Thermo-inducible expression of cloned early genes of bacteriophage Mu

Tn5 mutagenesis and insertion replacement in Azotobacter vinelandii

Tn5 insertion mutants of Azotobacter vinelandii were isolated using vectors pJB4JI (IncP) and pGS9 (IncN). A procedure to replace Tn5 (Kmr) by its nontransposing derivative Tn5-131 (Tcr) was developed. For the replacement, a ColEl derivative harboring Tn5-131 (pRZ131) was conjugally mobilized by the IncN plasmid pCU101 into A. vinelandii strains containing Tn5. Both plasmids are unable to be maintained in A. vinelandii, but the transient presence of pRZ131 allows recombination between the incoming and the resident Tn5 elements. Genetic and physical analysis showed that insertion replacements result in lower frequencies of Tn5-associated genomic rearrangements, thereby increasing the stability of Tn5-containing strains.

Kinetics and regulation of transcription of bacteriophage Mu

Mu transcription was analyzed by hybridization of [3H]uridine pulse-labeled RNA from heat-induced Mu lysogens to Mu DNA restriction fragments on nitrocellulose blots. Based on their time of appearance and dependence on Mu functions, we have defined three classes of transcripts: early, middle, and late. Replication-defective prophages containing A or B amber mutations or a deletion of the beta (right) end produced only early RNA derived from the left-most 8 to 10 kb of the Mu genome. A replication-proficient C amber mutant exhibited similar early transcription but at later times also produced middle transcripts from a region including C, which encodes the activator of late transcription. The C mutant did not produce late transcripts from the right-most 26 kb of the Mu genome encoding genes involved in phage morphogenesis and release. These results indicate that Mu DNA replication is required for efficient expression of middle RNA, which is itself required for expression of late transcripts. Amber mutations in essential genes other than A, B, and C had no significant effect on transcription except for polarity of one E mutation. Uninduced Mu c+ and Mu cts prophages produced very low levels of Mu-specific RNA derived from several regions including the c (immunity) gene and the region between genes B and C.

Identification of the bacteriophage Mu kil gene

The kil gene encoded in bacteriophage Mu DNA was previously shown to reside between the end of the B gene at 4.3 kb and the EcoRI site at 5.1 kb from the left end. To precisely map the kil gene within this region, two series of BAL-31 deletion derivatives were created: one removed Mu DNA rightward from the Hpal site (4.2 kb) and the other removed Mu DNA leftward from the EcoRI site. The deleted Mu DNA was subcloned into the expression vector pUC19 under lac promoter control and tested for the expression of the killing function following IPTG induction. Using DNA sequencing analysis, the Mu DNA in Kil+ and Kil- clones was precisely determined, and the kil gene was mapped to the first open reading frame beyond the B gene. The expression of the kil gene was sufficient to induce dramatic morphological changes: cells became enlarged and predominantly spherical, reminiscent of the phenotype of certain cell mutants.

Specificity of signal molecules in the activation of Agrobacterium virulence gene expression

The activation of the Agrobacterium virulence system is known to be induced by certain phenolic compounds. We have tested the vir-inducing ability of fifty compounds, by using a virB-lacZ gene fusion, and analysed the relationship between structure and activity of these compounds. In this way we have identified several new vir-inducers: coniferylalcohol, 3,5-dimethoxy-4-hydroxybenzene, homovanillic acid, ferulic acid, 3-ethoxy-4-hydroxybenzaldehyde and guaiacol, all of which are compounds with strong or moderate activity and four compounds with weak vir-inducing activity. In view of the specificity of vir-inducers, our data extended observations of others and enabled us to define the specific structural features of a vir-inducer molecule. In addition we show here that induction of the octopine Ti vir-genes is (i) optimal at 29 degrees C and totally abolished at 37 degrees C, and (ii) strongly inhibited at low concentrations of sodium chloride. The implications for plant transformation are discussed.

DNA replication studies with coliphage 186. II. Depression of host replication by a 186 gene

Using pre-labelling rather than pulse-labelling studies to determine rates of replication, we have shown that coliphage 186 infection is accompanied by a depression in host DNA replication. We have isolated mutants of the phage gene involved and mapped them in the early region of the phage genome. Sequencing the mutants ultimately led us to the identification of the gene that we have named the dhr gene.

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.

The effect of gamma-irradiation on mu DNA transposition and gene expression

Mobile genetic elements are a ubiquitous presence in the genomes of all well-studied organisms. The effect of genomic stress on the status and transposition of these elements has not, as yet, been extensively characterized. We have been using temperate, transposable bacteriophage Mu as a model system to examine the behavior of mobile genetic elements and have previously shown that many DNA-damaging agents did not induce a Mu prophage to enter the lytic cycle of multiple rounds of DNA transposition. To extend these results and to examine the possibility that they were a reflection of damage to the DNA substrate for Mu transposition, we have constructed a mini-Mu plasmid, pMD12, which contains the early region of Mu, flanked by both extremities required for transposition in cis, and the beginning of the transposase gene A fused in frame to the lacZ gene. This A'-lacZ fusion protein maintains beta-galactosidase enzymatic activity under the control of the expression of the Mu transposase A gene and thus, the capacity for Mu transposition can be easily monitored by assaying for beta-galactosidase. By measuring the amount of beta-galactosidase after various doses of gamma-irradiation, we found that doses of up to 75 krad had no effect on the expression of the Mu transposase gene A. This was confirmed by the lack of induction of a Mu prophage in strains containing a chromosomally inserted Mu genome. Although the plaque-forming units per colony-forming unit of strain CSH67, containing a chromosomally inserted lambda prophage, increased approximately 100-fold from 0 to 75 krad, no stimulation of induction of prophage Mu lytic growth was observed. We also found that plasmid pMD12 did not transpose and chromosomally associate upon gamma-irradiation. This supports the assertion that DNA-damaging agents, including gamma-rays, do not induce the transposition of prokaryotic mobile genetic elements.

Localization of the gam gene of bacteriophage mu and characterisation of the gene product

Using cloning techniques in conjunction with an in vitro assay for activity of the gam-coded protein (pgam), the gam gene has been located on a 930-bp fragment immediately to the right of an AccI site situated 5.75 kb from the left-hand end of the phage Mu genome. An analysis of the properties of pgam obtained from an overproducing clone indicates that it is a non-specific DNA-binding protein which interacts with linear duplex plasmid DNA having a variety of different termini and confers protection against exonuclease action (Gam function). It also stimulates the frequency with which linear plasmid DNA transforms Escherichia coli to antibiotic resistance (Sot function). The preliminary results reported here suggest that pgam is potentially a useful 'tool' in molecular biology, although the molecular details of pgam activity require further clarification.

Hek: an Escherichia coli function involved in functional expression of the kil gene of bacteriophage Mu

An Escherichia coli mutant has been isolated (Hek) in which the kil gene of bacteriophage Mu is not functionally expressed. The hek locus has been mapped between rpoD (66.2 min) and argR (69.5 min) on the E. coli chromosome. No influence of the hek mutation on phage or E. coli development could be detected.

Multiple factors and processes involved in host cell killing by bacteriophage Mu: characterization and mapping

The regions of bacteriophage Mu involved in host cell killing were determined by infection of a lambda-immune host with 12 lambda pMu-transducing phages carrying different amounts of Mu DNA beginning at the left end. Infecting lambda pMu phages containing 5.0 (+/- 0.2) kb or less of the left end of Mu DNA did not kill the lambda-immune host, whereas lambda pMu containing 5.1 kb did kill, thus locating the right end of the kil gene between approximately 5.0 and 5.1 kb. For the Kil+ phages the extent of killing increased as the multiplicity of infection (m.o.i.) increased. In addition, killing was also affected by the presence of at least two other regions of Mu DNA: one, located between 5.1 and 5.8 kb, decreased the extent of killing; the other, located between 6.3 and 7.9 kb, greatly increased host cell killing. Killing was also assayed after lambda pMu infection of a lambda-immune host carrying a mini-Mu deleted for most of the B gene and the middle region of Mu DNA. Complementation of mini-Mu replication by infecting B+ lambda pMu phages resulted in killing of the lambda-immune, mini-Mu-containing host, regardless of the presence or absence of the Mu kil gene. The extent of host cell killing increased as the m.o.i. of the infecting lambda pMu increased, and was further enhanced by both the presence of the kil gene and the region located between 6.3 and 7.9 kb. These distinct processes of kil-mediated killing in the absence of replication and non-kil-mediated killing in the presence of replication were also observed after induction of replication-deficient and kil mutant prophages, respectively.

Conditionally transposition-defective derivative of Mu d1(Amp Lac)

A Mu d1 derivative is described which is useful for genetic manipulation of Mu-lac fusion insertions. A double mutant of the specialized transducing phage Mu d1(Amp Lac c62ts) was isolated which is conditionally defective in transposition ability. The Mu d1 derivative, designated Mu d1-8(Tpn[Am] Amp Lac c62ts), carries mutations which virtually eliminate transposition in strains lacking an amber suppressor. In such strains, the Mu d1-8 prophage behaves like a standard transposon. It can be moved from one strain of Salmonella typhimurium to another by the general transducing phage P22 with almost 100% inheritance of the donor insertion mutation. When introduced into a recipient carrying supD, supE, or supF, 89 to 94% of the Ampr transductants were transpositions of the donor Mu d1-8, from the transduced fragment into new sites. The stability of Mu d1-8 in a wild-type, suppressor-free background was sufficient to permit use of the fusion to select constitutive mutations without prior isolation of deletions to stabilize the fusion. Fusion strains could be grown at elevated temperature without induction of the Mu d prophage. The transposition defect of Mu d1-8 was corrected by a plasmid carrying the Mu A and B genes.

Cloning and expression of the phage Mu A gene

We have cloned the phage Mu A gene, with and without the gene ner, under the control of the pL promoter of phage lambda in a multicopy plasmid vector. We demonstrate that plasmid-carrying cells are able to support growth of superinfecting Mu A am phages in a temperature-dependent fashion in a host strain carrying a defective lambda prophage which specifies the cI857-coded lambda repressor. In addition, we show that the presence of the ner gene reduces the efficiency of plating of the superinfecting phage. Analysis of proteins specified by the cloned Mu fragments indicates that two proteins, 70 and 33 kDal, are synthesized. The level of synthesis, compared to that of the vector-encoded beta-lactamase, was found to increase with temperature. This indicates that their transcription is driven by the pL promoter. The Mr of the 70-kDal protein is identical to that previously observed for pA.

Partial purification and properties of an exonuclease inhibitor induced by bacteriophage Mu-1

From an induced lysogen of bacteriophage Mu-1, we partially purified a substance of high molecular weight that blocks the action of several exonucleases on double-stranded DNA. The presence of the inhibitor in cell-free extracts is dependent on induction of a Mu prophage. The Mu-related inhibitor acts by binding to double-stranded DNA rather than by interacting with the DNase. The inhibitor protects linear duplex DNA of Mu, P22, and phi X174am3 from exonucleolytic degradation by recBC DNase and lambda exonuclease. Single-stranded DNA, however, is not protected by the inhibitor from degradation by either recBC DNase or exonuclease I. The inhibitor preparation contains a protein that binds to linear duplex DNA, but not to circular duplex DNA; ends are required for binding to occur. Single-stranded DNA is not a substrate for the binding protein. These and other results suggest that the binding protein and the inhibitor are the same activity.

Plasmid vectors derived from phage Mu allow direct selection of transformants containing cloned HindIII and PstI fragments

The vector plasmids pKN001 and pKN80 both contain the EcoRi.C fragment of E.coli phage Mu DNA which codes for a killing function that is efficiently expressed upon transformation into Mu-sensitive bacteria. By in vitro insertion of HindIII fragments at the single HindIII site of pKN80 or of PstI fragments at the single PstI site of pKN001 the killing function is inactivated. The resulting plasmids have a selective advantage over the religated vector when transformed into Mu-sensitive bacteria. More than 90% of the transformations contain hybrid plasmids. These results show the usefulness of Mu DNA containing plasmids pKN001 and pKN80 as vectors that allow the direct selection for recombinant plasmids.

Transcription of bacteriophage Mu. II. Transcription of the repressor gene

Using pBR322 as a vector, three plasmids were constructed, pGP2, pGP3, and pGP7, containing respectively 5, 100, 700-950, and 1,000 base pairs derived from the immunity end of bacteriophage Mu. All three plasmids contain a functional repressor gene coding for a thermosensitive repressor. RNAs produced when the DNA of these plasmids was used as template in in vitro RNA synthesis, were analysed by hybridization to the DNA of several lambda pMu transducing phages. In spite of the differences in length of the Mu fragments all three plasmids show the same amount of Mu specific l-strand transcription. Since the repressor gene comprises at least 70% of the Mu fragments of pGP3 and pGP7, these results indicate that the repressor gene c of bacteriophage Mu is transcribed on the l-strand. Analysis of in vivo RNA from cells harboring the plasmids pGP2, pGP3, or pGP7 also indicates that the repressor gene of phage Mu is transcribed on the l-strand, as all Mu-specific RNA extracted from these cells at 28 degrees C hybridizes with the l-strand of the first 3,100 basepairs from the Mu immunity end.