Kinetics and regulation of transcription of bacteriophage Mu

A phage-encoded anti-activator inhibits quorum sensing in Pseudomonas aeruginosa

The arms race between bacteria and phages has led to the evolution of diverse anti-phage defenses, several of which are controlled by quorum-sensing pathways. In this work, we characterize a quorum-sensing anti-activator protein, Aqs1, found in Pseudomonas phage DMS3. We show that Aqs1 inhibits LasR, the master regulator of quorum sensing, and present the crystal structure of the Aqs1-LasR complex. The 69-residue Aqs1 protein also inhibits PilB, the type IV pilus assembly ATPase protein, which blocks superinfection by phages that require the pilus for infection. This study highlights the remarkable ability of small phage proteins to bind multiple host proteins and disrupt key biological pathways. As quorum sensing influences various anti-phage defenses, Aqs1 provides a mechanism by which infecting phages might simultaneously dampen multiple defenses. Because quorum-sensing systems are broadly distributed across bacteria, this mechanism of phage counter-defense may play an important role in phage-host evolutionary dynamics.

Anti-CRISPR-Associated Proteins Are Crucial Repressors of Anti-CRISPR Transcription

Phages express anti-CRISPR (Acr) proteins to inhibit CRISPR-Cas systems that would otherwise destroy their genomes. Most acr genes are located adjacent to anti-CRISPR-associated (aca) genes, which encode proteins with a helix-turn-helix DNA-binding motif. The conservation of aca genes has served as a signpost for the identification of acr genes, but the function of the proteins encoded by these genes has not been investigated. Here we reveal that an acr-associated promoter drives high levels of acr transcription immediately after phage DNA injection and that Aca proteins subsequently repress this transcription. Without Aca activity, this strong transcription is lethal to a phage. Our results demonstrate how sufficient levels of Acr proteins accumulate early in the infection process to inhibit existing CRISPR-Cas complexes in the host cell. They also imply that the conserved role of Aca proteins is to mitigate the deleterious effects of strong constitutive transcription from acr promoters.

The phage Mu middle promoter Pm contains a partial UP element

There are three phases of transcription during lytic development of bacteriophage Mu: early, middle, and late. Transcription from the middle phase promoter P_m requires the activator protein Mor. In the presence of Mor, transcription from P_m is carried out by the Escherichia coli RNA polymerase holoenzyme containing σ⁷⁰. A Mor dimer binds to two 5-bp inverted repeats within a 16-bp element centered at −43.5 in P_m, replacing the normal −35 element contacted by RNA polymerase (RNAP). In this study random and targeted mutagenesis of the sequence upstream (−88 to −52) of the Mor binding site was performed to determine whether P_m also contains an UP element for binding of the RNAP α subunit, thereby stimulating transcription. The results demonstrated that mutations upstream of −57 had no effect on P_m activity in vivo, assayed by expression of lacZ fused downstream of a wild-type or mutant P_m. Mutations at positions −57 through −52 led to decreased transcription from P_m, consistent with the presence of an UP element. In DNase I footprinting and gel mobility shift assays, paired mutations at positions −55 and −54 did not affect Mor binding but decreased the synergistic binding of Mor with histidine tagged α (His-α), indicating that His-α binds to P_m in a sequence- and/or structure-specific manner. Taken together, these results demonstrate that P_m has a strong proximal UP element subsite, but lacks a distal subsite.

Unusual interaction of RNA polymerase with the bacteriophage Mu middle promoter Pm in the absence of its activator protein Mor

The bacteriophage Mu Mor activator protein is absolutely required for transcription from the Mu middle promoter P_m. However, when RNA polymerase (RNAP) was incubated with P_m DNA in the absence of Mor, a band at promoter position −51 was hypersensitive to DNase I cleavage, demonstrating an interaction of RNAP with the promoter DNA. The hypersensitivity was similar at four different lengths of P_m DNA assayed from −62 to +10, −62 to +46, −96 to +10, and −96 to +46. The hypersensitivity occurred equally well at 5°C, 15°C, and 30°C, indicating that it did not require open complex formation, which only occurred at 30°C. The −51 hypersensitivity at 5°C and 15°C was eliminated by the addition of heparin, consistent with the possibility that it arose by formation of unstable closed complexes of RNAP bound to P_m DNA. Generation of the hypersensitive band required the complete RNAP with its αCTDs, but neither the αCTD nor intact α were sufficient for the interaction and resulting hypersensitivity. There was no correlation between the level of hypersensitivity observed in vitro and the level of P_m activity in vivo, as assayed by the Mor-dependent production of β-galactosidase from a P_m-lacZ fusion. In an “order of addition” experiment, preincubation of P_m DNA with Mor followed by addition of RNAP led to the fastest open complex formation, whereas preincubation of P_m DNA with RNAP gave the slowest. These results support the conclusion that Mor recruits RNAP to P_m rather than reposition a prebound RNAP, as occurs for C-dependent repositioning of RNAP at the Mu late promoter P_mom.

Genetic analysis of phage Mu Mor protein amino acids involved in DNA minor groove binding and conformational changes

Gene expression during lytic development of bacteriophage Mu occurs in three phases: early, middle, and late. Transcription from the middle promoter, P(m), requires the phage-encoded activator protein Mor and the bacterial RNA polymerase. The middle promoter has a -10 hexamer, but no -35 hexamer. Instead P(m) has a hyphenated inverted repeat that serves as the Mor binding site overlapping the position of the missing -35 element. Mor binds to this site as a dimer and activates transcription by recruiting RNA polymerase. The crystal structure of the His-Mor dimer revealed three structural elements: an N-terminal dimerization domain, a C-terminal helix-turn-helix DNA-binding domain, and a β-strand linker between the two domains. We predicted that the highly conserved residues in and flanking the β-strand would be essential for the conformational flexibility and DNA minor groove binding by Mor. To test this hypothesis, we carried out single codon-specific mutagenesis with degenerate oligonucleotides. The amino acid substitutions were identified by DNA sequencing. The mutant proteins were characterized for their overexpression, solubility, DNA binding, and transcription activation. This analysis revealed that the Gly-Gly motif formed by Gly-65 and Gly-66 and the β-strand side chain of Tyr-70 are crucial for DNA binding by His-tagged Mor. Mutant proteins with substitutions at Gly-74 retained partial activity. Treatment with the minor groove- and GC-specific chemical chromomycin A(3) demonstrated that chromomycin prevented His-Mor binding but could not disrupt a pre-formed His-Mor·DNA complex, consistent with the prediction that Mor interacts with the minor groove of the GC-rich spacer in the Mor binding site.

Regional mutagenesis of the gene encoding the phage Mu late gene activator C identifies two separate regions important for DNA binding

Lytic development of bacteriophage Mu is controlled by a regulatory cascade and involves three phases of transcription: early, middle and late. Late transcription requires the host RNA polymerase holoenzyme and a 16.5-kDa Mu-encoded activator protein C. Consistent with these requirements, the four late promoters P_lys, P_I, P_P and P_mom have recognizable −10 hexamers but lack typical −35 hexamers. The C protein binds to a 16-bp imperfect dyad-symmetrical sequence element centered at −43.5 and overlapping the −35 region. Based on the crystal structure of the closely related Mor protein, the activator of Mu middle transcription, we predict that two regions of C are involved in DNA binding: a helix-turn-helix region and a β-strand region linking the dimerization and helix-turn-helix domains. To test this hypothesis, we carried out mutagenesis of the corresponding regions of the C gene by degenerate oligonucleotide-directed PCR and screened the resulting mutants for their ability to activate a P_lys-galK fusion. Analysis of the mutant proteins by gel mobility shift, β-galactosidase and polyacrylamide gel electrophoresis assays identified a number of amino acid residues important for C DNA binding in both regions.

Genome sequence comparison and superinfection between two related Pseudomonas aeruginosa phages, D3112 and MP22

A temperate transposable bacteriophage (MP22) was isolated from a Korean clinical isolate of Pseudomonas aeruginosa. It has a coliphage lambda-like morphology and a double-stranded DNA genome. The complete nucleotide sequence and annotation of the MP22 genome and its characteristics are presented. The MP22 genome is 36 409 bp long with a G+C content of 64.2 mol%. The genome contains 51 proposed ORFs, of which 48 (94 %) display synteny and significant nucleotide and protein sequence similarity to the corresponding ORFs of the closely related phage, D3112. Three of the predicted ORFs are unique proteins, whose functions are yet to be revealed. The phage c repressors exhibit striking dissimilarities and, when present as a single gene, did not show cross-immunity. In contrast, although an MP22 lysogen could be productively infected with D3112, MP22 could not grow on a D3112 lysogen, indicating a role of other D3112 genes in superinfection exclusion.

Crystallization and preliminary X-ray analysis of phage Mu activator protein C in a complex with promoter DNA

Bacteriophage Mu C protein is an activator of the four Mu late promoters that drive the expression of genes encoding DNA-modification as well as phage head and tail morphogenesis proteins. This report describes the purification and cocrystallization of wild-type and selenomethionine-substituted C protein with a synthetic late promoter P(sym), together with preliminary X-ray diffraction data analysis using SAD phasing. The selenomethionine peak data set was collected from a single crystal which diffracted to 3.1 A resolution and belonged to space group P4(1) or P4(3), with unit-cell parameters a = 68.9, c = 187.6 A and two complexes per asymmetric unit. The structure will reveal the amino acid-DNA interactions and any conformational changes associated with DNA binding.

Binding of the C-terminal domain of the alpha subunit of RNA polymerase to the phage mu middle promoter

The C-terminal domain of the alpha subunit (alpha CTD) of Escherichia coli RNA polymerase is often involved in transcriptional regulation. The alpha CTD typically stimulates transcription via interactions with promoter UP element DNA and transcriptional activators. DNase I footprinting and gel mobility shift assays were used to look for potential interaction of the alpha CTD with the phage Mu middle promoter P(m) and its activator protein Mor. Binding of RNA polymerase to P(m) in the presence of Mor resulted in production of a DNase I footprint downstream of Mor due to open complex formation and generation of a second footprint just upstream of the Mor binding site. Generation of the upstream footprint did not require open complex formation and also occurred in reactions in which the alpha CTD or His-alpha proteins were substituted for RNA polymerase. In gel mobility shift assays, the formation of a supershifted ternary complex demonstrated that Mor and His-alpha bind synergistically to P(m) DNA. Gel shift assays with short DNA fragments demonstrated that only the Mor binding site and a single upstream alpha CTD binding site were required for ternary complex formation. These results suggest that the alpha CTD plays a role in P(m) transcription by binding to P(m) DNA just upstream from Mor and making protein-protein interactions with Mor that stabilize the binding of both proteins.

Complete genomic sequence of bacteriophage B3, a Mu-like phage of Pseudomonas aeruginosa

Bacteriophage B3 is a transposable phage of Pseudomonas aeruginosa. In this report, we present the complete DNA sequence and annotation of the B3 genome. DNA sequence analysis revealed that the B3 genome is 38,439 bp long with a G+C content of 63.3%. The genome contains 59 proposed open reading frames (ORFs) organized into at least three operons. Of these ORFs, the predicted proteins from 41 ORFs (68%) display significant similarity to other phage or bacterial proteins. Many of the predicted B3 proteins are homologous to those encoded by the early genes and head genes of Mu and Mu-like prophages found in sequenced bacterial genomes. Only two of the predicted B3 tail proteins are homologous to other well-characterized phage tail proteins; however, several Mu-like prophages and transposable phage D3112 encode approximately 10 highly similar proteins in their predicted tail gene regions. Comparison of the B3 genomic organization with that of Mu revealed evidence of multiple genetic rearrangements, the most notable being the inversion of the proposed B3 immunity/early gene region, the loss of Mu-like tail genes, and an extreme leftward shift of the B3 DNA modification gene cluster. These differences illustrate and support the widely held view that tailed phages are genetic mosaics arising by the exchange of functional modules within a diverse genetic pool.

Crystal Structure of the Mor Protein of Bacteriophage Mu, a Member of the Mor/C Family of Transcription Activators

Transcription from the middle promoter, Pm, of bacteriophage Mu requires the phage-encoded activator protein Mor and bacterial RNA polymerase. Mor is a sequence-specific DNA-binding protein that mediates transcription activation through its interactions with the C-terminal domains of the alpha and sigma subunits of bacterial RNA polymerase. Here we present the first structure for a member of the Mor/C family of transcription activators, the crystal structure of Mor to 2.2-A resolution. Each monomer of the Mor dimer is composed of two domains, the N-terminal dimerization domain and C-terminal DNA-binding domain, which are connected by a linker containing a beta strand. The N-terminal dimerization domain has an unusual mode of dimerization; helices alpha1 and alpha2 of both monomers are intertwined to form a four-helix bundle, generating a hydrophobic core that is further stabilized by antiparallel interactions between the two beta strands. Mutational analysis of key leucine residues in helix alpha1 demonstrated a role for this hydrophobic core in protein solubility and function. The C-terminal domain has a classical helix-turn-helix DNA-binding motif that is located at opposite ends of the elongated dimer. Since the distance between the two helix-turn-helix motifs is too great to allow binding to two adjacent major grooves of the 16-bp Mor-binding site, we propose that conformational changes in the protein and DNA will be required for Mor to interact with the DNA. The highly conserved glycines flanking the beta strand may act as pivot points, facilitating the conformational changes of Mor, and the DNA may be bent.

Unusual transcriptional and translational regulation of the bacteriophage Mu mom operon

The bacteriophage Mu mom gene encodes a novel DNA modification that protects the viral genome against a wide variety of restriction endonucleases. Expression of mom is subject to a series of unusual regulatory controls. Transcription requires the action of a phage-encoded protein, C, which binds (probably as a dimer) the mom promoter from -33 to -52 (with respect to the transcription start site) in two adjacent DNA major grooves on one face of the helix. No apparent direct interaction between C and the host RNA polymerase (RNAP) is evident; however, C binding alters mom DNA conformation. In the absence of C, RNAP binds the mom promoter at a site that results in transcription in a direction away from the mom gene. The function of this transcription is unknown. An additional layer of transcriptional regulation complexity is due to the fact that the host Dam DNA-(N6-adenine)methyltransferase is required. Dam methylation of three closely spaced upstream GATC sequences is necessary to prevent binding by the host protein, OxyR, which acts as a repressor. Repression is not mediated by inhibition of C binding, but rather through interference with C-mediated recruitment of RNAP to the correct site. Translation of mom is regulated by the phage Com protein. Com is only 62 amino acids long and contains a zinc finger-like structure (coordinated by four cysteine residues) in the amino terminal domain. Com binds mom mRNA 5' to the mom open reading frame, whose translation start signals are contained in a stem-loop translation-inhibition-structure. Com binding to its target site (5' to and adjacent to the translation-inhibition-structure) results in a stable change in RNA secondary structure that exposes the translation start signals.

Bidirectional transcription in the mom promoter region of bacteriophage Mu

Transcription of the Mu mom operon requires activation by the phage gene product, C, a site-specific DNA binding protein. Previous in vivo and in vitro footprinting studies showed that Escherichia coli RNA polymerase (Esigma70=RNAP) bound the wild-type (wt) mom promoter (Pmom) region in the absence of C; this site, now designated momP2 (-11 to -64), is slightly upstream of, but overlapping with, momP1 (+16 to -49), the functional binding site for mom operon (rightward) transcription. The location/distribution of KMnO4-sensitive sites on the two DNA strands suggested that RNAP bound at momP2 was in an open-complex, but that transcription was in the opposite direction. Here, we used both runoff transcription and reverse transcriptase-primer extension sequencing to provide direct evidence that in the absence of C protein, RNAP carries out leftward transcription from momP2 both in vitro and in vivo. In addition, the 5' ends of these transcripts were mapped to the same upstream initiation site, -58G, relative to the initiation site of C-activated rightward transcription. We also present evidence that leftward transcription from momP2 requires RNAP recognition of an UP-element by the carboxyl-terminal domain of the alpha subunit.

Activation of bacteriophage Mu mom transcription by C protein does not require specific interaction with the carboxyl-terminal region of the alpha or sigma 70 subunit of Escherichia coli RNA polymerase

Late in its growth cycle, transcription of the phage Mu mom Promoter (Pmom) is activated by the phage gene product, C, a site-specific DNA binding protein. In vitro transcription analyses showed that this activation does not require specific contacts between C and the carboxyl-terminal region of the alpha or sigma 70 subunit of Escherichia coli RNA polymerase. Unexpectedly, these results are in contrast to those known for another Mu-encoded transcriptional activator, Mor, which has a high degree of sequence identity with C and appears to interact with the carboxyl termini of both alpha and sigma 70.

Distortion in the spacer region of Pm during activation of middle transcription of phage Mu

Transcription from the middle promoter, Pm, of phage Mu is initiated by Escherichia coli RNA polymerase holoenzyme (E sigma 70; RNAP) and the phage-encoded activator, Mor. Point mutations in the spacer region between the -10 hexamer and the Mor binding site result in changes of promoter activity in vivo. These mutations are located at the junction between a rigid T-tract and adjacent, potentially deformable G + C-rich DNA segment, suggesting that deformation of the spacer region may play a role in the transcriptional activation of Pm. This prediction was tested by using dimethyl sulfate and potassium permanganate footprinting analyses. Helical distortion involving strand separation was detected at positions -32 to -34, close to the predicted interface between Mor and RNAP. Promoter mutants in which this distortion was not detected exhibited a lack of melting in the -12 to -1 region and reduced promoter activity in vivo. We propose that complexes containing the distortion represent stressed intermediates rather than stable open complexes and thus can be envisaged as a transition state in the kinetic pathway of Pm activation in which stored torsional energy could be used to facilitate melting around the transcription start point.

Regulatory factors acting at the bacteriophage Mu middle promoter

Lytic development of bacteriophage Mu proceeds through three phases of transcription: early, middle, and late. Initiation of middle transcription from Pm requires the phage-encoded activator, Mor. An examination of the sequences surrounding the promoter revealed possible binding sites for Mu proteins A and c, as well as for Escherichia coli integration host factor. Promoter fragments containing 5' and 3' deletions were fused to the lacZ reporter gene and assayed for activity after induction of a Mu prophage or a plasmid-borne mor gene. Sequences upstream of position -62 and downstream of +10 were dispensable for promoter activity. In DNase I footprinting with both crude extract and purified protein, Mor protected Pm sequences from position -56 to -33. Mutations disrupting the dyad symmetry of the terminator of early transcription overlapping the Mor binding site did not reduce promoter activity, suggesting that the symmetry per se is not required for Mor binding or Pm activation. Purified Mu lysogenic repressor (c) also bound to Pm, overlapping the Mor binding site. Production of large amounts of repressor in vivo reduced Mor-dependent promoter activity nearly 10-fold. Promoters with mutations in the repressor binding site showed a reduction in this repressor-mediated inhibition of Pm activity.

The Escherichia coli DnaK chaperone machine and bacteriophage Mu late transcription

Bacteriophage Mu does not grow on temperature-sensitive E. coli dnaK mutants at elevated temperatures because of a defect in late transcription. As the Mu-encoded C protein is required for activation of transcription from the phage late promoters, we attempted to determine if DnaK and its accessory proteins DnaJ and GrpE are required for synthesis of C protein or at a later step. We found that the chaperones act in Mu late transcription beyond C-protein synthesis, and that C-protein stability is decreased in the mutant hosts. This suggests that the DnaK chaperone machine may be required for the proper folding and/or multimerization of C protein.

Identification and characterization of the terminators of the lys and P transcripts of bacteriophage Mu

Transcription during the lytic cycle of phage Mu occurs in three phases: early, middle, and late. Late transcription requires the Mu C protein and initiates at four promoters: Plys, PI, PP, and Pmom. Northern blot analysis of total RNA isolated 30 min after heat induction of Mu cts lysogens demonstrated that the full-length lys and P transcripts were approximately 7.6 and 6.3 kb long, respectively. The 3' ends of the lys and P transcripts were further localized by S1 nuclease mapping to intergenic regions between G and I and between U and U' in both the G(+) and G(-) orientations of the invertible G segment, respectively. As expected, when DNA fragments containing these termination regions were cloned into plasmids between Pgal and the galK gene, they showed efficient termination activity, even in a Rho-deficient background. Deletion analysis indicated that efficient termination required the presence of potential RNA stem-loop structures immediately preceding the RNA 3' ends. For the P transcript from phage with the G(-) orientation, full termination activity required both the region containing the stem-loop structure and upstream sequences. Taken together, these results suggest that the transcription termination sites of the lys and P transcripts are Rho-independent terminators.

Bacteriophage Mu Mor protein requires sigma 70 to activate the Mu middle promoter

Transcription during the bacteriophage Mu lytic cycle occurs in three phases: early, middle, and late. Middle transcription requires the early gene product Mor for its activation. Mor protein overproduction was accomplished by fusing the mor gene to an efficient phage T7 promoter and translation initiation region. A protein fraction highly enriched for Escherichia coli RNA polymerase (E sigma 70) from the Mor-overproducing strain was able to activate transcription from both the tac promoter (Ptac) and the Mu middle promoter (Pm) in vitro. Transcription initiation from Pm was Mor dependent, and the RNA 5' end was identical to that of in vivo RNA. Addition of anti-sigma 70 antibody to transcription reactions containing Ptac and Pm resulted in inhibition of transcription from both promoters; addition of purified sigma 70 restored transcription. These results indicate that Mor-dependent activation requires sigma 70 and therefore imply that Mor is not an alternate sigma factor. This conclusion was further substantiated by a reconstitution experiment with purified proteins in which all three components, Mor, sigma 70, and core RNA polymerase, were required for Pm-dependent transcription in vitro. The sigma 70 dependence of Mor-specific transcription and the amino acid sequence similarity between Mor and C (an activator for Mu late transcription) both support the hypothesis that Mor functions mechanistically as an activator protein.

A chromosomal expression vector for Escherichia coli based on the bacteriophage Mu

A new Escherichia coli expression vector with increased stability was developed based on bacteriophage Mu. Unlike traditional expression vectors, the vector described herein is chromosome based rather than existing as an autonomously replicating plasmid. The chromosomal location resulted in extreme stability of the vector even in the absence of selective pressure. Both replication and heterologous protein synthesis could be induced by temperature shift. Expression of the heterologous gene was controlled by the Mu middle promoter and was dependent on the presence of the transactivator, Mor, of the Mu middle promoter. Four proteins, beta-galactosidase, chloramphenicol acetyltransferase, porcine somatotropin and human growth hormone, were made from this vector at levels ranging from 5 to 20% of total cell protein. Expression from the middle promoter was highest when inductions were done in rich media. The expression of some genes varied in different strains.

Functionally distinct RNA polymerase binding sites in the phage Mu mom promoter region

Transcription of the phage Mu com/mom operon is trans-activated by another phage gene product, C, a site-specific DNA binding protein. To gain insight into the mechanism by which C activates transcription, we carried out footprinting analyses of Escherichia coli RNA polymerase (= RNAP) binding to various com-lacZ fusion plasmids. KMnO4-sensitive sites (diagnostic of the melted regions in open-complexes) and DNase I-sensitive sites were located by primer-extension analysis. The results are summarized as follows: (i) in vivo, in the absence of C, RNAP bound in the wild-type (wt) promoter region at a site designated P2; in vitro DNase I-footprinting showed that P2 extends from -74 to -24 with respect to transcription initiation. This overlaps a known strong C-binding site (at -35 to -54). RNAP bound at P2 appeared to be in an open-complex, as evidenced by the presence of KMnO4-hypersensitive sites. (ii) In contrast, when C was present in vivo, RNAP bound in the wt promoter region at a different site, designated P1, located downstream and partially overlapping P2. RNAP bound at P1 also appeared to be in an open-complex, as evidenced by the presence of KMnO4-hypersensitive sites. (iii) Two C-independent mutants, which initiate transcription at the same position as the wt, were also analyzed. In vivo, in the absence of C, RNAP bound mutant tin7 (contains a T to G substitution at -14) predominantly at P1; in vitro DNase I-footprinting showed that P1 extends from -56 to +21. With mutant tin6 (a 63 base-pair deletion removing P2, as well as part of P1 and the C-binding site from -35 to -54), RNAP bound to P1 independent of C. We conclude that P1 is the 'functional' RNAP binding site for mom-transcription initiation, and that C activates transcription by promoting binding at P1, while blocking binding at P2.

Identification of a positive regulator of the Mu middle operon

Transcription of bacteriophage Mu occurs in a regulatory cascade consisting of three phases: early, middle, and late. The 1.2-kb middle transcript is initiated at Pm and encodes the C protein, the activator of late transcription. A plasmid containing a Pm-lacZ operon fusion was constructed. beta-Galactosidase expression from the plasmid increased 23-fold after Mu prophage induction. Infection of plasmid-containing cells with lambda phages carrying different segment of the Mu early region localized the Pm-lacZ transactivation function to the region containing open reading frames E16 and E17. Deletion and linker insertion analyses of plasmids containing this region identified E17 as the transactivator; therefore we call this gene mor, for middle operon regulator. Expression of mor under the control of a T7 promoter and T7 RNA polymerase resulted in the production of a single polypeptide of 17 kDa as detected by sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Insertion of a linker into mor substantially reduced the ability of Mu to form plaques. When growth of the mor mutant was assayed in liquid, lysis was delayed by about 50 min and the burst size was approximately one-fifth that of wild-type Mu. The mor requirement for plaque formation and normal growth kinetics was abolished when C protein was provided in trans, indicating that the primary function of Mor is to provide sufficient C for late gene expression. Comparison of the predicted amino acid sequence of Mor with other proteins revealed that Mor and C share substantial amino acid sequence homology.

Activation of the bacteriophage Mu lys promoter by Mu C protein requires the sigma 70 subunit of Escherichia coli RNA polymerase

Bacteriophage Mu C protein, a product of the middle operon, is required for activation of the four Mu late promoters. To address its mechanism of action, we overproduced the approximately 16.5-kilodalton C protein from a plasmid containing the C gene under the control of a phage T7 promoter and ribosome-binding site. A protein fraction highly enriched for Escherichia coli RNA polymerase (E sigma 70) and made from the overproducing strain was able to activate transcription in vitro from both the tac promoter (Ptac) and a Mu late promoter, Plys. The behavior of Plys was similar in vivo and in vitro; under both conditions, transcription was C dependent and the RNA 5' ends were identical. When anti-sigma 70 antibody was added to C-dependent transcription reactions containing both Ptac and Plys templates, transcription from both promoters was inhibited; transcription was restored by the addition of excess E sigma 70. This result suggests that C-dependent activation of Plys requires sigma 70. Further supporting evidence was provided by a reconstitution experiment in which an E sigma 70-depleted fraction containing C was unable to activate transcription from Plys unless both purified sigma 70 and core polymerase were added. These results strongly suggest that C is not a new sigma factor but acts as an activator for E sigma 70-dependent transcription.