Cloning and biological characterization of the immunity region of Escherichia coli phage Mu

Localization and regulation of bacteriophage Mu promoters

Mu promoters active during the lytic cycle were located by isolating RNA at various times after induction of Mu prophages, radiolabeling it by capping in vitro, and hybridizing it to Mu DNA fragments on Southern blots. Signals were detected from four new promoters in addition to the previously characterized Pe (early), PcM (repressor), and Pmom (late) promoters. A major signal upstream of C was first observed at 12 min and intensified thereafter with RNA from cts and C amber but not replication-defective prophages; these characteristics indicate that this signal arises from a middle promoter, which we designate Pm. With 20- and 40-min RNA, four additional major signals originated in the C-lys, F-G-I, N-P, and com-mom regions. These signals were missing with RNA from C amber and replication-defective prophages and therefore reflected the activity of late promoters, one of which we presume was Pmom. Uninduced lysogens showed weak signals from five regions, one from the early regulatory region, three between genes B and lys, and one near the late genes K, L, and M. The first of these probably resulted from PcM activity; the others remain to be identified.

Pathogen-derived resistance to viral infection using a negative regulatory molecule

The principle of pathogen-derived resistance (the use of pathogen-derived genes to interfere with the pathogenic process and thereby confer disease resistance to the host) has been put forward as a broadly applicable conceptual tool for use in the genetic engineering of resistance to pathogens and parasites. It was previously predicted that four mechanisms of pathogen-derived resistance could be established using the bacteriophage QB and its host, Escherichia coli, as a model system. This paper demonstrates and helps ellucidate the first of these mechanisms by using a viral regulatory protein, the QB coat protein, to block viral replication. The QB coat protein gene was transferred to susceptible E. coli. Expression of this gene had no obvious detrimental effect on the host. Low-level, constitutive expression of the coat protein conditions very high levels of resistance to QB infection. The resulting resistance is not associated with RNA interference or loss of pili as attachment sites, and does not appear to be associated with premature encapsidation. This low-level expression of the QB coat protein also produces an intermediate level of resistance to the closely related phage SP, but fails to protect against the unrelated phage f2. Thus the resistance does not result from a generalized antiviral host response induced by the presence of the coat protein. We conclude that the QB coat protein blocks viral infection, as was predicted, due to its action as a negative regulatory molecule. The use of negative regulatory molecules may provide an effective mechanism for use in the genetic engineering of pathogen-derived resistance.

The bacteriophage Mu gene B product is incorporated into the inner membrane of Escherichia coli

By using the Triton X-114 method, it could be shown that an Mr 33 000 protein expressed from the left-end EcoRI-C fragment of phage Mu behaves like a hydrophobic protein. Fusion of a DNA region coding for this protein to the controllable pL promoter of phage lambda results in the production of large amounts of the 33-kDal protein under inducing conditions. This protein can be isolated in association with the inner membrane of Escherichia coli cells. Based on complementation studies it was concluded that the 33-kDal protein is the product of the early gene B.

Mapping of binding sites for Mu repressor and ner product within the left-end EcoRI. C fragment of the Mu genome

Bacterial cells containing the ner gene of phage Mu inserted into pBR322 express a binding activity with specificity for the left-end EcoRI.C fragment of Mu DNA. Crude extracts containing either Mu repressor or ner protein have been used to localize binding sites on TaqI subfragments of the EcoRI.C fragment. There are at least 3 distinct binding sites for the Mu repressor and 1 binding site for the ner protein on the EcoRI. C fragment. The possible biological function of these binding sites is discussed.

AvaII and BglI restriction maps of bacteriophage Mu

The AvaII and BglI restriction maps of bacteriophage Mu were derived by restriction analysis of a series of plasmid clones containing segments of Mu DNA which, in combination, covered the entire Mu genome. The plasmids analyzed included pKN36, pKN54, pKN62, pKN50, pKN35, pKN27, pKN48, pKN82, and pKN56 from the collection of W. Schumann and E. G. Bade, and pCM02, a newly constructed plasmid containing the rightmost internal EcoRI-PstI fragment of Mu DNA. BglI cuts Mu DNA at 23 sites, producing 24 fragments which range in size from 0.05 kb up to the approximately 7-kb fragment derived from the right end. AvaII cuts Mu DNA at 17 sites (including 2 within the G segment), producing fragments which range in size from 0.17 to 8.9 kb. The derived maps were confirmed by results of hybridization of 32P-labeled, nick-translated plasmid DNA to AvaII- and BglI-digested Mu DNAs. Evidence for modification of one of the AvaII sites in E. coli was obtained.

The products of gene A of the related phages Mu and D108 differ in their specificities

By recombination between different mutants of mutator phages Mu and D108, we isolated a set of viable hybrids. The structure of the hybrids was analyzed by digestion with different restriction enzymes. Genetic studies show that hybrids which carry the left end of the Mu genome complement a mini-Mu deleted from within the A gene as well as Mu while hybrids with the left end of the D108 genome or D108 do not. Vice versa, hybrids with the left end of the D108 genome or D108, but not hybrids with the left end of the Mu genome or Mu complement a mini-D108 deleted from within the A gene. The nucleotide sequence of the A gene of Mu and its equivalent on D108 are mainly similar except on their left end. These observations demonstrate that the two pA products, although only partially different, have different specificities.

Nucleotide sequence of the immunity region of bacteriophage Mu

The leftmost 1590 bp of Mu DNA covering the immunity region have been sequenced. This region encodes the cI repressor, the cII or ner function and the beginning of gene A. An open reading frame extends from position 863 to 342 on the l-strand corresponding to cI protein with a molecular weight of 19212. It is preceded by a sequence resembling a promoter. To the right of the HindIII site an open reading frame extends from position 1099 to 1323 corresponding to cII or ner protein (molecular weight of 8505) followed by the beginning of gene A at position 1328. Between position 863 and 1099 promoters for leftward and rightward transcription and operator-like structures can be recognized in the sequence. The promoter for rightward transcription overlaps with the HindIII site and coincides with a RNA polymerase binding site as demonstrated by electron microscopy.

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.

Construction of an HpaI and HindII plasmid vector allowing direct selection of transformants harboring recombinant plasmids

The construction of the vector plasmid PKN80 is described, which can be used as HpaI or HindII cloning vehicle with direct selection on transformants harboring hybrid plasmids. pKN80 carries the EcoRI.C fragment of phage Mu DNA coding for a killing function which is efficiently expressed upon transformation of pKN80 into Mu-sensitive bacteria. Cloning of DNA fragments at the single HpaI site of pKN80 results in insertional inactivation of the killing function. Whereas religated pKN80 molecules yielded only a few transformants, the transformation efficiency had been increased by a factor of at least ten when HpaI fagments of lambda DNA were added to the linearized vector prior to ligation. More than 90% of the transformants tested containted hybrid plasmids.