead and phage production was greatly reduced. The assembly defect was transferred to competent B. subtilis by transformation and transduction. PBS1 transduction showed that the vam locus was linked to Tn917 located at 317 degrees on the B. subtilis chromosome.

Mutant prohead RNAs in the in vitro packaging of bacteriophage phi 29 DNA-gp3

The 174-base prohead RNA encoded by bacteriophage phi 29 of Bacillus subtilis, essential for packaging of the DNA-gp3 (DNA-gene product 3) complex, was expressed efficiently from the cloned gene. Computer programs for RNA structure analysis were used to fold hypothetical RNA mutants and thus to target mutagenesis of the RNA for studies of structure and function. Five mutants of the RNA were then produced by oligonucleotide-directed mutagenesis that were altered in the primary sequence at selected sites; two of these mutants were predicted to be altered in secondary structure from a model established previously by a phylogenetic analysis. The binding of the 32P end-labeled mutant RNAs to RNA-free proheads was comparable with that of the wild-type RNA. However, the capability of the mutant RNAs to reconstitute RNA-free proheads for DNA-gp3 packaging in the defined in vitro system and for assembly of phage in RNA-free extracts was variable, depending upon the alteration. Changes of highly conserved bases that retained the predicted secondary structure of the RNA model were tolerated to a much greater extent than changes predicted to alter the RNA secondary structure.

RNA dependence of the bacteriophage phi 29 DNA packaging ATPase

The activity of the DNA packaging adenosine triphosphatase (ATPase) of the Bacillus subtilis bacteriophage phi 29 is dependent upon prohead RNA. The 174 nucleotide viral-encoded RNA is positioned on the head-tail connector at the portal vertex of the phi 29 precursor shell (prohead). Here, the RNA interacts with the ATP-binding gene 16 product (gp16) to constitute the DNA-packaging ATPase and initiate DNA packaging in vitro. Both the prohead connector (gene 10 product, gp10) and gp16 may utilize an RNA recognition motif characteristic of a number of RNA-associated proteins, and the binding of gp16 by proheads shields the prohead RNA from RNase A. The ATPase activity of gp16 is stimulated fourfold by RNA and tenfold by proheads with RNA. RNA is needed continuously for the gp16/RNA ATPase activity and is essential for the gp16/prohead ATPase activity. The prohead, with its connector, RNA and associated gp16 in an assembly-regulated configuration, hydrolyzes ATP and drives phi 29 DNA translocation.

Role of RNA in bacteriophage phi 29 DNA packaging

A novel bacteriophage phi 29 RNA of 174 nucleotides is essential for the in vitro packaging of the DNA-terminal protein complex into proheads. The RNA, bound to the prohead portal vertex (connector), participates in assembly and function of the DNA translocating ATPase and in recognition of the DNA left-end during the course of the packaging reaction. The RNA is present in related phages and varies widely in primary sequence, but its secondary structure, as deduced by phylogenetic analysis, is both highly conserved and unique among small RNAs.

In vitro packaging of bacteriophage phi 29 DNA restriction fragments and the role of the terminal protein gp3

Restriction fragments of bacteriophage phi 29 DNA-gp3 (DNA-gene product 3 complex) were packaged in a completely defined in vitro system that included purified proheads, the DNA packaging protein gp16 and ATP. Both left and right end DNA-gp3 fragments were packaged in this system, in contrast to the oriented and selective packaging of left end DNA-gp3 fragments in extracts; left ends could be packaged quantitatively in the defined system, while the packaging efficiency of right ends was generally about threefold lower. In addition, certain internal (non-end) DNA fragments were packaged at efficiencies of about 10% to 15%. Digestion of the gp3 with trypsin or proteinase K reduced the packaging of whole-length DNA by a factor of 2 or 4, respectively, and removal of the gp3 from whole-length DNA or end fragments with piperidine reduced packaging to the level of internal fragments. Though the terminal protein gp3 was non-essential for DNA translocation in the defined system, it stimulated packaging of left and right end fragments, and stabilized packaging of the left end. The packaging of end and internal DNA fragments of the related phage M2Y into phi 29 proheads was similar to that of phi 29 DNA fragments, and certain fragments of lambda DNA were packaged at the efficiency of the internal phi 29 DNA fragments. Selective packaging of DNA-gp3 left ends was restored by the addition of bacterial cell extracts or glycerol to the defined system, and these packaging conditions discriminated between phi 29 and M2Y DNAs that have distinct terminal proteins.

Cleaving the prohead RNA of bacteriophage phi 29 alters the in vitro packaging of restriction fragments of DNA-gp3

In vitro packaging of restriction fragments of the bacteriophage phi 29 DNA-gp3 (DNA-gene product 3 complex) in the defined system was dependent on prohead RNA. Truncated prohead RNAs were obtained by in situ RNase A digestion, isolated and sequenced. Proheads having the intact 174 base RNA were compared to proheads having RNAs of 120, 95, 71, 69 or 54 bases for the capacity to package the DNA-gp3 left and right ends and internal (non-end) fragments generated by the restriction enzymes EcoRI, HpaI and BstNI. Proheads with the 174 or 120 base RNAs packaged both left and right ends; internal fragments were packaged more efficiently by proheads with the 120 base RNA. Proheads with the 95 base RNA packaged DNA-gp3 left ends and internal fragments efficiently, but lost the capacity to package right ends. Only internal fragments were packaged by proheads with the 71 base RNA, and proheads having 69 or 54 base RNAs were inactive. RNA-free proheads were effectively reconstituted with purified 174 and 120 base RNAs to produce particles similar in biological activity to the proheads from which the RNAs were isolated. The 95 base RNA was the smallest RNA of the group that could reconstitute the prohead and direct fragment packaging, although packaging was inefficient. Alteration of the specificity of DNA fragment packaging with truncated prohead RNAs has delineated RNA domains that function in DNA-gp3 recognition and prohead binding.