Constitutive expression of bacteriophage P2 early genes resulting from a tandem duplication

Physical mapping of bacteriophage P2 mutations and their relation to the genetic map

Three new deletion mutants and an insertion mutant of E. coli bacteriophage P2, del2, vir79, del4 and sig5, were mapped by the electron microscope heteroduplex method. The deletions were found to cover 45.5-51.6%, 75.6-76.7% and 92.3-99.3% respectively of P2 DNA while sig5 represented a 3.7% insertion at 78.6% from the left end. The region covering 75.9-76.7% of P2 DNA is also deleted in the two previously characterized immunity insensitive variants of P2, vir22 and Hy dis. This region may identify the portion of the genome responsible for immunity. The physical and genetic maps of P2 were previously found to be colinear with respect to the two mutations vir22 and vir37. This relationship is confirmed by the position of del2.

Further physical characterization of deletion and substitution mutants affecting the control of lysogeny in bacteriophage P2

A deletion of phage P2, del6 (L.E. Bertani, 1980), thought to remove the structural gene int, and a deletion/substitution, vir94, thought to remove genes int, C and cox, were mapped by electron microscopy, using the heteroduplex technique. Four independent deletion/substitution mutations, all affecting the regulatory region of P2, were compared in all possible combinations with the same technique: two showed sequence homology in their substitution DNA. The results confirm the model proposed for the origin of these mutants, analogous to that for the origin of transducing variants in phage lambda, but suggest in first approximation that the exchange between the P2 DNA and the chromosome of the host bacterium may occur at several different bacterial sites. A map of the regulatory region of P2, based on all data available from the study of deletions and insertions, is presented.

Tandem pentuplication of a DNA segment in a derivative of bacteriophage P2: its use in the study of the mechanism of DNA annealing

From a tandem duplication mutant of phage P2, triplication, quadruplication and pentuplication forms were derived. They were recognized by decreased virion heat stability resulting from the increase in DNA content, and were confirmed by electron microscope heteroduplex mapping. These forms of partially repeated DNA are quite stable in P2 because of the low level of recombination typical of this phage. Under conditions normally employed for full DNA renaturation, these high order repeat chromosomes gave often incomplete renaturation over the repeated segments. Based on current models for DNA renaturation, several predictions were made and tested. The results, although not quantitatively exhaustive, indicated that base pairing proceeding from a nucleation site was sufficiently slow to allow a second nucleation to occur with a fair probability over a length of a few thousand base pairs.

A restriction endonuclease cleavage map of bacteriophage P2

A restriction endonuclease cleavage map of phage P2 was constructed. The enzymes used and, within parenthesis, the number of their cleavage sites on the P2 lg cc DNA molecule were: AvaI(3), BalI(1), BAMI(3), BglII(3), HaeIII (more than 40; only three were mapped), HindIII(0), HpaI(10), KpnI(3), PstI(3), SalI(2) and SmaI(2). The EcoRI cleavage sites (3), as determined earlier, were used as reference points for this study. The DNAs of a variety of P2 mutants carrying chromosomal aberrations (dell, del2, del3, del6, vir22, vir37(2), vir79 and vir94) were also similarly examined.

Isolation and characterization of tox mutants of corynebacteriophage beta

Seventeen nontoxinogenic (tox) mutants of corynebacteriophage beta have been isolated by using a tissue culture screening technique. The mutants fall into four major classes. Two of the classes, I and II, appear to contain missense and nonsense mutants, respectively. However, classes III and IV have not been previously described. Class III mutants produce two proteins (CRMs) seriologically related to diphtheria toxin, but efforts to demonstrate the presence of more than one tox gene have been successful. Class IV mutants are phenotypically CRM-, failing to produce any detectable protein serologically related to diphtheria toxin. Genetic studies indicate that the mutations in class IV strains are not in a gene distinct form the structural gene for toxin, and that the CRM- strains retain at least a portion of that gene. A natural phage isolate, gamma, behaves in a completely parallel fashion to the class IV mutants. The production of tox+ recombinants through recombination of various pairs of tox phage mutants has been demonstrated. The implications of these findings for the natural history of diphtheria are discussed.

Interaction of P2 bacteriophage with the dnaB gene of Escherichia coli

The dnaB gene product of Escherischia coli is required for multiplication of temperate phage P2. At 37 C in dnaB-ts mutnats, P2 will not plaque and gives a very small burst of progeny. P2 mutants have been isolated which can grow well enough to plaque under these conditions. This type of phage mutant is cis dominant, and one such mutant (P2rlb1) has been mapped near the left end of the early gene B and to the right of the cox4 (excision) mutation. The rlb1 mutation does not lie at the replication origin, but may affect transcription in the early region, which includes the replication origin. It may also represent a site on the P2 DNA which interacts with the dnaB gene product.

Deletions in bacteriophage P2. Circularity of the genetic map and its orientation relative to the DNA denaturation map

Several types of viable chromosomal deletions of bacteriophage P2 were isolated. One type gives the immunity insensitive phenotype and may extend to the genes for the immunity repressor (C) and for integrative recombination (int). Two other types delete genes (old and fun) known to be active in the lysogenic state. For such deletion mutants the relationship between particle density and DNA length was established. The deletions were located in respect to previously mapped genes and the results were compared with electron microscopical studies (by Inman and collaborators) of the P2 chromosome. It is concluded that the best representation of the genetic map of P2 is circular. The cohesive ends of the linear P2 DNA molecule are most likely formed between genes old and Q. Except for the neighborhood of gene old, the previously published, linear genetic map of P2 (Lindahl) is colinear with the melting map of the P2 chromosome (Inman). Preliminary evidence for some specific recombination event often accompanying integrative recombination between phage chromosomes is presented.

Tandem duplication in bacteriophage P2: electron microscopic mapping

The physical position of vir37, a new immunity-insensitive mutant of Escherichia coli bacteriophage P2, was mapped by the electron microscopic heteroduplex method. In P2 vir37, a segment equivalent to 2.8% of P2 DNA is added. The addition was characterized as a tandem duplication of the segment occurring between 77.2 and 80.0% from the left end of P2 DNA (the right half of P2 DNA is arbitrarily defined, from denaturation map studies, as the half richer in A + T). The point of addition of the duplicated segment (the "novel-joint") was, thus, 80.0% from the left end of P2 DNA. On the basis of previous studies on P2 vir22, it was tentatively concluded that the physical and genetic maps of P2 are colinear. This conclusion is now further supported by physical and genetic data on P2 vir37.