On the control of transcription of bacteriophage Mu

Restoration of ligase activity in E. coli K12 lig ts7 strain by bacteriophage Mu and cloning of a DNA fragment harbouring the Mu 'lig' gene

Restoration of ligase activity has been observed in E. coli K12 ligts7 strain lysogenic for Mu, in presence as well in absence of lysogenic immunity. This restoration consist in phenotypic reversal of temperature sensitivity of E. coli ligts7 which also regain the ability to sustain the complete growth cycle of T4 lig-phages. It is possible to put under the control of the gal operon the expression of the viral gene responsible for the restoration effect. This new gene of Mu has been named 'lig'. A 5 kb fragment responsible for the reported effects and localized between genes gam and lys of Mu genome has been cloned in pBR322. This recombinant plasmid used for transforming ligts7 strain restores in it normal behaviour for ligation of Okazaki pieces.

Association of Mu-containing plasmids with the Escherichia coli chromosome upon prophage induction

To determine the structure of a prophage-containing plasmid during Mu transposition, we have monitored the physical state of pSC101[unk]Mucts after thermoinduction. We have also examined the fate of a mini Mu plasmid constructed in vitro by deleting 27 kilobases from the center of the Mu prophage in pSC101[unk]Mucts. At various times after prophage induction, DNA was extracted from Mu or mini Mu plasmid-containing strains and subjected to electrophoresis in low concentration agarose gels followed by transfer of the DNA to nitrocellulose paper. Separate hybridization with (32)P-labeled pSC101 and Mu DNA revealed the position of the plasmids and the replication of Mu DNA. At times after induction when Mu replication was clearly visible, Mu and mini Mu plasmids were found to migrate with Escherchia coli DNA. This Mu-specific association requires the phage coded A and B proteins. Electron microscopy has shown that some of the associated DNA is comprised of circular plasmid molecules which appear to be in contact with the chromosomal DNA. These structures may represent intermediates or end products of the replication-integration process. The finding that Mu and mini Mu plasmids do not give rise to any detectable excision products and apparently remain intact during Mu transposition supports our proposal that the predominant event after Mu induction is the replication of Mu DNA in situ to generate integrative intermediates.

Mapping of the modification function of temperate phage Mu-1

Using internal deletions in the Mu genome, we have mapped the gene coding for Mu modification in the beta segment of Mu DNA.

Thermo-inducible expression of cloned early genes of bacteriophage Mu

An EcoRI fragment, containing approx. 5100 base pairs (bp) of the immunity-end of bacteriophage Mu, was inserted into the multicopy plasmid pMB9 by in vitro recombination. The expression of early Mu genes, located on the cloned fragment, is thermo-inducible because of the presence of the ts mutation in gene c. The isolation of a transformant harbouring the recombinant plasmid, pGP1, was possible only when expression of Mu genes was prevented. pGP1 can be maintained at 28 degrees C at high copy number, but at 42 degrees C the pGP1 containing cells are killed due to the expression of the kil gene of Mu. The following Mu genes are present on pGP1: the ner gene, the integration and replication genes A and B, the cim gene, and the kil gene. pGP1 containing cells do not show Gam and Sot activity at 42 degrees C, therefore the leftmost EcoRI site on the Mu DNA is located between genes kil and gam or sot, or within the gam or sot gene.

Use of RP4-prime plasmids constructed in vitro to promote a polarized transfer of the chromosome in Escherichia coli and Rhizobium meliloti

RP4-prime plasmids containing chromosomal fragments of either Escherichia coli or Rhizobium meliloti were constructed in vitro. When introduced into E. coli or R. meliloti respectively, they promoted a polarized transfer of the chromosome as demonstrated either by the gradient of transfer of various markers or by the study of the genetic constitution of recombinants. In E. coli, mobilization was shown to be dependent upon the presence of a functional rec A system. Inheritance of markers was due to their integration into the chromosome of the recipient as shown by the need for a functional rec A system in the recipient E. coli or by mobilization of recessive markers in R. meliloti. The system described could be applied to genetic mapping in any Gram negative bacteria.

Kinetics of Mu DNA synthesis

Mu specific DNA synthesis starts at 10 min after infection. All essentail amber mutants of Mu were tested for the ability to replicate in a non permissive host. Except for the amber mutants A and B, which were already known to be blocked in Mu DNA synthesis (Wijffelman et al., 1974), all the other mutants showed normal Mu DNA replication. Using mitomycin C-treated cells Mu DNA synthesis was found to start at about 20 min after induction. However using the much more sensitive method of DNA-RNA hybridization, it was found that the DNA synthesis starts already at 10 min after induction, and that at 20 min after induction about 7 copies of the Mu DNA are present per cell.

Defective prophages of bacteriophage Mu

A method is described for the isolation of thermoinducible defective Mu lysogens. Four of these defective lysogens were studied more extensively. By marker-rescue experiments it was shown that the strain harbouring the smallest defective prophage contains the immunity gene cts and the genes A and B; the strain with the largest defective prophage still contains all the known essential genes of Mu, A to S (see Fig. 1). After induction at 43 degrees C all the defective lysogens are killed, whereas no lysis occurs. Although in all the thermoinducible defective lysogens the A and B gene products could be demonstrated by complementation, these gene products are not responsible for the killing of the host, suggesting the presence of another unknown early gene product of Mu. The level of complementation of a mutation in gene A is reduced by the presence in the cell of another defective Mu prophage containing the "G" beta part of Mu. This effect on A gene complementation is markedly enhanced when the defective prophage, containing the "G" beta part, is located on an episome instead of on the chromosome. Complementation of late genes by a defective prophage located on the chromosome, is extremely low or undetectable. A stimulation of complementation by a factor of 10 to 40 was found when the same defective prophage was situated on a F' factor. A possible explanation for this "episome" effect will be discussed.

Events following prophage Mu induction

Escherichia coli strains lysogenic for a thermoinducible Mu prophage (Mu cts62) undergo rapid lysis about 50 min after heat induction. Induction of Mu cts62 apparently causes damage to the host sequences in which Mu is inserted. The normal expression of A, BU, and X genes of Mu is needed for this specific deleterious effect on the prophage-containing host sequences. Mu deoxyribonucleic acid can be shown to reintegrate extensively at different sites on the host genome during the lytic cycle after prophage induction or after infection of sensitive cells by clear-plaque mutants of Mu. We estimate that approximately 10 copies of Mu deoxyribonucleic acid are inserted per chromosome during vegetative growth. The episome rescue method for detecting vegetative Mu deoxyribonucleic acid insertion, in which an episome is transferred from the lytically infected cells to F- receipient cells, can be applied to study Mu integration without requiring the host cells to survive. It also provides an easy system to isolate Mu insertions in transmissible episomes and plasmids.

Model for the enchancement of lambde-gal integration into partially induced Mu-1 lysogens

Temperate phage Mu-1, which is able to integrate at random in its host chromosome, is also able to mediate integration of other circular deoxyribonucleic acid, as a lambda-gal mutant unable to integrate by itself. After mixed infection with lambda-gal and Mucplus, galplus transductants are recovered that have the lambda-gal integrated in any circular permutation, sandwiched between two complete Mu genomes in the same orientation, the whole Mu-lambda-gal-Mu structure being found at any location in the bacterial chromosome. Here we show that such a lambda-gal can integrate in an induced Mu lysogen. In this case the lambda-gal is again in any circular permutation, between two Mu in the same orientation, but it is always located at the site of the original Mu prophage, and the two surrounding Mu have always the same genotype as the original Mu prophage. Active Mu replication functions are not essential for that process to occur. This suggests that bacterial replication may generate two Mu copies that in some way can regenerate a Mu attachment site that recombines with the lambda-gal. A model is presented that accounts for these observations, may be helpful for understanding some complex features of Mu development, and may possibly offer a basis for explaining spontaneous duplications.

Transcription of bacteriophage mu. An analysis of the transcription pattern in the early phase of phage development

It has previously been shown that the transcription of Mu is asymmetric and takes place on the heavy DNA strand (Bade, 1972; Wijffelman et al., 1974). The direction of transcription of Mu has now been determined by RNA-DNA hybridizations between purified Mu-RNA and the separated strands of lambda-Mu hybrid phages. The direction of transcription is from the c-gene (immunity gene) end of the heavy strand to the beta-end (immunity distal end) (Fig. 1). Thermo-inducible, defective Mu lysogens, in which the prophage is deleted from the beta-end, have a normal early transcription pattern, but the increase of RNA at later times is absent. A defective lysogen, which contains only the immunity gene c and the genes A and B, still has an early transcription pattern similar to that of the wild-type. Therefore, we conclude that the early RNA is transcribed from that region of the Mu genome. The early Mu-RNA synthesis is negatively regulated with a minimum rate of transcription at 9 minutes after induction. Before the onset of the late RNA synthesis, at about 22 minutes there is a rather long period in which the rate of Mu-RNA synthesis slowly increases. Using DNA strands of lambda-Mu hybrids which contain only that part of the Mu-DNA on which the early RNA synthesis takes place, we have determined that during the first half in the intermediate phase only early genes are transcribed. The amount of Mu-RNA synthesized by a Mu prophage carrying the X-mutation, which influences the excision of Mu, is greatly reduced. Negative regulation of early transcription occurs normally in this mutant.

Temperature-sensitive mutants of bacteriophage mu

Temperature-sensitive mutants of bacteriophage Mu, which grow at 32 C but not at 42 C, have been isolated. These mutants fall into two groups. Group 1 mutants fail to lyse host cells at nonpermissive temperatures, whereas lysis occurs normally with the group 2 mutants. All of the group 1 mutants apparently belong to the cistrons mapping to the left of gene C, whereas the group 2 mutants have lesions in various genes between D and S.