Allan Campbell

Sensitive mutants of bacteriophage λ

Abstract Mutants of bacteriophage λ have been isolated whose plaque-forming ability is more sensitive than that of wild-type λ to high temperature, to extreme pH, or to the presence of suppressor genes in the bacterial host. A single mutation can determine sensitivity to one or more of these unfavorable conditions. Such mutations can occur within eighteen distinct genetic loci; mutations imparting different degrees and types of sensitivities are found within the same locus. Different suppressor mutations in bacterial genes are allele specific rather than locus specific. One of the phage loci has been identified as controlling the production of the lytic enzyme. It is suggested that all the sensitive mutants synthesize abnormal proteins, which are less stable or less active under extreme conditions than are the proteins of wild-type λ.

The steric effect in lysogenization by bacteriophage lambda: I. Lysogenization of a partially diploid strain of Escherichia coli K12

Abstract A strain of Escherichia coli K12 has been isolated which is diploid for the chromosomal region containing the gal gene and the lambda attachment site. This diploidy is not related to the presence or absence of the F factor. The two potential sites for lambda prophage attachment are distinguishable from each other by their linkage with gal + and gal − , respectively, in a strain heterozygous for gal . This linkage can be seen either by examining haploid segregants (which arise at a rate of 4 × 10 −2 per generation) or in bacterial crosses. The two sites are lysogenized with equal ease.

The steric effect in lysogenization by bacteriophage lambda: II. Chromosomal attachment of the b2 mutant

Abstract A strain of Escherichia coli K12 partially diploid for the prophage attachment site of bacteriophage lambda was mixedly infected with lambda and lambda b 2 . In the double lysogens formed, the two prophages were always located at the same site. In double lysogens from mixed infection with two b 2 + strains, the two prophages were generally at different sites. It is concluded that the ability of b 2 + to allow lysogenization by b 2 is due to structural interactions rather than (or conceivably in addition to) physiological ones.

Segregants from lysogenic heterogenotes carrying recombinant lambda prophages.

Abstract The segregation from lysogenic heterogenotes in which λ and λ dg are distinguished by several genetic markers has been examined. Recombination between the two prophages is correlated with loss of λ dg. The results suggest that the prophage genetic map is a circular permutation of the map for the vegetative phage.

The relationship between heritable physical and genetic properties of selected gal− and gal+ transducing λdg

Abstract Relative density measurements are made on λ dg differing either in their ability to transduce different galactose sites or their ability to recombine with different sus mutants by centrifuging the HFT lysates of different λ dg together in a density gradient. Of each pair of gal − λdg , the denser is always the more complete phage. When a gal + and a gal − transducer are broken in the same phage region, the gal + transducer is always the denser. The physical distance between some of the regions within the λ genome is approximated in terms of the size of λs DNA component.

The steric effect in lysogenization by bacteriophage lambda: III. Superinfection of monolysogenic derivatives of a strain diploid for the prophage attachment site

A partially diploid strain of Escherichia coli monolysogenic for bacteriophage lambda was superinfected with another lambda mutant. A low frequency of doubly lysogenic survivors was observed. In all those examined (3838), the second phage had added at the same chromosomal site as the original prophage rather than at the initially vacant site. We conclude that there is a strong physiological effect depressing prophage integration in the lysogenic cell, but that the purely structural effect of the presence of prophage is to facilitate, rather than to hinder, lysogenization.

Genetic recombination between λ prophage and irradiated λ dg phage

Abstract Galactose negative cells of Escherichia coli made lysogenic for multiple mutants of phage λ have been exposed to λ dg irradiated at various doses up to about 8 phage lethal hits. The pattern of rescue of phage markers among the heterogenotic transductants indicates that recombination occurs according to the genetic map of the prophage rather than that of the vegetative phage. The proportion of transductants that are heterogenotic rather than stable depends on the extent of the dg region, especially at high doses of UV. Heterogenote formation is lower and more highly UV sensitive with λ dg mutants that have few or no known genes at the left end of the lineom. From this we conclude that the free λ dg particle probably has physical ends corresponding to the ends of the vegetative map, and that the ends become joined prior to lysogenization and marker rescue.

On the mechanism of the recombinational event in the formation of transducing phages

Abstract Defective, galactose-transducing λ bacteriophages have been studied which have acquired a genetic marker by recombination with an active λ phage. The precise amount of genetic material recognizably derived from the phage has not been observed to change during the course of such recombination. Comparison with suitable control crosses allows the conclusion that the transducing phage does not arise by equational recombination within regions of good homology between phage and host.

The steric effect in lysogenization by bacteriophage λ: IV. Superinfection of nonimmune lysogens

Abstract A strain of Escherichia coli diploid for the λ attachment site was made lysogenic for the heteroimmune hybrid phage λ imm434 or the deletion-defective prophage λ cryptic. When these strains were superinfected with wild-type λ, most of the double lysogens produced carried the superinfecting phage at the initially unoccupied site ( trans to the original prophage). This contrasts with superinfection by λ of λ-immune lysogens, where all addition is cis . In mixed superinfection of imm434 lysogens with wild-type λ and λ imm434 , the presence of the λ phage allows trans addition of the λ imm434 , which would otherwise go cis to its homoimmune prophage. This suggests that immunity blocks the production of some diffusible factor that promotes integration rather than rendering the phage genome an unsuitable substrate for the integration process. This factor is equated with the phage-specific “integrase” because an int mutant is unable to promote trans addition of λ imm434 in an imm434 lysogen. Of the λ-21 hybrid phages tested, only the one which attaches at the λ site complements λ int . This indicates that the int function is site specific.