Similarity of vegetative map and prophage map of bacteriophage Mu-1

A novel illegitimate recombination event: precise excision and reintegration with the Mu gem mutant prophage

The bacteriophage Mu is known to insert its DNA more or less randomly within the Escherichia coli chromosome, as do transposable elements, but unlike the latter, precise excision of the prophage, thereby restoring the original sequence, is not observed with wild-type Mu, although it has been reported with certain defective mutants. We show here that the mutant prophage Mu gem2ts can excise precisely from at least three separate loci -- malT, lac and thyA (selected as Mal+, Lac+ and Thy+, respectively). This excision occurs under permissive conditions for phage development, is observed in fully immune (c+) lysogens, and is independent of RecA and of Mu transposase. Mu gemts2 excision is invariably accompanied by reintegration of a Mu gem2ts prophage elsewhere in the chromosome. In the case of Mal+ revertants, this prophage is systematically located at 94 min on the E. coli chromosome. Mu gem2ts excision therefore sheds some light on the long-standing paradox of the lack of precise Mu excision.

Morphogenetic structures present in lysates of amber mutants of bacteriophage Mu

The Mu phage particle is structurally similar to that of the T-even phages, consisting of an icosahedral head and contractile tail. This study continues an analysis of the morphogenesis of the Mu phage particle by defining the structural defects resulting from mutations in specific Mu genes. Defective lysates produced by induction of 55 amber mutants, representing 24 essential genes, were examined in the electron microscope and categorized into eight classes based on the observed phage-related structures. (1) Mutations in genes lys, F and G, and some H mutations, did not cause a visible alteration in particle structure. (2) Mutants defective in genes A, B, and C produced no detectable phage structures, consistent with their lack of production of late RNA. (3) Extracts defective in genes L, M, Y, N, P, Q, V, W, and R contained only head structures, and these appeared normal. (4) K-defective mutants accumulated free heads as well as free tails which were longer than normal and variable in length. (5) Tails which appeared normal were the only structures found in T- and some I-defective extracts. (6) Free tails and empty heads accumulated in D-, E-, and some I- and H-defective extracts. These heads were as much as 16% smaller than normal heads. The heads found in some I amber lysates had a protruding neck-like structure and unusually thick shells suggestive of a scaffolding-like structure. (7) Defects in gene J resulted in the accumulation of unattached tails and full heads. (8) Previous analysis of lysates produced by inversion-defective gin mutants fixed in the G(+) orientation demonstrated that S and U mutants produced particles lacking tail fibers (F.J. Grundy and M.M. Howe (1984), Virology 134, 296-317). In these experiments with Gin+ phages S and U mutants produced apparently normal phage particles. Presumably the tail fiber defects were masked by the production of S' and U' proteins by G(-) phages in the population.

The isolation and characterisation of a plaque-forming derivative of bacteriophage Mu carrying a fragment of Tn3 conferring ampicillin resistance

We have isolated a plaque-forming derivative of phage Mu which carries a determinant for ApR. The biological properties of this MuAp phage are similar to those of normal Mu. Its genome contains a 1.1 kb substitution where Mu DNA from the right end of the G region has been replaced by a similar length of DNA from the transposon Tn3. This fragment of Tn3 DNA carries the ApR gene, but is no longer capable of independent transposition.

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.

Mutational alteration of a nitrogen-fixing bacterium to sensitivity to infection by bacteriophage Mu: isolation of nif mutations of Klebsiella pneumoniae M5al induced by Mu

The nitrogen-fixing bacterium Klebsiella pneumoniae M5al is not sensitive to infection by bacteriophage Mu. A mutant of K. pneumoniae that is sensitive to Mu infection was isolated. Several Mu-induced auxotrophic mutations of K. pneumoniae including nif, trp, and rtl were isolated and genetically characterized. Evidence is presented that the Mu-induced mutations of nif arise as the result of insertion of Mu within (or near) the nif operon(s). The rtl locus, which determines the ability to utilize ribitol as a carbon source, was found to be linked to nif loci.

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.

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.