A truncated form of the bacteriophage Mu B protein promotes conservative integration, but not replicative transposition, of Mu DNA

Transposable Phage Mu

Transposable phage Mu has played a major role in elucidating the mechanism of movement of mobile DNA elements. The high efficiency of Mu transposition has facilitated a detailed biochemical dissection of the reaction mechanism, as well as of protein and DNA elements that regulate transpososome assembly and function. The deduced phosphotransfer mechanism involves in-line orientation of metal ion-activated hydroxyl groups for nucleophilic attack on reactive diester bonds, a mechanism that appears to be used by all transposable elements examined to date. A crystal structure of the Mu transpososome is available. Mu differs from all other transposable elements in encoding unique adaptations that promote its viral lifestyle. These adaptations include multiple DNA (enhancer, SGS) and protein (MuB, HU, IHF) elements that enable efficient Mu end synapsis, efficient target capture, low target specificity, immunity to transposition near or into itself, and efficient mechanisms for recruiting host repair and replication machineries to resolve transposition intermediates. MuB has multiple functions, including target capture and immunity. The SGS element promotes gyrase-mediated Mu end synapsis, and the enhancer, aided by HU and IHF, participates in directing a unique topological architecture of the Mu synapse. The function of these DNA and protein elements is important during both lysogenic and lytic phases. Enhancer properties have been exploited in the design of mini-Mu vectors for genetic engineering. Mu ends assembled into active transpososomes have been delivered directly into bacterial, yeast, and human genomes, where they integrate efficiently, and may prove useful for gene therapy.

Mu insertions are repaired by the double-strand break repair pathway of Escherichia coli

Mu is both a transposable element and a temperate bacteriophage. During lytic growth, it amplifies its genome by replicative transposition. During infection, it integrates into the Escherichia coli chromosome through a mechanism not requiring extensive DNA replication. In the latter pathway, the transposition intermediate is repaired by transposase-mediated resecting of the 5′ flaps attached to the ends of the incoming Mu genome, followed by filling the remaining 5 bp gaps at each end of the Mu insertion. It is widely assumed that the gaps are repaired by a gap-filling host polymerase. Using the E. coli Keio Collection to screen for mutants defective in recovery of stable Mu insertions, we show in this study that the gaps are repaired by the machinery responsible for the repair of double-strand breaks in E. coli—the replication restart proteins PriA-DnaT and homologous recombination proteins RecABC. We discuss alternate models for recombinational repair of the Mu gaps.

Transposon activity shapes genome structure and evolution. The movement of these elements generates target site duplications as a result of staggered cuts in the target made initially by the transposase. For replicative transposons, the single-stranded gaps generated after the initial strand transfer event are filled by target-primed replication. However, the majority of known transposable elements transpose by a non-replicative mechanism. Despite a wealth of information available for the mechanism of transposase action, little is known about how the cell repairs gaps left in the wake of transposition of these majority elements. Phage Mu is unique in using both replicative and non-replicative modes of transposition. Our study finds that during its non-replicative pathway, the gaps created by Mu insertion are repaired by the primary machinery for double-strand break repair in E. coli, not by gap-filling polymerases as previously thought. This first report of specific host processes involved in repair of transposon insertions in bacteria is likely to have a broad significance, given also that double-strand break repair pathways have been implicated in repair of the retroviral and Line retroelement insertions.

Chromosomal integration mechanism of infecting mu virion DNA

DNA transposition is central to the propagation of temperate phage Mu. A long-standing problem in Mu biology has been the mechanism by which the linear genome of an infecting phage, which is linked at both ends to DNA acquired from a previous host, integrates into the new host chromosome. If Mu were to use its well-established cointegrate mechanism for integration (single-strand nicks at Mu ends, joined to a staggered double-strand break in the target), the flanking host sequences would remain linked to Mu; target-primed replication of the linear integrant would subsequently break the chromosome. The absence of evidence for chromosome breaks has led to speculation that infecting Mu might use a cut-and-paste mechanism, whereby Mu DNA is cut away from the flanking sequences prior to integration. In this study we have followed the fate of the flanking DNA during the time course of Mu infection. We have found that these sequences are still attached to Mu upon integration and that they disappear soon after. The data rule out a cut-and-paste mechanism and suggest that infecting Mu integrates to generate simple insertions by a variation of its established cointegrate mechanism in which, instead of a "nick, join, and replicate" pathway, it follows a "nick, join, and process" pathway. The results show similarities with human immunodeficiency virus integration and provide a unifying mechanism for development of Mu along either the lysogenic or lytic pathway.

Visualizing the assembly and disassembly mechanisms of the MuB transposition targeting complex

MuB, a protein essential for replicative DNA transposition by the bacteriophage Mu, is an ATPase that assembles into a polymeric complex on DNA. We used total internal reflection fluorescence microscopy to observe the behavior of MuB polymers on single molecules of DNA. We demonstrate that polymer assembly is initiated by a stochastic nucleation event. After nucleation, polymer assembly occurs by a mechanism involving the sequential binding of small units of MuB. MuB that bound to A/T-rich regions of the DNA assembled into large polymeric complexes. In contrast, MuB that bound outside of the A/T-rich regions failed to assemble into large oligomeric complexes. Our data also show that MuB does not catalyze multiple rounds of ATP hydrolysis while remaining bound to DNA. Rather, a single ATP is hydrolyzed, then MuB dissociates from the DNA. Finally, we show that "capping" of the enhanced green fluorescent protein-MuB polymer ends with unlabeled MuB dramatically slows, but does not halt, dissociation. This suggests that MuB dissociation occurs through both an end-dependent mechanism and a slower mechanism wherein subunits dissociate from the polymer interior.

Effect of mutations in the C-terminal domain of Mu B on DNA binding and interactions with Mu A transposase

Bacteriophage Mu transposition requires two phage-encoded proteins, the transposase, Mu A, and an accessory protein, Mu B. Mu B is an ATP-dependent DNA-binding protein that is required for target capture and target immunity and is an allosteric activator of transpososome function. The recent NMR structure of the C-terminal domain of Mu B (Mu B223-312) revealed that there is a patch of positively charged residues on the solvent-exposed surface. This patch may be responsible for the nonspecific DNA binding activity displayed by the purified Mu B223-312 peptide. We show that mutations of three lysine residues within this patch completely abolish nonspecific DNA binding of the C-terminal peptide (Mu B223- 312). To determine how this DNA binding activity affects transposition we mutated these lysine residues in the full-length protein. The full-length protein carrying all three mutations was deficient in both strand transfer and allosteric activation of transpososome function but retained ATPase activity. Peptide binding studies also revealed that this patch of basic residues within the C-terminal domain of Mu B is within a region of the protein that interacts directly with Mu A. Thus, we conclude that this protein segment contributes to both DNA binding and protein-protein contacts with the Mu transposase.

DNA gyrase requirements distinguish the alternate pathways of Mu transposition

The MuA transposase mediates transposition of bacteriophage Mu through two distinct mechanisms. The first integration event following infection occurs through a non-replicative mechanism. In contrast, during lytic growth, multiple rounds of replicative transposition amplify the phage genome. We have examined the influence of gyrase and DNA supercoiling on these two transposition pathways using both a gyrase-inhibiting drug and several distinct gyrase mutants. These experiments reveal that gyrase activity is not essential for integration; both lysogens and recombination intermediates are detected when gyrase is inhibited during Mu infection. In contrast, gyrase inhibition causes severe defects in replicative transposition. In two of the mutants, as well as in drug-treated cells, replicative transposition is almost completely blocked. Experiments probing for formation of MuA-DNA complexes in vivo reveal that this block occurs very early, during assembly of the transposase complex required for the catalytic steps of recombination. The findings establish that DNA structure-based signals are used differently for integrative and replicative transposition. We propose that transposase assembly, the committed step for recombination, has evolved to depend on different DNA /architectural signals to control the reaction outcome during these two distinct phases of the phage life cycle.

Target immunity during Mu DNA transposition. Transpososome assembly and DNA looping enhance MuA-mediated disassembly of the MuB target complex

The Mu transpososome can distinguish between proximal and distal DNA during the selection of a site for transposition. This phenomenon, termed target immunity, involves MuA-stimulated removal of MuB oligomers from sites near the Mu genome. Using a combination of ensemble and single-molecule fluorescence methods, we show that the MuA tetramer can stably associate with the DNA-bound MuB oligomer and is more efficient than monomeric MuA at stimulating the dissociation of MuB from DNA. In addition, we demonstrate that DNA looping is essential for efficient disassembly of the MuB oligomer. We propose a model in which the MuA tetramer forms a multivalent complex with the MuB oligomer and catalyzes the processive removal of MuB from DNA.

Dynamics of a protein polymer: the assembly and disassembly pathways of the MuB transposition target complex

MuB assembles into a polymer on DNA in the presence of ATP and is directly involved in the selection of an appropriate site on the Escherichia coli chromosome for the insertion of the bacteriophage Mu genome. We have developed an assay using fluorescently tagged proteins to monitor the polymeric state of MuB via fluorescence resonance energy transfer. We show that polymer assembly is initiated by the formation of an ATP-MuB complex. MuB then self-associates into a protomer before binding to DNA. Upon binding to DNA, a dramatic increase in energy transfer is observed, suggesting a conformational change within MuB. Polymer disassembly is much slower than assembly and is greatly stimulated by the MuA transposase. Additionally, MuB is readily exchanged between polymers, and ATP hydrolysis is directly coupled to polymer disassembly. Our data support a model in which a combination of rapid polymer assembly, MuA-mediated disassembly, followed by rapid reassembly of the polymer allows MuB to sample multiple DNA targets until an appropriate site is located for the insertion of the bacteriophage genome.

Handoff from recombinase to replisome: insights from transposition

Bacteriophage Mu replicates as a transposable element, exploiting host enzymes to promote initiation of DNA synthesis. The phage-encoded transposase MuA, assembled into an oligomeric transpososome, promotes transfer of Mu ends to target DNA, creating a fork at each end, and then remains tightly bound to both forks. In the transition to DNA synthesis, the molecular chaperone ClpX acts first to weaken the transpososome's interaction with DNA, apparently activating its function as a molecular matchmaker. This activated transpososome promotes formation of a new nucleoprotein complex (prereplisome) by yet unidentified host factors [Mu replication factors (MRF alpha 2)], which displace the transpososome in an ATP-dependent reaction. Primosome assembly proteins PriA, PriB, DnaT, and the DnaB--DnaC complex then promote the binding of the replicative helicase DnaB on the lagging strand template of the Mu fork. PriA helicase plays an important role in opening the DNA duplex for DnaB binding, which leads to assembly of DNA polymerase III holoenzyme to form the replisome. The MRF alpha 2 transition factors, assembled into a prereplisome, not only protect the fork from action by nonspecific host enzymes but also appear to aid in replisome assembly by helping to activate PriA's helicase activity. They consist of at least two separable components, one heat stable and the other heat labile. Although the MRF alpha 2 components are apparently not encoded by currently known homologous recombination genes such as recA, recF, recO, and recR, they may fulfill an important function in assembling replisomes on arrested replication forks and products of homologous strand exchange.

Differential role of the Mu B protein in phage Mu integration vs. replication: mechanistic insights into two transposition pathways

The Mu B protein is an ATP-dependent DNA-binding protein and an allosteric activator of the Mu transposase. As a result of these activities, Mu B is instrumental in efficient transposition and target-site choice. We analysed in vivo the role of Mu B in the two different recombination reactions performed by phage Mu: non-replicative transposition, the pathway used during integration, and replicative transposition, the pathway used during lytic growth. Utilizing a sensitive PCR-based assay for Mu transposition, we found that Mu B is not required for integration, but enhances the rate and extent of the process. Furthermore, three different mutant versions of Mu B, Mu BC99Y, Mu BK106A, and Mu B1-294, stimulate integration to a similar level as the wild-type protein. In contrast, these mutant proteins fail to support Mu growth. This deficiency is attributable to a defect in formation of an essential intermediate for replicative transposition. Biochemical analysis of the Mu B mutant proteins reveals common features: the mutants retain the ability to stimulate transposase, but are defective in DNA binding and target DNA delivery. These data indicate that activation of transposase by Mu B is sufficient for robust non-replicative transposition. Efficient replicative transposition, however, demands that the Mu B protein not only activate transposase, but also bind and deliver the target DNA.

The solution structure of the C-terminal domain of the Mu B transposition protein

Mu B is one of four proteins required for the strand transfer step of bacteriophage Mu DNA transposition and the only one where no high resolution structural data is available. Structural work on Mu B has been hampered primarily by solubility problems and its tendency to aggregate. We have overcome this problem by determination of the three-dimensional structure of the C-terminal domain of Mu B (B(223-312)) in 1.5 M NaCl using NMR spectroscopic methods. The structure of Mu B(223-312) comprises four helices (backbone r.m.s.d. 0.46 A) arranged in a loosely packed bundle and resembles that of the N-terminal region of the replication helicase, DnaB. This structural motif is likely to be involved in the inter-domainal regulation of ATPase activity for both Mu A and DnaB. The approach described here for structural determination in high salt may be generally applicable for proteins that do not crystallize and that are plagued by solubility problems at low ionic strength.

Organization and dynamics of the Mu transpososome: recombination by communication between two active sites

Movement of transposable genetic elements requires the cleavage of each end of the element genome and the subsequent joining of these cleaved ends to a new target DNA site. During Mu transposition, these reactions are catalyzed by a tetramer of four identical transposase subunits bound to the paired Mu DNA ends. To elucidate the organization of active sites within this tetramer, the subunit providing the essential active site DDE residues for each cleavage and joining reaction was determined. We demonstrate that recombination of the two Mu DNA ends is catalyzed by two active sites, where one active site promotes both cleavage and joining of one Mu DNA end. This active site uses all three DDE residues from the subunit bound to the transposase binding site proximal to the cleavage site on the other Mu DNA end (catalysis in trans). In addition, we uncover evidence that the catalytic activity of these two active sites is coupled such that the coordinated joining of both Mu DNA ends is favored during recombination. On the basis of these results, we propose that the DNA joining stage requires a cooperative transition within the transposase-DNA complex. The cooperative utilization of active sites supplied in trans by Mu transposase provides an example of how mobile elements can ensure concomitant recombination of distant DNA sites.

Mu gem2ts DNA integration is not necessary for induction of synchrony of cell division in Escherichia coli K12

The gem2ts mutant of bacteriophage Mu induced synchrony of cell division on bacteria surviving infection. Induction of synchronous growth could also be observed as a response to the entire infected bacterial population, as in the case of infection of hic mutants, a peculiar class of gyrB alleles. After Mu wild-type or Mu gem2ts infection of hic mutants, there was a lack of viral DNA integration and replication, while phage gene expression (including that of A gene, coding for the transposase) seemed to be quite normal. These data indicate that the mechanism of bacterial synchronization induced by Mu gem2ts does not require integration nor replication of the phage DNA.

ClpX and MuB interact with overlapping regions of Mu transposase: implications for control of the transposition pathway

Transposition of phage Mu is catalyzed by an extremely stable transposase-DNA complex. Once recombination is complete, the Escherichia coli ClpX protein, a member of the Clp/Hsp100 chaperone family, initiates disassembly of the complex for phage DNA replication to commence. To understand how the transition between recombination and replication is controlled, we investigated how transposase-DNA complexes are recognized by ClpX. We find that a 10-amino-acid peptide from the carboxy-terminal domain of transposase is required for its recognition by ClpX. This short, positively charged peptide is also sufficient to convert a heterologous protein into a ClpX substrate. The region of transposase that interacts with the transposition activator, MuB protein, is also defined further and found to overlap with that recognized by ClpX. As a consequence, MuB inhibits disassembly of several transposase-DNA complexes that are intermediates in recombination. This ability of MuB to block access to transposase suggests a mechanism for restricting ClpX-mediated remodeling to the proper stage during replicative transposition. We propose that overlap of sequences involved in subunit interactions and those that target a protein for remodeling or destruction may be a useful design for proteins that function in pathways where remodeling or degradation must be regulated.

Disassembly of the Mu transposase tetramer by the ClpX chaperone

Mu transposition is promoted by an extremely stable complex containing a tetramer of the transposase (MuA) bound to the recombining DNA. Here we purify the Escherichia coli ClpX protein, a member of a family of multimeric ATPases present in prokaryotes and eukaryotes (the Clp family), on the basis of its ability to remove the transposase from the DNA after recombination. Previously, ClpX has been shown to function with the ClpP peptidase in protein turnover. However, neither ClpP nor any other protease is required for disassembly of the transposase. The released MuA is not modified extensively, degraded, or irreversibly denatured, and is able to perform another round of recombination in vitro. We conclude that ClpX catalyzes the ATP-dependent release of MuA by promoting a transient conformational change in the protein and, therefore, can be considered a molecular chaperone. ClpX is important at the transition between the recombination and DNA replication steps of transposition in vitro; this function probably corresponds to the essential contribution of ClpX for Mu growth. Deletion analysis reveals that the sequence at the carboxyl terminus of MuA is important for disassembly by ClpX and can target MuA for degradation by ClpXP in vitro. These data contribute to the emerging picture that members of the Clp family are chaperones specifically suited for disaggregating proteins and are able to function with or without a collaborating protease.

MuB protein allosterically activates strand transfer by the transposase of phage Mu

The MuA and MuB proteins collaborate to mediate efficient transposition of the phage Mu genome into many DNA target sites. MuA (the transposase) carries out all the DNA cleavage and joining steps. MuB stimulates strand transfer by activating the MuA-donor DNA complex through direct protein-protein contact. The C-terminal domain of MuA is required for this MuA-MuB interaction. Activation of strand transfer occurs irrespective of whether MuB is bound to target DNA. When high levels of MuA generate a pool of free MuB (not bound to DNA) or when chemical modification of MuB impairs its ability to bind DNA, MuB still stimulates strand transfer. However, under these conditions, intramolecular target sites are used exclusively because of their close proximity to the MuA-MuB-donor DNA complex.

Two mutations of phage mu transposase that affect strand transfer or interactions with B protein lie in distinct polypeptide domains

Two mutations within the transposase (the A protein) gene of phage Mu with distinct effects on DNA transposition have been studied. The first mutation maps to the central domain (domain II) of A, a protein consisting of three major structural domains. The variant protein is normal in synapsis and cleavage of Mu ends but is temperature-sensitive in the strand transfer reaction, joining the Mu ends to target DNA. The second mutation is a deletion at the C terminus (within domain III); on the basis of genetic studies, the mutant protein is predicted to have lost the ability to interact with the Mu B protein. The B protein, in conjunction with A, promotes efficient intermolecular transposition, while inhibiting intramolecular transposition. We show that the purified mutant protein is proficient in intramolecular, but not intermolecular transposition in vitro. The interactions between A and B proteins have been followed by a proteolysis assay. The chymotrypsin sensitivity of the interdomainal Phe221-Ser222 peptide bond within the bidomainally organized B protein is exquisitely modulated by ATP, DNA and A protein. The sensitive or "open" state of this bond in native B protein becomes partially "open" upon binding of ATP by B, attains a "closed" or resistant configuration upon binding of DNA in presence of ATP, and is rendered "open" again upon addition of the A protein. In this test for the interaction of A protein with B protein-DNA complex, the domain II mutant behaves like wild-type A protein. However, the domain III mutant fails to restore chymotrypsin susceptibility of the Phe221-Ser222 bond.

Bacteriophage Mu sites and functions involved in the inhibition of lambda::mini-Mu growth

To better understand the nature of the mini-Mu-directed process which results in inhibition of lambda::mini-Mu growth we characterized spontaneous deletion mutants of the lambda::mini-Mu phage. On the basis of analysis of the deletion endpoints, mini-Mu replication functions, and integration and inhibition properties, the lambda::mini-Mu deletion mutants were divided into five classes which define the Mu sites and functions involved in lambda::mini-Mu growth inhibition. Class 1 mutants, which still exhibit lambda::mini-Mu growth inhibition, collectively delete all the Mu late functions encoded by the mini-Mu. Class 2 and 5 mutants, which show cis-dominant defects in inhibition and integration, delete the right and left mini-Mu attachment sites, respectively. Phages of Classes 3 and 4, which delete the Mu B or A and B genes, respectively, show recessive defects in growth inhibition. The properties of these mutants define the Mu replication functions, A and B, and the Mu attachment sites as essential for the inhibition of lambda::mini-Mu growth. The observation that the sites and functions essential for Mu replication also have requisite roles in the inhibition of lambda::mini-Mu growth suggests that inhibition results from mini-Mu-promoted replicative interference of lambda::mini-Mu development.

Secondary structural features of the bacteriophage Mu-encoded A and B transposition proteins

The role of the bacteriophage Mu-encoded A and B proteins is to direct the transposition of Mu DNA. These are the first active DNA transposition proteins to have been purified and their mechanism of action at the biochemical level is under intensive study. Structural studies on these proteins, however, have lagged behind their biochemical characterization. We report here near- and far-u.v. c.d. spectra for these proteins and their secondary structural features derived from these data. The Mu A protein appears to be composed of primarily beta-sheet (40%) with 24% alpha-helix, 9% beta-turn and 27% random coil. In contrast, the Mu B protein contains 55% alpha-helix with only 13% beta-sheet and 3+ beta-turn and 29% random coil. The near-u.v. c.d. spectrum of the A protein was not unusual; however, the profile of the B protein suggested either buried or restricted chromophores within the protein or short-range interactions between aromatic residues.

Characterization of amber mutations in bacteriophage Mu transposase: a functional analysis of the protein

We have characterized a series of amber mutations in the A gene of bacteriophage Mu encoding the phage transposase. We tested different activities of these mutant proteins either in a sup0 strain or in different sup bacteria. In conjunction with the results described in the accompanying paper by Bétermier et al. (1989) we find that the C-terminus of the protein is not absolutely essential for global transposase function, but is essential for phage growth. Specific binding to Mu ends is defined by a more central domain. Our results also reinforce the previous findings (Bétermier et al., 1987) that more than one protein may be specified by the A gene.

Target immunity of Mu transposition reflects a differential distribution of Mu B protein

A DNA molecule carrying Mu end DNA sequence(s) is a poor target in the Mu DNA strand-transfer reaction, a phenomenon which is referred to as "target immunity." We find that Mu B protein stimulates intermolecular strand-transfer by binding to the target DNA. Our results show that a differential distribution of Mu B protein between "immune" and "non-immune" DNA molecules is responsible for target immunity; in the presence of Mu A protein and ATP, Mu B protein dissociates preferentially from immune DNA molecules. Hydrolysis of ATP is implicated in establishing the differential distribution of Mu B protein between immune and non-immune DNA molecules in the presence of Mu A protein; nonhydrolyzable ATP gamma S can support an efficient strand-transfer reaction even with a target DNA that is immune in a reaction with ATP.

Lysogenization of Escherichia coli him+, himA, and himD hosts by bacteriophage Mu

The possible outcomes of infection of Escherichia coli by bacteriophage Mu include lytic growth, lysogen formation, nonlysogenic surviving cells, and perhaps simple killing of the host. The influence of various parameters, including host himA and himD mutations, on lysogeny and cell survival is described. Mu does not grow lytically in or kill him bacteria but can lysogenize such hosts. Mu c+ lysogenizes about 8% of him+ bacteria infected at low multiplicity at 37 degrees C. The frequency of lysogens per infected him+ cell diminishes with increasing multiplicity of infection or with increasing temperature over the range from 30 to 42 degrees C. In him bacteria, the Mu lysogenization frequency increases from about 7% at low multiplicity of infection to approach a maximum where most but not all cells are lysogens at high multiplicity of infection. Lysogenization of him hosts by an assay phage marked with antibiotic resistance is enhanced by infection with unmarked auxiliary phage. This helping effect is possible for at least 1 h, suggesting that Mu infection results in formation of a stable intermediate. Mu immunity is not required for lysogenization of him hosts. We argue that in him bacteria, all Mu genomes which integrate into the host chromosome form lysogens.

In vivo mutagenesis of bacteriophage Mu transposase

We devised a method for isolating mutations in the bacteriophage Mu A gene which encodes the phage transposase. Nine new conditional defective A mutations were isolated. These, as well as eight previously isolated mutations, were mapped with a set of defined deletions which divided the gene into 13 100- to 200-base-pair segments. Phages carrying these mutations were analyzed for their ability to lysogenize and to transpose in nonpermissive hosts. One Aam mutation, Aam7110, known to retain the capacity to support lysogenization of a sup0 host (M. M. Howe, K. J. O'Day, and D. W. Shultz, Virology 93:303-319, 1979) and to map 91 base pairs from the 3' end of the gene (R. M. Harshey and S. D. Cuneo, J. Genet. 65:159-174, 1987) was shown to be able to complement other A mutations for lysogenization, although it was incapable of catalyzing either the replication of Mu DNA or the massive conservative integration required for phage growth. Four Ats mutations which map at different positions in the gene were able to catalyze lysogenization but not phage growth at the nonpermissive temperature. Phages carrying mutations located at different positions in the Mu B gene (which encodes a product necessary for efficient integration and lytic replication) were all able to lysogenize at the same frequency. These results suggest that the ability of Mu to lysogenize is not strictly correlated with its ability to perform massive conservative and replicative transposition.

Phage Mu transposase: deletion of the carboxy-terminal end does not abolish DNA-binding activity

We demonstrate that a specific site on the transposase protein, pA, of bacteriophage Mu is highly susceptible to proteolytic cleavage. Cleavage is observed in a minicell system on solubilisation with the non-ionic detergent Triton X-100 or following addition of a solubilised minicell preparation to pA synthesised in a cell-free coupled transcription/translation system. Cleavage occurs at the carboxy-terminal end of the protein and generates a truncated polypeptide of 64 kDa, pA*, which retains some of the DNA-binding properties of pA. These results suggest that pA may be divided into functional domains for DNA binding and for interaction with the proteins involved in phage replication.

Specificity of bacteriophage Mu excision

To study the excision of bacteriophage Mu at the DNA sequence level, the Mu-derived phage lambda placMu3 was transposed to the transcribed but non-translated leader region of a plasmid-borne tetracycline (tet) resistance gene. Revertants (excision products) were then selected by Tet+ restoration of Tet+ and characterized. Of 21 independent Tet+ revertants, 17 contained simple deletions of most or all of lambda placMu3, while the other four contained more complex rearrangements in which one end of lambda placMu3 had been transposed, and most of the prophage had been deleted. The deletion endpoints were found in short direct repeats in each of the complex rearrangements and in 11 of the 17 simple deletion excisants. The results suggest models of slipped mispairing of template and nascent DNA strands facilitated by proteins of the Mu transposition machinery.

Transpososomes: stable protein-DNA complexes involved in the in vitro transposition of bacteriophage Mu DNA

We report that two types of stable protein-DNA complexes, or transpososomes, are generated in vitro during the Mu DNA strand transfer reaction. The Type 1 complex is an intermediate in the reaction. Its formation requires a supercoiled mini-Mu donor plasmid, Mu A and HU protein, and Mg2+. In the Type 1 complex the two ends of Mu are held together, creating a figure eight-shaped molecule with two independent topological domains; the Mu sequences remain supercoiled while the vector DNA is relaxed because of nicking. In the presence of Mu B protein, ATP, target DNA, and Mg2+, the Type 1 complex is converted into the protein-associated product of the strand transfer reaction. In this Type 2 complex, the target DNA has been joined to the Mu DNA ends held in the synaptic complex at the center of the figure eight. Supercoils are not required for the latter reaction.

Inhibition of bacterial segregation by early functions of phage mu and association of replication protein B with the inner cell membrane

Infection of Mu-sensitive bacteria with a recombinant lambda phage that carries the EcoRI.C fragment from the immunity end of wild type Mu DNA causes filamentous growth. Transmission electron microscopy revealed that the cell-division cycle was inhibited at, or prior to, the initiation of septation. The filamentation does not occur after infection of Mu-immune bacteria or after infection with a phage carrying the same EcoRI.C fragment, but with an IS1 insertion in gene B of Mu, showing that either gpB and/or some non-essential functions (e.g. kil) mapping downstream from the insertion are required for the inhibition of cell division. These data and previously published evidence suggest that in the "killing" of E. coli K12 by early Mu functions expressed from the cloned EcoRI.C fragment, two components have to be distinguished: one, a highly efficient elimination of plasmid DNA carrying the early Mu genes, and second, a series of interactions with host functions conducent to an inhibition of cell division. It is suggested that functions normally involved in the SOS reaction participate in the inhibition of cell division by early Mu functions. Infected bacteria synthesize the replication protein B (MR 33000) of Mu, which was found by cell fractionation experiments to be associated with the inner cell membrane. The role of this association for filamentous growth and for the integrative replication of the phage is discussed. The recombinant phage might be useful as a tool for the study of the E. coli cell division cycle.