Restructuring the bacteriophage f1 genome: expression of gene VIII in the intergenic space

Structure, Biology, and Applications of Filamentous Bacteriophages

The closely related Escherichia coli Ff filamentous phages (f1, fd, and M13) have taken a fantastic journey over the past 60 years, from the urban sewerage from which they were first isolated, to their use in high-end technologies in multiple fields. Their relatively small genome size, high titers, and the virions that tolerate fusion proteins make the Ffs an ideal system for phage display. Folding of the fusions in the oxidizing environment of the E. coli periplasm makes the Ff phages a platform that allows display of eukaryotic surface and secreted proteins, including antibodies. Resistance of the Ffs to a broad range of pH and detergents facilitates affinity screening in phage display, whereas the stability of the virions at ambient temperature makes them suitable for applications in material science and nanotechnology. Among filamentous phages, only the Ffs have been used in phage display technology, because of the most advanced state of knowledge about their biology and the various tools developed for E. coli as a cloning host for them. Filamentous phages have been thought to be a rather small group, infecting mostly Gram-negative bacteria. A recent discovery of more than 10 thousand diverse filamentous phages in bacteria and archaea, however, opens a fascinating prospect for novel applications. The main aim of this review is to give detailed biological and structural information to researchers embarking on phage display projects. The secondary aim is to discuss the yet-unresolved puzzles, as well as recent developments in filamentous phage biology, from a viewpoint of their impact on current and future applications.

Filamentous Phage: Structure and Biology

Ff filamentous phage (fd, M13 and f1) of Escherichia coli have been the workhorse of phage display technology for the past 30 years. Dominance of Ff over other bacteriophage in display technology stems from the titres that are about 100-fold higher than any other known phage, efficacious transformation ensuring large library size and superior stability of the virion at high temperatures, detergents and pH extremes, allowing broad range of biopanning conditions in screening phage display libraries. Due to the excellent understanding of infection and assembly requirements, Ff phage have also been at the core of phage-assisted continual protein evolution strategies (PACE). This chapter will give an overview of the Ff filamentous phage structure and biology, emphasizing those properties of the Ff phage life cycle and virion that are pertinent to phage display applications.

Simulation of the M13 life cycle I: Assembly of a genetically-structured deterministic chemical kinetic simulation

To expand the quantitative, systems level understanding and foster the expansion of the biotechnological applications of the filamentous bacteriophage M13, we have unified the accumulated quantitative information on M13 biology into a genetically-structured, experimentally-based computational simulation of the entire phage life cycle. The deterministic chemical kinetic simulation explicitly includes the molecular details of DNA replication, mRNA transcription, protein translation and particle assembly, as well as the competing protein-protein and protein-nucleic acid interactions that control the timing and extent of phage production. The simulation reproduces the holistic behavior of M13, closely matching experimentally reported values of the intracellular levels of phage species and the timing of events in the M13 life cycle. The computational model provides a quantitative description of phage biology, highlights gaps in the present understanding of M13, and offers a framework for exploring alternative mechanisms of regulation in the context of the complete M13 life cycle.

Continuous microfluidic assortment of interactive ligands (CMAIL)

Finding an interactive ligand-receptor pair is crucial to many applications, including the development of monoclonal antibodies. Biopanning, a commonly used technique for affinity screening, involves a series of washing steps and is lengthy and tedious. Here we present an approach termed continuous microfluidic assortment of interactive ligands, or CMAIL, for the screening and sorting of antigen-binding single-chain variable antibody fragments (scFv) displayed on bacteriophages (phages). Phages carrying native negative charges on their coat proteins were electrophoresed through a hydrogel matrix functionalized with target antigens under two alternating orthogonal electric fields. During the weak horizontal electric field phase, phages were differentially swept laterally depending on their affinity for the antigen, and all phages were electrophoresed down to be collected during the strong vertical electric field phase. Phages of different affinity were spatially separated, allowing the continuous operation. More than 10⁵ CFU (colony forming unit) antigen-interacting phages were isolated with ~100% specificity from a phage library containing 3 × 10⁹ individual members within 40 minutes of sorting using CMAIL. CMAIL is rapid, sensitive, specific, and does not employ washing, elution or magnetic beads. In conclusion, we have developed an efficient and cost-effective method for isolating and sorting affinity reagents involving phage display.

On the interaction of colicin E3 with the ribosome

Colicin E3 is a protein that kills Escherichia coli cells by a process that involves binding to a surface receptor, entering the cell and inactivating its protein biosynthetic machinery. Colicin E3 kills cells by a catalytic mechanism of a specific ribonucleolytic cleavage in 16S rRNA at the ribosomal decoding A-site between A1493 and G1494 (E. coli numbering system). The breaking of this single phosphodiester bond results in a complete cessation of protein biosynthesis and cell death. The inactive E517Q mutant of the catalytic domain of colicin E3 binds to 30S ribosomal subunits of Thermus thermophilus, as demonstrated by an immunoblotting assay. A model structure of the complex of the ribosomal subunit 30S and colicin E3, obtained via docking, explains the role of the catalytic residues, suggests a catalytic mechanism and provides insight into the specificity of the reaction. Furthermore, the model structure suggests that the inhibitory action of bound immunity is due to charge repulsion of this acidic protein by the negatively charged rRNA backbone

Crystal structure of colicin E3: implications for cell entry and ribosome inactivation

Colicins kill E. coli by a process that involves binding to a surface receptor, entering the cell, and, finally, intoxicating it. The lethal action of colicin E3 is a specific cleavage in the ribosomal decoding A site. The crystal structure of colicin E3, reported here in a binary complex with its immunity protein (IP), reveals a Y-shaped molecule with the receptor binding domain forming a 100 A long stalk and the two globular heads of the translocation domain (T) and the catalytic domain (C) comprising the two arms. Active site residues are D510, H513, E517, and R545. IP is buried between T and C. Rather than blocking the active site, IP prevents access of the active site to the ribosome.

The rational design of a 'type 88' genetically stable peptide display vector in the filamentous bacteriophage fd

Filamentous bacteriophages are particularly efficient for the expression and display of combinatorial random peptides. Two phage proteins are often employed for peptide display: the infectivity protein, PIII, and the major coat protein, PVIII. The use of PVIII typically requires the expression of two pVIII genes: the wild-type and the recombinant pVIII gene, to generate mosaic phages. 'Type 88' vectors contain two pVIII genes in one phage genome. In this study a novel 'type 88' expression vector has been rationally designed and constructed. Two factors were taken into account: the insertion site and the genetic stability of the second pVIII gene. It was found that selective deletion of recombinant genes was encountered when inserts were cloned into either of the two non-coding regions of the phage genome. The deletions were independent of recA yet required a functional F-episome. Transcription was also found to be a positive factor for deletion. Taking the above into account led to the generation of a novel vector, designated fth1, which can be used to express recombinant peptides as pVIII chimeric proteins in mosaic bacteriophages. The fth1 vector is not only genetically stable but also of high copy number and produces high titers of recombinant phages.

Cloning and sequencing of genes encoding the TthHB8I restriction and modification enzymes: comparison with the isoschizomeric TaqI enzymes

Genes encoding the TthHB8I restriction and modification (R-M) system from Thermus thermophilus HB8 (recognition sequence T decreases CGA) were cloned in Escherichia coli. The genes have the same transcriptional orientation, with the last 13 codons of the methyltransferase (MTase) overlapping the first 13 codons of the endonuclease (ENase). Nucleotide sequence analysis of the TthHB8I ENase revealed a single chain of 263 amino acid (aa) residues that share a 77% identity with the corrected isoschizomeric TaqI ENase. Likewise, the Tth MTase (428 aa) shares a 79% identity with the corrected sequence of the TaqI MTase. This high degree of aa conservation suggests a common origin between the Taq and Tth R-M systems. However, codon usage and G+C content for the R-M genes differed markedly from that of other cloned Thermus genes. This suggests that these R-M genes were only recently introduced into the genus Thermus.

Correlation between insertion mutant activities and amino acid sequence identities of the TaqI and TthHB8 restriction endonucleases

A two-codon insertion mutagenesis method has been generalized. Over two dozen insertion mutants throughout the gene encoding TaqI restriction endonuclease were constructed and activity was characterized. All mutants with activity either cleaved or nicked the canonical T decreases CGA recognition sequence. Some insertion mutants created duplication of gene regions, termed Gemini proteins, which still retained activity. The correlation between mutants with poor activity and the regions of shared amino acid identity between the isoschizomeric TaqI and TthHB8I suggests these regions are involved in DNA recognition and/or catalysis.

Cloning, overexpression and nucleotide sequence of a thermostable DNA ligase-encoding gene

Thermostable DNA ligase has been harnessed for the detection of single-base genetic diseases using the ligase chain reaction [Barany, Proc. Natl. Acad. Sci. USA 88 (1991) 189-193]. The Thermus thermophilus (Tth) DNA ligase-encoding gene (ligT) was cloned in Escherichia coli by genetic complementation of a ligts 7 defect in an E. coli host. Nucleotide sequence analysis of the gene revealed a single chain of 676 amino acid residues with 47% identity to the E. coli ligase. Under phoA promoter control, Tth ligase was overproduced to greater than 10% of E. coli cellular proteins. Adenylated and deadenylated forms of the purified enzyme were distinguished by apparent molecular weights of 81 kDa and 78 kDa, respectively, after separation via sodium dodecyl sulfate-polyacrylamide-gel electrophoresis.

Genetic analysis of the filamentous bacteriophage packaging signal and of the proteins that interact with it

The single-stranded DNA of filamentous phages (f1, fd, M13, Ike) contains a region that can fold into a hairpin structure that serves to earmark the DNA for encapsidation. Second-site suppressor mutants of f1 that can compensate for deletion of this packaging signal have been isolated and characterized. The mutations lie in three genes, two that encode virion proteins located at the end of the particle that is first to emerge from the cell, the end at which the packaging signal is located, and the third in a gene whose product is required for assembly but which is not itself a part of the virion. Analysis of base substitution and deletion mutations in the packaging signal suggests that both structural and sequence elements are important to its proper function.

Variable electrostatic interaction between DNA and coat protein in filamentous bacteriophage assembly

A restriction fragment carrying the major coat protein gene (gene VIII) was excised from the DNA of the class I filamentous bacteriophage fd, which infects Escherichia coli. This fragment was cloned into the expression plasmid pKK223-3, where it came under the control of the tac promoter, generating plasmid pKf8P. Bacteriophage fd gene VIII was similarly cloned into the plasmid pEMBL9+, enabling it to be subjected to site-directed mutagenesis. By this means the positively charged lysine residue at position 48, one of four positively charged residues near the C terminus of the protein, was turned into a negatively charged glutamic acid residue. The mutated fd gene VIII was cloned back from the pEMBL plasmid into the expression plasmid pKK223-3, creating plasmid pKE48. In the presence of the inducer isopropyl-beta-D-thiogalactoside, the wild-type and mutated coat protein genes were strongly expressed in E. coli TG1 cells transformed with plasmids pKf8P and pKE48, respectively, and the product procoat proteins underwent processing and insertion into the E. coli cell inner membrane. A net positive charge of only 2 on the side-chains in the C-terminal region is evidently sufficient for this initial stage of the virus assembly process. However, the mutated coat protein could not encapsidate the DNA of bacteriophage R252, an fd bacteriophage carrying an amber mutation in its own gene VIII, when tested on non-suppressor strains of E. coli. On the other hand, elongated hybrid bacteriophage particles could be generated whose capsids contained mixtures of wild-type (K48) and mutant (E48) subunits. This suggests that the defect in assembly may occur at the initiation rather than the elongation step(s) in virus assembly. Other mutations of lysine-48 that removed or reversed the positive charge at this position in the C-terminal region of the coat protein were also found to lead to the production of commensurately longer bacteriophage particles. Taken together, these results indicate direct electrostatic interaction between the DNA and the coat protein in the capsid and support a model of non-specific binding between DNA and coat protein subunits with a stoicheiometry that can be varied during assembly.

Cloning and expression of the filamentous bacteriophage Pf1 major coat protein gene in Escherichia coli. Membrane protein processing and virus assembly

A restriction fragment carrying the major coat protein gene (gene VIII) was excised from the replicative form (RF) DNA of the class II filamentous bacteriophage Pf1, which infects Pseudomonas aeruginosa. This fragment was cloned into the expression plasmid pKK223-3, where it came under the control of the tac promoter. In transformed Escherichia coli JM101 cells, in the presence of the inducer isopropyl-beta-D-thiogalactoside, the bacteriophage Pf1 gene was strongly expressed. The bacteriophage Pf1 coat protein displays the same pattern of negatively charged N-terminal region, hydrophobic middle region and positively charged C-terminal region as that of its counterpart in the class I bacteriophage fd, which infects E. coli, but otherwise the two proteins have no sequence homology. However, the Pf1 procoat protein was found to undergo processing and insertion into the E. coli cell inner membrane, like its fd counterpart, demonstrating that this part of the assembly process is the same for these different bacteriophages. The complete transcriptional unit, incorporating the tac promoter and rrnB transcription terminators flanking the Pf1 coat protein gene, was excised from the expression plasmid and cloned into the intergenic space of bacteriophage R252, an fd bacteriophage that carries an amber mutation in its own major coat protein gene. The Pf1 coat protein gene was again well expressed in infected E. coli cells but the chimeric bacteriophage had growth properties identical to those of the parent bacteriophage R252 on suppressor and non-suppressor strains of E. coli. The class I bacteriophage Pf1 coat protein evidently cannot be recognized by the class I bacteriophage assembly complex at or in the E. coli cell inner membrane, either at the point of initiation of assembly or during the elongation process.

Interaction between gene II protein and the DNA replication origin of bacteriophage f1

The origin of DNA replication of the filamentous bacteriophage f1 binds its initiator protein (gene II protein) in vitro to form a complex that can be trapped on nitrocellulose filters. The binding occurs with both superhelical form DNA and linear DNA fragments. A number of defective mutants of the origin were tested for the ability to bind gene II protein. The region of DNA required for the binding is around a second palindrome downstream from the palindrome that contains the DNA replication initiation site. It overlaps, but is not identical to, the region required for the nicking reaction by the protein. The nicking site itself was dispensable for the binding. In vivo, a number of defective deletion mutants of the origin, when in a plasmid, inhibited growth of superinfecting phage if the intracellular level of gene II protein was low. In addition, these defective origins inhibited the activity of the functional phage origin located on the same replicon. The domain of the DNA sequence required for inhibition in vivo was consistent with that for the binding in vitro.

Type II restriction endonucleases cleave single-stranded DNAs in general

Restriction endonucleases (13 out of 18 species used for the test) were certified to cleave single-stranded(ss)DNA. Such enzymes as AvaII, HaeII, DdeI, AluI, Sau3AI, AccII,TthHB8I and HapII were newly reported to cleave ssDNA. A model to account for the cleavage of ssDNA by restriction enzymes was proposed with supportive data. The essential part of the model was that restriction enzymes preferentially cleave transiently formed secondary structures (called canonical structures) in ssDNA composed of two recognition sequences with two fold rotational symmetry. This means that a restriction enzyme can cleave ssDNAs in general so far as the DNAs have the sequences of restriction sites for the enzyme, and that the rate of cleavage depends on the stabilities of canonical structures.

Two-codon insertion mutagenesis of plasmid genes by using single-stranded hexameric oligonucleotides

An efficient method for introducing two codons into a cloned gene has been applied to studying functional regions of the pBR322-encoded tetracycline-resistance gene and beta-lactamase (ampicillin-resistance) gene. Single-stranded hexameric linkers are inserted into a preexisting cohesive end restriction site to create a new (six-base recognition) restriction site. Insertion mutations are enriched by using biochemical selection or are selected by using a kanamycin-resistance cassette (biological selection). Phenotypes of insertion mutations isolated in the tetracycline-resistance gene support the hypothesis that it is comprised of two domains connected by a central hinge. Mutations in the beta-lactamase gene are temperature sensitive and demonstrate altered sensitivity to various beta-lactams and inhibitors.

A rho-dependent transcription termination signal in bacteriophage f1

The bacteriophage f1 intergenic region distal to gene IV encodes a rho-dependent transcription termination signal. Terminator function in vivo and in vitro is dependent upon active Escherichia coli rho protein, although the RNA 3' ends detected in vivo differ from those seen in vitro. The minimal sequence required for terminator function in a heterologous plasmid system encompasses approximately 100 nucleotides distal to gene IV, which can be drawn as a large hairpin structure. The in vivo rho-dependent 3' end occurs within this sequence, while the in vitro rho-dependent 3' ends occur just distal to it. In vivo in a rho mutant host, f1 transcripts pass through the rho-dependent sites and stop within a sequence of high potential secondary structure near the f1 origin of DNA replication. This sequence alone causes transcription termination in the heterologous plasmid system in vivo. In vitro in the absence of rho protein, transcription does not terminate within this sequence. The RNA 3' ends detected in these studies do not occur within A + T-rich sequences.

Co-evolution of a filamentous bacteriophage and its defective interfering particles

Serial passage of bacteriophage f1 at high multiplicities of infection results in the appearance of defective deletion mutants (miniphage) that harbor a tandem reiteration of regions of the f1 genome near the origin of DNA replication. These miniphage interfere with the growth of wild-type f1, and cause a sharp decrease of the viable phage titer. Upon further passage, however, the titer increases again. Viable phage variants (maxiphage) appear which harbor the same tandem reiteration of DNA as the miniphage. The maxiphage are more resistant than the wild type to interference by the miniphage. In the absence of miniphage the maxiphage grow at the same rate as the wild type. The structure of the DNA reiteration gradually changes during further passage. Miniphage and maxiphage follow, in parallel, a similar course of changes in the pattern of reiteration. In miniphage the reiterations change while the deletions are conserved. Serial passage of maxiphage quickly yields miniphage, which harbor a reiteration identical to that of the parental maxiphage. Both reiteration and deletion are relevant to the mechanism of interference by miniphage. Thus serial passage of the filamentous phage affords an experimental system to study evolution of a DNA genome in test tubes. Possible mechanisms of the interference by miniphage are discussed.

Transcription in bacteriophage f1-infected Escherichia coli. Messenger populations in the infected cell

Transcription of bacteriophage f1 DNA in vivo occurs in two independent regions. They are separated from one another by a strong terminator just downstream from gene VIII on one side, and by the filamentous phage intergenic space on the other. One of these regions contains genes II, V, VII, IX and VIII, and is actively transcribed. In this region there are a number of promoters but only one effective terminator. Thus, most of the RNAs that come from this region overlap and share sequences close to the termination site. The other region, which contains genes III, VI, I and IV, is transcribed much less actively. This region gives rise to a long (approximately 4 X 10(3) bases) RNA that covers the entire region, and several RNAs that overlap in the region closest to their 5' termini. Several other RNAs appear to overlap only with the 4 X 10(3) base transcript. Thus, not only the frequency but the organization of transcription differs in the two portions of the genome.

Genetic transformation of Streptococcus pneumoniae by DNA cloned into the single-stranded bacteriophage f1

A Staphylococcus aureus plasmid derivative, pFB9, coding for erythromycin and chloramphenicol resistance was cloned into the filamentous Escherichia coli phage f1. Recombinant phage-plasmid hybrids, designated plasmids, were isolated from E. coli and purified by transformation into Streptococcus pneumoniae. Single-stranded DNA was prepared from E. coli cells infected with two different plasmids, fBB101 and fBB103. Introduction of fully or partially single-stranded DNA into Streptococcus pneumoniae was studied, using a recipient strain containing an inducible resident plasmid. Such a strain could rescue the donor DNA marker. Under these marker rescue conditions, single-stranded fBB101 DNA gave a 1% transformation frequency, whereas the double-stranded form gave about a 31% frequency. Transformation of single-stranded fBB101 DNA was inhibited by competing double-stranded DNA and vice versa, indicating that single-stranded DNA interacts with the pneumococcus via the same binding site as used by double-stranded DNA. Heteroduplexed DNA containing the marker within a 70- or 800-base single-stranded region showed only slightly greater transforming activity than pure single-stranded DNA. In the absence of marker rescue, both strands of such imperfectly heteroduplexed DNA demonstrated transforming activity. Pure single-stranded DNA demonstrated low but significant transforming activity into a plasmid-free recipient pneumococcus.

Effects of bacteriophage f1 gene III protein on the host cell membrane

Plasmids which encode bacteriophage f1 coat protein genes VIII and III are responsible for a number of unusual properties suggesting that they have a drastic effect on the bacterial outer membrane. Analysis of several such recombinant plasmids and selection of mutant plasmids unable to cause this effect established that the properties were caused by gene III protein or its amino-terminal fragment.

The filamentous phage (Ff) as vectors for recombinant DNA--a review

Derivatives of filamentous phage, f1, fd, and M13, useful as cloning vectors are listed, and procedures for their use are reviewed. Methods for growing phage, preparing single- and double-stranded DNA, and cloning are given in the "cook-book" form. These procedures minimize the practical problem often associated with filamentous-phage cloning, i.e., deletion of inserts.

Staphylococcal plasmids that replicate and express erythromycin resistance in both Streptococcus pneumoniae and Escherichia coli

Plasmid pSA5700 from Staphylococcus aureus coding for erythromycin (EmR) and chloramphenicol (CmR) resistance was transformed into Streptococcus pneumoniae. High-copy-number and EmR constitutive mutants of this plasmid were isolated. Transformation frequencies in S. pneumoniae as high as 70% were obtained with a constitutive plasmid as donor DNA, into a recipient cell containing a resident, inducible, high-copy-number plasmid. With the aid of these high frequencies, the site of constitutive mutations could be mapped via a simple marker rescue technique that uses purified restriction endonuclease-generated fragments. One of the EmR constitutive mutants, pFB9, a plasmid originating from a Gram-positive host, was shown to replicate and express EmR and CmR in a Gram-negative organism, Escherichia coli. Four derivatives of pFB9 containing large (0.6-0.9 megadalton) insertion sequences that arose spontaneously in E. coli demonstrated unusual transforming activity, as well as enhanced EmR, in E. coli. The inserted elements mapped to the region in front of the EmR gene. Three of these inserted elements had the size and restriction patterns of insertion sequence IS1, IS2, and IS5. Plasmid pFB9 and derivatives are useful for isolation of new insertion sequences and for comparison of gene expression and illegitimate recombination between Gram-positive and Gram-negative species.

One and two codon insertion mutants of bacteriophage f1

Simple methods for introducing one or two extra codons of genetic information into the f1 genome in vitro have been devised. The methods use various combinations of enzymes to insert three or six base-pairs into the RF1 DNA of the bacteriophage. Since such insertions do not cause frameshifts in coding regions, a number of these mutants are viable. Several such mutants were mapped and characterized. The methods described and variations of them can be applied to other circular DNA genomes.

Origin of DNA replication of bacteriophage f1 as the signal for termination

Restriction fragments that contain the origin of DNA replication of bacteriophage f1 were inserted in vitro into circular f1 DNA molecules to form genomes that contain two origins. This DNA was used to transfect Escherichia coli. Analyses of the DNA of the progeny phage indicated that one origin and the DNA segment located between the two origins in the infecting DNA molecules had been eliminated. This result is interpreted to mean that the nucleotide sequence of the origin for plus (viral)-strand synthesis also serves as the signal for the termination of DNA synthesis.