Bacteriophage P2 and P4 morphogenesis: protein processing and capsid size determination

Structure of the Capsid Size-Determining Scaffold of "Satellite" Bacteriophage P4

P4 is a mobile genetic element (MGE) that can exist as a plasmid or integrated into its Escherichia coli host genome, but becomes packaged into phage particles by a helper bacteriophage, such as P2. P4 is the original example of what we have termed "molecular piracy", the process by which one MGE usurps the life cycle of another for its own propagation. The P2 helper provides most of the structural gene products for assembly of the P4 virion. However, when P4 is mobilized by P2, the resulting capsids are smaller than those normally formed by P2 alone. The P4-encoded protein responsible for this size change is called Sid, which forms an external scaffolding cage around the P4 procapsids. We have determined the high-resolution structure of P4 procapsids, allowing us to build an atomic model for Sid as well as the gpN capsid protein. Sixty copies of Sid form an intertwined dodecahedral cage around the T = 4 procapsid, making contact with only one out of the four symmetrically non-equivalent copies of gpN. Our structure provides a basis for understanding the sir mutants in gpN that prevent small capsid formation, as well as the nms "super-sid" mutations that counteract the effect of the sir mutations, and suggests a model for capsid size redirection by Sid.

Bacteriophage P2

P2 is the original member of a highly successful family of temperate phages that are frequently found in the genomes of gram-negative bacteria. This article focuses on the organization of the P2 genome and reviews current knowledge about the function of each open reading frame.

Pirates of the Caudovirales

Molecular piracy is a biological phenomenon in which one replicon (the pirate) uses the structural proteins encoded by another replicon (the helper) to package its own genome and thus allow its propagation and spread. Such piracy is dependent on a complex web of interactions between the helper and the pirate that occur at several levels, from transcriptional control to macromolecular assembly. The best characterized examples of molecular piracy are from the E. coli P2/P4 system and the S. aureus SaPI pathogenicity island/helper system. In both of these cases, the pirate element is mobilized and packaged into phage-like transducing particles assembled from proteins supplied by a helper phage that belongs to the Caudovirales order of viruses (tailed, dsDNA bacteriophages). In this review we will summarize and compare the processes that are involved in molecular piracy in these two systems.

Assembly of bacteriophage 80α capsids in a Staphylococcus aureus expression system

80α is a temperate, double-stranded DNA bacteriophage of Staphylococcus aureus that can act as a "helper" for the mobilization of S. aureus pathogenicity islands (SaPIs), including SaPI1. When SaPI1 is mobilized by 80α, the SaPI genomes are packaged into capsids that are composed of phage proteins, but that are smaller than those normally formed by the phage. This size determination is dependent on SaPI1 proteins CpmA and CpmB. Here, we show that co-expression of the 80α capsid and scaffolding proteins in S. aureus, but not in E. coli, leads to the formation of procapsid-related structures, suggesting that a host co-factor is required for assembly. The capsid and scaffolding proteins also undergo normal N-terminal processing upon expression in S. aureus, implicating a host protease. We also find that SaPI1 proteins CpmA and CpmB promote the formation of small capsids upon co-expression with 80α capsid and scaffolding proteins in S. aureus.

Structure and size determination of bacteriophage P2 and P4 procapsids: function of size responsiveness mutations

Bacteriophage P4 is dependent on structural proteins supplied by a helper phage, P2, to assemble infectious virions. Bacteriophage P2 normally forms an icosahedral capsid with T=7 symmetry from the gpN capsid protein, the gpO scaffolding protein and the gpQ portal protein. In the presence of P4, however, the same structural proteins are assembled into a smaller capsid with T=4 symmetry. This size determination is effected by the P4-encoded protein Sid, which forms an external scaffold around the small P4 procapsids. Size responsiveness (sir) mutants in gpN fail to assemble small capsids even in the presence of Sid. We have produced large and small procapsids by co-expression of gpN with gpO and Sid, respectively, and applied cryo-electron microscopy and three-dimensional reconstruction methods to visualize these procapsids. gpN has an HK97-like fold and interacts with Sid in an exposed loop where the sir mutations are clustered. The T=7 lattice of P2 has dextro handedness, unlike the laevo lattices of other phages with this fold observed so far.

Bacteriophage protein-protein interactions

Bacteriophages T7, λ, P22, and P2/P4 (from Escherichia coli), as well as ϕ29 (from Bacillus subtilis), are among the best-studied bacterial viruses. This chapter summarizes published protein interaction data of intraviral protein interactions, as well as known phage-host protein interactions of these phages retrieved from the literature. We also review the published results of comprehensive protein interaction analyses of Pneumococcus phages Dp-1 and Cp-1, as well as coliphages λ and T7. For example, the ≈55 proteins encoded by the T7 genome are connected by ≈43 interactions with another ≈15 between the phage and its host. The chapter compiles published interactions for the well-studied phages λ (33 intra-phage/22 phage-host), P22 (38/9), P2/P4 (14/3), and ϕ29 (20/2). We discuss whether different interaction patterns reflect different phage lifestyles or whether they may be artifacts of sampling. Phages that infect the same host can interact with different host target proteins, as exemplified by E. coli phage λ and T7. Despite decades of intensive investigation, only a fraction of these phage interactomes are known. Technical limitations and a lack of depth in many studies explain the gaps in our knowledge. Strategies to complete current interactome maps are described. Although limited space precludes detailed overviews of phage molecular biology, this compilation will allow future studies to put interaction data into the context of phage biology.

Assembly of bacteriophage P2 capsids from capsid protein fused to internal scaffolding protein

Most tailed bacteriophages with double-stranded DNA genomes code for a scaffolding protein, which is required for capsid assembly, but is removed during capsid maturation and DNA packaging. The gpO scaffolding protein of bacteriophage P2 also doubles as a maturation protease, while the scaffolding activity is confined to a 90 residue C-terminal "scaffolding" domain. Bacteriophage HK97 lacks a separate scaffolding protein; instead, an N-terminal "delta" domain in the capsid protein appears to serve an analogous role. We asked whether the C-terminal scaffolding domain of gpO could work as a delta domain when fused to the gpN capsid protein. Varying lengths of C-terminal sequences from gpO were fused to the N-terminus of gpN and expressed in E. coli. The presence of just the 41 C-terminal residues of gpO increased the fidelity of assembly and promoted the formation of closed shells, but the shells formed were predominantly small, 40 nm shells, compared to the normal, 55 nm P2 procapsid shells. Larger scaffolding domains fused to gpN caused the formation of shells of varying size and shape. The results suggest that while fusing the scaffolding protein to the capsid protein assists in shell closure, it also restricts the conformational variability of the capsid protein.

Functional domains of the bacteriophage P2 scaffolding protein: identification of residues involved in assembly and protease activity

Bacteriophage P2 encodes a scaffolding protein, gpO, which is required for correct assembly of P2 procapsids from the gpN major capsid protein. The 284 residue gpO protein also acts as a protease, cleaving itself into an N-terminal fragment, O, that remains in the capsid following maturation. In addition, gpO is presumed to act as the maturation protease for gpN, which is N-terminally processed to N, accompanied by DNA packaging and capsid expansion. The protease activity of gpO resides in the N-terminal half of the protein. We show that gpO is a classical serine protease, with a catalytic triad comprised of Asp 19, His 48 and Ser 107. The C-terminal 90 amino acids of gpO are required and sufficient for capsid assembly. This fragment contains a predicted alpha-helical segment between residues 197 and 257 and exists as a multimer in solution, suggesting that oligomerization is required for scaffolding activity. Correct assembly requires the C-terminal cysteine residue, which is most likely involved in transient gpN interactions. Our results suggest a model for gpO scaffolding action in which the N-terminal half of gpO binds strongly to gpN, while oligomerization of the C-terminal alpha-helical domain of gpO and transient interactions between Cys 284 and gpN lead to capsid assembly.

Incorporation of scaffolding protein gpO in bacteriophages P2 and P4

Scaffolding proteins act as chaperones for the assembly of numerous viruses, including most double-stranded DNA bacteriophages. In bacteriophage P2, an internal scaffolding protein, gpO, is required for the assembly of correctly formed viral capsids. Bacteriophage P4 is a satellite phage that has acquired the ability to take control of the P2 genome and use the P2 capsid protein gpN to assemble a capsid that is smaller than the normal P2 capsid. This size determination is dependent on the P4 external scaffolding protein Sid. Although Sid is sufficient to form morphologically correct P4-size capsids, the P2 internal scaffolding protein gpO is required for the formation of viable capsids of both P2 and P4. In most bacteriophages, the scaffolding protein is either proteolytically degraded or exits intact from the capsid after assembly. In the P2/P4 system, however, gpO is cleaved to an N-terminal fragment, O(*), that remains inside the mature capsid after DNA packaging. We previously showed that gpO exhibits autoproteolytic activity, which is abolished by removal of the first 25 amino acids. Co-expression of gpN with this N-terminally truncated version of gpO leads to the production of immature P2 procapsid shells. Here, we use protein analysis and mass spectroscopy to show that P2 and P4 virions as well as procapsids isolated from viral infections contain O(*) and that cleavage occurs between residues 141 and 142 of gpO. By co-expression of gpN with truncated gpO proteins, we show that O(*) binds to gpN and retains the proteolytic activity of gpO and that the C-terminal 90 residues of gpO (residues 195-284) are sufficient to promote the formation of P2-size procapsids. Using mass spectrometry, we have also identified the head completion protein gpL in the virions.

The gpQ portal protein of bacteriophage P2 forms dodecameric connectors in crystals

Double-stranded bacteriophages code for a protein called a connector or portal protein that serves as the entry and exit portal for DNA during genome packaging and ejection, as well as the connection point between heads and tails, and possibly as a nucleator for capsid assembly. The gpQ connector protein from bacteriophage P2 has been overexpressed in Escherichia coli and purified by sucrose gradient centrifugation. Negative stain electron microscopy and image analysis revealed a 135 A diameter dodecameric ring structure with a central 25 A hole. The connector showed a strong propensity to aggregate at low ionic strength and would form microcrystalline structures in solution. Consequently, the connectors were crystallized by hanging-drop vapor diffusion against low ionic strength buffer. Two crystal forms were observed: a P4(1)22 form with unit cell parameters a=b=96.33 A and c=454.42 A that diffracted X-rays to 4.5 A resolution and an I222 crystal form with a=168.86 A, b=171.88 A and c=168.68 A that diffracted to 4.1A resolution. Self-rotation functions confirmed the presence of 12-fold symmetry in the crystals.

Assembly of bacteriophage P2 and P4 procapsids with internal scaffolding protein

Assembly of the E. coli bacteriophage P2 into an icosahedral capsid with T = 7 symmetry is dependent on the gpN capsid protein, the gpQ connector protein and the gpO internal scaffolding protein. In the presence of the P4-encoded protein Sid, the same proteins are assembled into a smaller capsid with T = 4 symmetry. Although gpO has long been expected to act as an internal scaffolding protein, it has not been possible to produce P2 procapsids efficiently in vitro or in vivo due to a failure to express gpO at high levels. In this study, we find that full-length gpO undergoes proteolytic degradation within 1 h of induction of expression. However, a truncated version of gpO lacking the N-terminal 25 amino acids (Odelta25) is stably expressed at high levels and is able to direct the formation of P2 size procapsids. In the presence of Sid, Odelta25 is incorporated into P4 procapsids, showing that Sid overrides the effect of gpO on capsid size determination.

Displacements of prohead protease genes in the late operons of double-stranded-DNA bacteriophages

Most of the known prohead maturation proteases in double-stranded-DNA bacteriophages are shown, by computational methods, to fall into two evolutionarily independent clans of serine proteases, herpesvirus assemblin-like and ClpP-like. Phylogenetic analysis suggests that these two types of phage prohead protease genes displaced each other multiple times while preserving their exact location within the late operons of the phage genomes.

Cleavage leads to expansion of bacteriophage P4 procapsids in vitro

Proteolytic cleavage of the structural proteins is an important part of the maturation process for most bacteriophages and other viruses. In the double-stranded DNA bacteriophages this cleavage is associated with DNA packaging, capsid expansion, and scaffold removal. To understand the role of protein cleavage in the expansion of bacteriophages P2 and P4, we have experimentally cleaved P4 procapsids produced by overexpression of the capsid and scaffolding proteins. The cleavage leads to particle expansion and scaffold removal in vitro. The resulting expanded capsid has a thin-shelled structure similar, but not identical, to that of mature virions.

The structure of P4 procapsids produced by coexpression of capsid and external scaffolding proteins

The double-stranded DNA bacteriophage P4 has a T = 4 icosahedral arrangement of the gpN capsid protein derived from the P2 helper phage. The precursor procapsids in addition contain an external scaffold made up of the P4-encoded Sid protein. High yields of pure P4 procapsids have been obtained by coexpressing the gpN and Sid proteins from a chimeric plasmid. Biochemical measurements show that the ratio of gpN to Sid in the procapsids is 2:1, corresponding to 120 copies of Sid per procapsid particle. A reconstruction of the P4 procapsid, made from 213 particle images to an effective resolution of about 21 A, greatly improves on the previously determined P4 procapsid structures. The structure shows a T = 4 capsid shell and a unique tandem arrangement of 120 copies of chilli-shaped Sid monomers, which form trimers and dimers on the procapsid surface.

Capsid size determination in the P2-P4 bacteriophage system: suppression of sir mutations in P2's capsid gene N by supersid mutations in P4's external scaffold gene sid

The sid gene of the P2-dependent phage P4 provides an external scaffold so P2 N gene encoded protomers assemble as T = 4 capsids rather than as P2's T = 7 capsids. Mutations (sir) in the middle of N interfere with Sid's function. We describe a new P4 mutant class, nms ("supersid") mutations, which direct also P2 sir to provide small capsids. Three different nms mutations were located near the sid end, commingled with sid(-) mutations. Suppression of sir by nms is not allele-specific. Our results favor this interpretation of capsid size control: (i) sir mutations reduce pN protomer flexibility and thereby interfere with the generation of T = 4 compatible hexons; (ii) the C-termini of Sid molecules link up when forming the scaffold; nms mutations strengthen these Sid-Sid contacts and thus allow the scaffold to force even sir-type protomers to form T = 4 compatible hexons. Some related findings concern suppression of N ts mutations by P4.

In vitro assembly of bacteriophage P4 procapsids from purified capsid and scaffolding proteins

Bacteriophage P4 is a satellite virus of bacteriophage P2, which has acquired the ability to utilize the structural gene products of P2 to assemble its own capsid. The normal P2 capsid has a T = 7 icosahedral structure comprised of the gpN-derived capsid protein, whereas the capsid produced under the control of P4 has a smaller, T = 4 structure. The protein responsible for this size determination is the P4-coded gene product Sid, which forms an external scaffold on the P4 procapsid. Using an in vitro assembly system, we show that gpN and Sid can coassemble into procapsid-like particles, indistinguishable from those produced in vivo, in the absence of any other gene products. The fidelity of the assembly reaction is enhanced by the inclusion of PEG and has a pH optimum between 8.0 and 8.5. Analysis of the assembly properties of truncated versions of Sid and gpN suggests that the amino-terminal part of Sid is involved in gpN binding, while the carboxyl-terminal part forms trimeric Sid-Sid interactions, and that the first 31 amino acids of gpN are required for binding to Sid as well as for size determination.

The complete nucleotide sequence of phi CTX, a cytotoxin-converting phage of Pseudomonas aeruginosa: implications for phage evolution and horizontal gene transfer via bacteriophages

phi CTX is a cytotoxin-converting phage isolated from Pseudomonas aeruginosa. In this study, we determined the complete nucleotide sequence of the phi CTX phage genome. The precise genome size was 35,538 bp with 21 base 5'-extruding cohesive ends. Forty-seven open reading frames (ORFs) were identified on the phi CTX genome, including two previously identified genes, ctx and int. Among them, 15 gene products were identified in the phage particle by protein microsequencing. The most striking feature of the phi CTX genome was an extensive homology with the coliphage P2 and P2-related phages; more than half of the ORFs (25 ORFs) had marked homology to P2 genes with 28.9-65.8% identity. The gene arrangement on the genome was also highly conserved for the two phages, although the G + C content and codon usage of most phi CTX genes were similar to those of the host P. aeruginosa chromosome. In addition, phi CTX was found to share several common features with P2, including the morphology, non-inducibility, use of lipopolysaccharide core oligosaccharide as receptor and Ca(2+)-dependent receptor binding. These findings indicate that phi CTX is a P2-like phage well adapted to P. aeruginosa, and provide clear evidence of the intergeneric spread and evolution of bacteriophages. Furthermore, comparative analysis of genome structures of phi CTX, P2 and other P2 relatives revealed the presence of several hot-spots where foreign DNAs, including the cytotoxin gene, were inserted. They appear to be deeply concerned in the acquisition of various genes that are horizontally transferred by bacteriophage infection.

Bacteriophage P2 and P4 morphogenesis: structure and function of the connector

The connector, the structure located between the bacteriophage capsid and tail, is interesting from several points of view. The connector is in many cases involved in the initiation of the capsid assembly process, functions as a gate for DNA transport in and out of the capsid, and is, as implied by the name, the structure connecting a tail to the capsid. Occupying a position on a 5-fold axis in the capsid and connected to a coaxial 6-fold tail, it mediates a symmetry mismatch between the two. To understand how the connector is capable of all these interactions its structure needs to be worked out. We have focused on the bacteriophage P2/P4 connector, and here we report an image reconstruction based on 2D crystalline layers of connector protein expressed from a plasmid in the absence of other phage proteins. The overall design of the connector complies well with that of other phage connectors, being a toroid structure having a conspicuous central channel. Our data suggests a 12-fold symmetry, i.e., 12 protrusions emerge from the more compact central part of the structure. However, rotational analysis of single particles suggests that there are both 12- and 13-mers present in the crude sample. The connectors used in this image reconstruction work differ from connectors in virions by having retained the amino-terminal 26 amino acids normally cleaved off during the morphogenetic process. We have used different late gene mutants to demonstrate that this processing occurs during DNA packaging, since only mutants in gene P, coding for the large terminase subunit, accumulate uncleaved connector protein. The suggestion that the cleavage might be intimately involved in the DNA packaging process is substantiated by the fact that the fragment cleaved off is highly basic and is homologous to known DNA binding sequences.

Cloning and characterization of bacteriophage-like DNA from Haemophilus somnus homologous to phages P2 and HP1

In an attempt to identify and characterize components of a heme uptake system of Haemophilus somnus, an Escherichia coli cosmid library of H. somnus genomic DNA was screened for the ability to bind hemin (Hmb+). The Hmb+ phenotype was associated with a 7,814-bp HindIII fragment of H. somnus DNA that was subcloned and sequenced. Thirteen open reading frames (orfs) were identified, all transcribed in one direction, and transposon mutagenesis identified orf7 as the gene associated with the Hmb+ phenotype. Orf7 (178 amino acids) has extensive homology with the lysozymes of bacteriophages P-A2, P21, P22, PZA, phi-29, phi-vML3, T4, or HP1. The orf7 gene complemented the lytic function of the K gene of phage P2 and the R gene of phage lambda. A lysozyme assay using supernatants from whole-cell lysates of E. coli cultures harboring plasmid pRAP501 or pGCH2 (both of which express the orf7 gene product) exhibited significant levels of lysozyme activity. The orf6 gene upstream of orf7 has the dual start motif common to the holins encoded by lambdoid S genes, and the orf6 gene product has significant homology to the holins of phages HP1 and P21. When expressed from a tac promoter, the orf6 gene product caused immediate cell death without lysis, while cultures expressing the orf7 gene product grew at normal rates but lysed immediately after the addition of chloroform. Based on this data, we concluded that the Hmb+ phenotype was an artifact resulting from the expression of cloned lysis genes which were detrimental to the E. coli host. The DNA flanking the cloned lysis genes contains orfs that are similar to structural and DNA packaging genes of phage P2. Polyclonal antiserum against Orf2, which is homologous to the major capsid precursor protein (gpN) of phage P2, detected a 40,000-M(r) protein expressed from pRAP401 but did not detect Orf2 in H. somnus, lysates. The phage-like DNA was detected in the serum-susceptible preputial strains HS-124P and HS-127P but was absent from the serum-resistant preputial strains HS-20P and HS-22P. Elucidation of a potential role for this cryptic prophage in the H. somnus life cycle requires more study.

The complete nucleotide sequence of bacteriophage HP1 DNA

The complete nucleotide sequence of the temperate phage HP1 of Haemophilus influenzae was determined. The phage contains a linear, double-stranded genome of 32 355 nt with cohesive termini. Statistical methods were used to identify 41 probable protein coding segments organized into five plausible transcriptional units. Regions encoding proteins involved in recombination, replication, transcriptional control, host cell lysis and phage production were identified. The sizes of proteins in the mature HP1 particle were determined to assist in identifying genes for structural proteins. Similarities between HP1 coding sequences and those in databases, as well as similar gene organizations and control mechanisms, suggest that HP1 is a member of the P2-like phage family, with strong similarities to coliphages P2 and 186 and some similarity to the retronphage Ec67.

Peptide presentation by bacteriophage P4

This article focuses on bacteriophage P4 as a potential peptide display phage by exploring the possibility of using the P4 capsid decoration component, Psu, as a peptide carrier protein. Psu is non-essential for P4 growth but it enhances the stability of the P4 capsid by binding to its exterior. We have constructed a unique SacI cloning site in the beginning of the psu gene. This site changes the third amino acid of Psu from Ser to Leu. This substitution does not destroy the binding of Psu to the P4 capsid. A synthetic oligonucleotide encoding a 10-amino acid peptide whose sequence is part of the human p62c-myc protein, has been inserted into the SacI site. The Psuc-myc shows full capsid binding activity and reacts with monoclonal antibodies directed against the c-myc peptide. These results pave the way for the further development of a peptide display system based on bacteriophage P4.

Electron microscopic studies on intracellular phage development--history and perspectives

This review is centered on the applications of thin sections to the study of intracellular precursors of bacteriophage heads. Results obtained with other preparation methods are included in so far as they are essential for the comprehension of the biological problems. This type of work was pioneered with phage T4, which contributed much to today's understanding of morphogenesis and form determination. The T4 story is rich in successes, but also in many fallacies. Due to its large size, T4 is obviously prone to preparation artefacts such as emptying, flattening and others. Many of these artefacts were first encountered in T4. Artefacts are mostly found in lysates, however, experience shows that they are not completely absent from thin sections. This can be explained by the fact that permeability changes induced by fixatives occur. The information gained from T4 was profitably used for the study of other phages. They are included in this review as far as electron microscopic studies played a major role in the elucidation of their morphogenetic pathways. Research on phage assembly pathways and form determination is a beautiful illustration for the power of the integrated approach which combines electron microscopy with biochemistry, genetics and biophysics. As a consequence, we did not restrict ourselves to the review of electron microscopic work but tried to integrate pertinent data which contribute to the understanding of the molecular mechanisms acting in determining the form of supramolecular structures.

Mechanisms of genome propagation and helper exploitation by satellite phage P4

Temperate coliphage P2 and satellite phage P4 have icosahedral capsids and contractile tails with side tail fibers. Because P4 requires all the capsid, tail, and lysis genes (late genes) of P2, the genomes of these phages are in constant communication during P4 development. The P4 genome (11,624 bp) and the P2 genome (33.8 kb) share homologous cos sites of 55 bp which are essential for generating 19-bp cohesive ends but are otherwise dissimilar. P4 turns on the expression of helper phage late genes by two mechanisms: derepression of P2 prophage and transactivation of P2 late-gene promoters. P4 also exploits the morphopoietic pathway of P2 by controlling the capsid size to fit its smaller genome. The P4 sid gene product is responsible for capsid size determination, and the P2 capsid gene product, gpN, is used to build both sizes. The P2 capsid contains 420 capsid protein subunits, and P4 contains 240 subunits. The size reduction appears to involve a major change of the whole hexamer complex. The P4 particles are less stable to heat inactivation, unless their capsids are coated with a P4-encoded decoration protein (the psu gene product). P4 uses a small RNA molecule as its immunity factor. Expression of P4 replication functions is prevented by premature transcription termination effected by this small RNA molecule, which contains a sequence that is complementary to a sequence in the transcript that it terminates.

The polarity suppression factor of bacteriophage P4 is also a decoration protein of the P4 capsid

We show that the product of the polarity suppression (psu) gene from bacteriophage P4 associates with P4 capsids. This association can occur when Psu is (i) provided in vivo from the P4 genome or from a plasmid or (ii) provided in vitro by mixing viable phage particles with Psu protein. Psu is unable to associate with the larger capsid of P4's helper phage P2. Discrimination of the P4 and P2 capsids by Psu appears to be independent of the presence of the P4 genome in the capsid, since P2 size capsids filled with P4 DNA cannot accommodate Psu association. P4 psu particles devoid of Psu are less stable than P4 particles carrying Psu. These results indicate that, in addition to its antitermination activity at Rho-dependent terminators, Psu is also a decoration protein that stabilizes the P4 capsids.