Inversion induced by temperature bacteriophage mu-1 in the chromosome of Escherichia coli K-12

Reversal of Mu gem2ts-induced mutations

Mutations induced by the integration of a Mu gem2ts mutant prophage can revert at frequencies around 1 x 10(-6), more than 10(4)-fold higher than that obtained with Mu wild-type. Several aspects characterize Mu gem2ts precise excision: (i) the phage transposase is not involved; (ii) the RecA protein is not necessary; and (iii) revertants remain lysogenic with the prophage inserted elsewhere in the host genome. In addition, prophage re-integration seems to be non-randomly distributed, whereas Mu insertion into the host genome is a transposition event without any sequence specificity. In this paper, we describe that the site of re-integration somehow depends on the original site of insertion. Two alternative models are proposed to explain the strong correlation between donor and receptor sites.

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.

Large inversion in Escherichia coli K-12 1485IN between inversely oriented IS3 elements near lac and cdd

A companion study has shown that the inversion carried by strain 1485IN has one terminus between lac and proC and the other between his and cdd of the normal strain. Starting with this mapping data, we have done molecular work demonstrating that the inversion occurred by recombination between inversely oriented two IS3 elements, one present near lac and the other near the cdd locus; i.e., the inversion is IN(is3B-is3E). Evidence supporting this conclusion includes: (i) Normal and inversion strains share two short regions with identical restriction maps. One of these regions is near lac and the other near cdd. (ii) IS3 homology was detected in each of the terminus regions of both the normal and inversion strains. (iii) The sequence on one side of the original IS3 element near lac has been exchanged with the sequence on one side of the IS3 near cdd. Whether the inversion has occurred by one event of homologous recombination between the two IS3 elements or has been caused by involvement of IS3 elements on an F factor is discussed. Another rearrangement, probably related to inversion and deletion, was detected between the IS3 and cdd of the inversion strain.

Mapping by transposons of the inversion termini in Escherichia coli K-12 strain 1485IN

Strain 1485IN carries a chromosomal inversion which corresponds to 35% of the chromosome and includes proC, trp and his genes. The termini of the inversion lie between the lac and proC loci and between his and cdd of the normal strain. Using Tn10 and Tn5 in transduction crosses between the normal and inversion strains, the termini were mapped to sites located approximately 0.25 min and 1.6 min away from proC and his, respectively within a region of roughly 4 kb long. The crosses where the normal strains carrying Tn10 near the terminus are donors and the inversion strain is a recipient, yielded unusual Tetr His- recombinants, which arose from illegitimate recombination leading to the replacement of a chromosomal his+ region with a transducing fragment carrying proC. Another rearrangement was detected between the normal and inversion strains in a region outside the inverted segment near the cdd locus.

Physical characterization of mini-mu and mini-D108

Several derivatives of phages Mu and D108 have been isolated that carry an internal deletion generated by one of the IS1 components of a Tn9 transposon located in the A, B, or S gene of the prenatal phage. The deletions remove most of the lytic functions of the phage but leave intact either genes A and B or gene A and the left and the right end of the phages. These deleted derivatives, called mini-Mu and mini-D108, were physically characterized by electron microscopy and digestion with restriction enzymes. Mini-Mu and mini-D108, which carry an antibiotic resistance marker, are described and some of their genetic properties are summarized in the paper by Toussaint et al. (1981).

Use of bacteriophage transposon Mu d1 to determine the orientation for three proC-linked phosphate-starvation-inducible (psi) genes in Escherichia coli K-12

We have previously used the bacteriophage transposon Mu d1 (which encodes the lacZY structural genes but without their promoter) to construct strains that have lacZY fused to phosphate-regulated promoters in Escherichia coli K-12. Among 18 identified phosphate-starvation-inducible (psi) genes, three (the phoA and two new genes: psiF and psiG) are closely linked to the proC region. The gene order (clockwise) is phoA psiF proC psiG phoB phoR. Using these mutants containing Mu d1 insertions, we devised and tested a new method to determine their orientation. In this procedure, mutants with deletions that are selectable by their ability to grow at 42 degrees C are tested for the presence of Mu d1 and of neighboring genes. Some difficulties arose during analysis of suspected deletion-containing strains derived from Mu d1 lysogens (which also contained a Tn5 element) that were caused by Mu d1 and transposon transpositions and other possible genome rearrangements. Nevertheless, we have shown that the phoA and psiF genes are transcribed clockwise and the psiG gene is transcribed counterclockwise towards proC. Because phoA, but not psiF, gene expression requires the phoB+ (positive regulator) gene product, the phoA and psiF genes do not constitute an operon. On the other hand, the psiG:lacZ fusion-bearing strain may have a fusion to the promoter-distal end of the phoB gene. This implies that phoB expression is phosphate regulated. We believe that this method may be useful in general to elucidate the direction of gene transcription.

A mechanism of DNA transposition

Bacteriophage Mu and many other transposable elements undergo transposition by a process that involves replication of the element. We describe here a mechanism by which such integrative replication may take place. We hve examined electron microscopically the DNA structures generated in host cells after Mu induction and have deduced the following steps in the transposition process, (i) Association. A protein-mediated association is brought about between the transposable element and the target DNA. (ii) Attachment. One end of the element is nicked and attached to a site that undergoes a double-stranded cleavage. (iii) Roll-in replication. While one strand of the target DNA is linked to the nicked strand of the element, the complementary strand of the target DNA is used as a primer for replication into the element such that the replicating DNA is threaded through the replication complex. (iv) Roll-in termination. When the distal end of the element arrives at the replication complex, replication is terminated. The roll-in replication mechanism can also explain laying down of tandem repeats--i.e., amplification of circular DNA sequences.