Reversal of Mu gem2ts-induced mutations

Abstract: Mutations induced by the integration of a Mu gem 2ts mutant prophage can revert at frequencies around 1 × 10⁻⁶, more than 10⁴-fold higher than that obtained with Mu wild-type. Several aspects characterize Mu gem 2ts 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.     

Characterization of functionally important sites in the bacteriophage Mu transposase protein

The 663 amino acid Mu transposase protein is absolutely required for Mu DNA transposition. Mutant proteins were constructed in vitro in order to locate regions of transposase that may be important for the catalysis of DNA transposition. Deletions in the A gene, which encodes the transposase, yielded two stable mutant proteins that aid in defining the end-specific DNA-binding domain. Linker insertion mutagenesis at eight sites in the Mu A gene generated two proteins, FF6 and FF14 (resulting from two and four amino acid insertions, respectively, at position 408), which were thermolabile for DNA binding in vitro at 43°C. However, transposition activity in vivo was severely reduced for all mutant proteins at 37°C, except those with insertions at positions 328 and 624. In addition, site-specific mutagenesis was performed to alter tyrosine 414, which is situated in a region that displays amino acid homology to the active sites of a number of nicking/closing enzymes. Tyrosine 414 may reside within an important, yet non-essential, site of transposase, as an aspartate-substituted protein had a drastically reduced frequency of transposition, while the remaining mutants yielded reduced, but substantial, frequencies of Mu transposition in vivo.     

The bacteriophage D108 Ner repressor binds a conformationally distinct operator

The Ner protein encoded by the transposable coliphage D108, an 8.6 kDa λ Cro-like repressor, binds to an operator spanning 50 bp of DNA. The distinguishing features of this operator are two perfect 11-bp inverted repeats (5′-CCGTGAGCTAC-3′) that are separated by an 8-bp AT-rich spacer. Hyperreactivity of the ner operator to potassium permanganate and the hydroxyl radical indicate that the AT-rich spacer assumes a variant conformation consistent with a bend. Using an electrophoretic mobility shift assay, we demonstrated that Ner does not display significant affinity for a single 11-bp site. Furthermore, DNase I protection analysis and circular-permutation binding assays reveal that alterations in the length and sequence of the AT-rich spacer that separates the 11-bp inverted repeats significantly alter Ner-operator interactions, and demonstrate that the intrinsically bent ner operator is conformationally altered upon protein binding. Received: 29 September 1999 / Accepted: 21 December 1999     

Transposition studies of mini-Mu plasmids constructed from the chemically synthesized ends of bacteriophage Mu

We describe below the chemical synthesis of the right and left ends of bacteriophage Mu and characterize the activity of these synthetic ends in mini-Mu transposition. Mini-Mu plasmids were constructed which carry the synthetic Mu ends together with the Mu A and B genes under control of the bacteriophage λ p_L promoter. Derepression of p_L leads to a high frequency of mini-Mu transposition (5.6 × 10⁻²) which is dependent on the presence of the Mu ends and the Mu A and B proteins. Five deletion mutants in the Mu ends were tested in the mini-Mu transposition system and their effects on transposition are described.     

Neighboring plasmid sequences can affect Mini-Mu DNA transposition in the absence of expression of the bacteriophage Mu semi-essential early region

Bacteriophage Mu DNA, like other transposable elements, requires DNA sequences at both extremities to transpose. It has been previously demonstrated that the transposition activity of various transposons can be influenced by sequences outside their ends. We have found that alterations in the neighboring plasmid sequences near the right extremity of a Mini-Mu, inserted in the plasmid pSC101, can exert an influence on the efficiency of Mini-Mu DNA transposition when an induced helper Mu prophage contains a polar insertion in its semi-essential early region (SEER). The SEER of Mu is known to contain several genes that can affect DNA transposition, and our results suggest that some function(s), located in the SEER of Mu, may be required for optimizing transposition (and thus, replication) of Mu genomes from restrictive locations during the lytic cycle.     

Involvement of transposable elements in the formation of hybrid phages between bacteriophage P1 and the R plasmid NR1

Homologous recombination between IS1 elements present on both replicons, P1 and NR1, resulted in P1-NR1 cointegrates and P1-RTF and P1-r-det phages. Cointegration between P1 and NR1-B, and NR1 derivative with multiple DNA rearrangements including insertion of the transposable element γδ, was also mediated by reciprocal recombination in IS1 sequences. However, all 4 hybrids studied carried deletions promoted by γδ residing on NR1-B. Further IS1-mediated deletions on the hybrid genomes resulted in plaque-forming P1Cm phages.     

Regulation of bacteriophage Mu transposition

Bacteriophage Mu is a transposon and a temperate phage which has become a paradigm for the study of the molecular mechanism of transposition. As a prophage, Mu has also been used to study some aspects of the influence of the host cell growth phase on the regulation of transposition. Through the years several host proteins have been identified which play a key role in the replication of the Mu genome by successive rounds of replicative transposition as well as in the maintenance of the repressed prophage state. In this review we have attempted to summarize all these findings with the purpose of emphasizing the benefit the virus and the host cell can gain from those phage-host interactions.     

Mechanisms involved in the formation of plaque-forming derivatives from over-sized hybrid phages between bacteriophage P1 and the R plasmid NR1

Three case histories document how subsequent events of genomic rearrangements and selection interplay in the evolution of infectious bacteriophage genomes carrying acquired genes. Two of the phages studied were plaque-forming P1CmTc recombinants derived from P1Cm1 and P1Tc1, both of which are hybrids between phage P1 and the R plasmid NR1. In the formation of the P1CmTc4 genome a postulated intermediate underwent IS1-mediated deletion formation. From the same intermediate P1CmTc1 must have evolved by IS1-mediated inversion followed by homologous recombination with a parental phage DNA. The third case documents formation of the P1Cm2 genome by “illegitimate” intramolecular recombination in the genome of P1-r-det, a hybrid between P1 and NR1.     

In vitro and in vivo manipulations of bacteriophage Mu DNA: cloning of Mu ends and construction of mini-Mu's carrying selectable markers

Recombinant plasmids carrying one or both ends of the bacteriophage Mu genome were constructed by molecular cloning. Transposable mini-Mu's with selectable markers (ampicillin resistance, kanamycin resistance or the entire lac operon of Escherichia coli) inserted between the Mu ends were also constructed. As a source of lac operon DNA, a pBR322 derivative with a 27 kb insert containing the lac operon was constructed. The plasmids with both ends of Mu (mini-Mu's) conferred full Mu immunity upon the host cells. However, the same mini-Mu's containing kan or lac inserts were defective in immunity. A summary of the construction and physical characterization, including restriction endonuclease cleavage maps and some of the biological properties of the plasmids, is presented.     

Generation in vitro of deletions in the broad host range plasmid RK2 using phage Mu insertions and a restriction endonuclease

Several non-lethal deletions of the broad host range plasmid RK2 (molecular weight of 37.6 . 10(6) have been produced in vitro. The method employed relied on the single HindIII restriction nuclease site in RK2 and the ability of phage Mu to insert and thereby add new HindIII restriction sites at various positions in the plasmid. The deleted plasmids have in each case lost kanamycin (Km) resistance, and in two cases are defective in self-transmissibility. The method used to reduce the size of the RK2 plasmid also results in the cloning of each of the two ends of the Mu phage DNA on the plasmid derivatives.     

Polymorphism of DNA conformation inside the bacteriophage capsid

Double-stranded DNA bacteriophage genomes are packaged into their icosahedral capsids at the highest densities known so far (about 50 % w:v). How the molecule is folded at such density and how its conformation changes upon ejection or packaging are fascinating questions still largely open. We review cryo-TEM analyses of DNA conformation inside partially filled capsids as a function of the physico-chemical environment (ions, osmotic pressure, temperature). We show that there exists a wide variety of DNA conformations. Strikingly, the different observed structures can be described by some of the different models proposed over the years for DNA organisation inside bacteriophage capsids: either spool-like structures with axial or concentric symmetries, or liquid crystalline structures characterised by a DNA homogeneous density. The relevance of these conformations for the understanding of DNA folding and unfolding upon ejection and packaging in vivo is discussed.     

Analysis of the killing effect of Mu ligts mutants on host cells

Infection of Escherichia coli with the mutant lig ts2 of bacteriophage Mu at a temperature nonpermissive for this mutant is lethal for the host cells. This effect is insensitive to phage immunity of the host cells, to inhibitors of protein synthesis and is not suppressed in trans in bacterial strains producing the Lig⁺ active protein. These data suggest that the killing effect of this mutant is different from the other kil functions identified in Mu [1].     

Developmental regulation of excision timing of Mutator transposons of maize: Comparison of standard lines and an early excision bzl::Mu1 line

Three characteristics of standard Mutator lines reflect developmental regulation: new mutants usually involve single gametes, somatic excision is restricted to terminal cell divisions during tissue development, and germinal excision is rare. By selection for earlier (larger) somatic sectors in the aleurone, a Mutator line was identified that exhibits a dramatic elevation in somatic excision frequency during the first three nuclear divisions of the endosperm and more than a 10-fold increase in germinal reversion from the bzl::Mul reporter gene. The programming of early sectoring is dominant in crosses with Mutator lines containing diverse reporter alleles. Germinal reversion is biased 5- to 10-fold for events through the pollen compared to the ear. The timing of germinal excision in the tassel is late because somatic excision sectors in the anthers are small; however, 98% of the germinal revertants are concordant. These observations indicate that in the early sectoring line Mu excision usually occurs before the mitotic divisions that separate gametic nuclei and may be restricted to the early stages of microsporogenesis. © 1992 Wiley-Liss, Inc.     

The cis-acting DNA sequences required in vivo for bacteriophage Mu helper-mediated transposition and packaging

The 37,000 bp double-stranded DNA genome of bacteriophage Mu behaves as a plaque-forming transposable element of Escherichia coli. We have defined the cis-acting DNA sequences required in vivo for transposition and packaging of the viral genome by monitoring the transposition and maturation of Mu DNA-containing pSC101 and pBR322 plasmids with an induced helper Mu prophage to provide the trans-acting functions. We found that nucleotides 1 to 54 of the Mu left end define an essential domain for transposition, and that sequences between nucleotides 126 and 203, and between 203 and 1,699, define two auxiliary domains that stimulate transposition in vivo. At the right extremity, the essential sequences for transposition require not more than the first 62 base pairs (bp), although the presence of sequences between 63 and 117 bp from the right end increases the transposition frequency about 15-fold in our system. Finally, we have delineated the pac recognition site for DNA maturation to nucleotides 32 to 54 of the Mu left end which reside inside of the first transposase binding site (L1) located between nucleotides 1–30. Thus, the transposase binding site and packaging domains of bacteriophage Mu DNA can be separated into two well-defined regions which do not appear to overlap.Abbreviations attL attachment site left - attR attachment site right - bp base pairs - Kb kilobase pair - nt nucleotide - Pu Purine - Py pyrimidine - Tn transposable element State University of New York, Downstate Medical Center, Brooklyn, NY 11204 USA     

Restriction endonuclease BamHI cleaves bacteriophage Mu DNA within cistrons E and F

We have cleaved phage Mu DNA with restriction endonucleases EcoRI and BamHI and have cloned three specific DNA fragments from the middle of the Mu genome into vector plasmid pBR322. By marker rescue experiments, we have determined that the two BamHI cleavage sites in Mu DNA occur within cistrons E and F.     

The cell wall receptor for bacteriophage Mu G(−) in Erwinia and Escherichia coli C

Abstract Adsorption of bacteriophage Mu with its invertible DNA segment in the G(−) orientation requires a terminal glucose residue for binding to the core lipopolysaccharide (LPS) of Gram-negative bacteria. Analysis of a Mu-resistant mutant shows that the receptor for Mu G(−) in Erwinia B374 is a Glc-β1,6-Glc disaccharide. A spontaneously occurring host-range mutant, Mu G(−)h101, grows on Escherichia coli C. The loss of the terminal β1,3-linked glucose from the LPS of E. coli C leads to resistance to the phage Mu. These mutants are also resistant to phage P1 and D108 which have largely homologous G segments. This shows that Mu G(+) and G(−) phage particles differ with respect to their cell-wall receptors in the type of glycosidic linkage of a terminal glucose residue: α1, 2 for G(+) and β1,6 for G(−).     

Characterization of a Tc1-like transposable element in zebrafish (Danio rerio)

We have characterized Tdr1, a family of Tc1-like transposable elements found in the genome of zebrafish (Danio rerio). The copy number and distribution of the sequence in the zebrafish genome have been determined, and by these criteria Tdr1 can be classified as a moderately repetitive, interspersed element. Examination of the sequences and structures of several copies of Tdr1 revealed that a particular deletion derivative, 1250 by long, of the transposon has been amplified to become the dominant form of Tdr1. The deletion in these elements encompasses sequences encoding the N-terminal portion of the putative Tdr1 transposase. Sequences corresponding to the deleted region were also detected, and thus allowed prediction of the nucleotide sequence of a hypothetical full-length element. Well conserved segments of Tc1-like transposons were found in the flanking regions of known fish genes, suggesting that these elements have a long evolutionary history in piscine genomes. Tdr1 elements have long, 208 by inverted repeats, with a short DNA motif repeated four times at the termini of the inverted repeats. Although different from that of the prototype C. elegans transposon Tc1, this inverted repeat structure is shared by transposable elements from salmonid fish species and two Drosophila species. We propose that these transposons form a subgroup within the Tc1-like family. Comparison of Tc1-like transposons supports the hypothesis that the transposase genes and their flanking sequences have been shaped by independent evolutionary constraints. Although Tc1-like sequences are present in the genomes of several strains of zebrafish and in salmonid fishes, these sequences are not conserved in the genus Danio, thus raising the possibility that these elements can be exploited for gene tagging and genome mapping.