Relationship Between Prophage Induction and Transformation in Haemophilus influenzae
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The interaction between transformation and prophages of HP1 c 1, S2, and a defective phage of Haemophilus influenzae has been investigated by measurement of (i) the effect of prophage on transformation frequency and (ii) the effect of transformation on phage induction. The presence of any of the prophages does not appreciably alter transformation frequencies in various Rec + and Rec − strains. However, exposure of competent lysogens to transforming deoxyribonucleic acid (DNA) may induce phage but only in Rec + strains, which are able to integrate transforming DNA into their genome. Transformation of Rec + lysogens with DNA irradiated with ultraviolet (UV) light causes the production of even more phage than results from unirradiated DNA, but this indirect UV induction is not as effective as direct induction by UV irradiation of lysogens. Both types of UV induction are influenced by the repair capacity of the host. Wild-type cells contain a prophage and can be induced by transformation to produce a defective phage, which kills a small fraction of the cells. Defective phage in wild-type cells are also induced by H. parainfluenzae DNA, and a much larger fraction of the cells is killed. Strain BC200, which is highly transformable but is not inducible for defective phage, is not killed by H. parainfluenzae DNA, suggesting that wild-type cells are killed by killed by this DNA because of phage induction. A minicell-producing mutant, LB11, has been isolated. Some phage induction occurs in this strain when the cells are made competent, unlike the wild type. A large majority of LB11 cells surviving the competence regime are killed by exposure to transforming DNA.Keywords:
Prophage
Lysogen
Lysogenic cycle
Temperateness
Phagemid
Ultraviolet light
The most significant difference between bacteriophages functionally and ecologically is whether they are purely lytic (virulent) or temperate. Virulent phages can only be transmitted horizontally by infection, most commonly with the death of their hosts. Temperate phages can also be transmitted horizontally, but upon infection of susceptible bacteria, their genomes can be incorporated into that of their host’s as a prophage and be transmitted vertically in the course of cell division by their lysogenic hosts. From what we know from studies with the temperate phage Lambda and other temperate phages, in laboratory culture, lysogenic bacteria are protected from killing by the phage coded for by their prophage by immunity; where upon infecting lysogens, the free temperate phage coded by their prophage is lost. Why are lysogens not also resistant as well as immune to the phage coded by their prophage since immunity does not confer protection against virulent phages? To address this question, we used a mathematical model and performed experiments with temperate and virulent mutants of the phage Lambda in laboratory culture. Our models predict and experiments confirm that selection would favor the evolution of resistant and immune lysogens, particularly if the environment includes virulent phage that shares the same receptors as the temperate. To explore the validity and generality of this prediction, we examined 10 lysogenic Escherichia coli from natural populations. All 10 were capable of forming immune lysogens, but their original hosts were resistant to the phage coded by their prophage.
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CRISPR-Cas systems offer an immune mechanism through which prokaryotic hosts can acquire heritable resistance to genetic parasites, including temperate phages. Co-transcriptional DNA and RNA targeting by type III-A CRISPR-Cas systems restricts temperate phage lytic infections while allowing lysogenic infections to be tolerated under conditions where the prophage targets are transcriptionally repressed. However, long-term consequences of this phenomenon have not been explored. Here we show that maintenance of conditionally tolerant type III-A systems can produce fitness costs within populations of Staphylococcus aureus lysogens. The fitness costs depend on the activity of prophage-internal promoters and type III-A Cas nucleases implicated in targeting, can be more severe in double lysogens, and are alleviated by spacer-target mismatches which do not abrogate immunity during the lytic cycle. These findings suggest that persistence of type III-A systems that target endogenous prophages could be enhanced by spacer-target mismatches, particularly among populations that are prone to polylysogenization.
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Cultures of Bacillus subtilis lysogenic for the temperate bacteriophage SPβ release "betacin," a bacteriocinlike substance that inhibits B. subtilis strains which do not carry this phage. Production of betacin is blocked by mutations in the bet gene on the prophage and a second phage gene, tol, is apparently involved in making the lysogen itself tolerant to betacin. Mutations in a bacterial gene betR, located on the B. subtilis chromosome between metC and pyrD, render nonlysogens tolerant to betacin.
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Prophage was induced when strains of Bacillus subtilis 168 lysogenic for phi105c4 were grown to competence and exposed to specific bacterial DNAs. The time course of phage production was similar to that observed for mitomycin C induction of wild-type prophage. Induction was directly dependent upon DNA concentration up to levels which were saturating for the transformation of bacterial auxotrophic markers. The extent of induction varied with the source of DNA. The burst of phage induced by DNA isolated from a W23 strain of B. subtilis was fivefold less than that induced by DNA from B. subtilis 168 strains, while B. licheniformis DNA was completely inactive. This order of inducing activity was correlated with the ability of the respective DNAs to transform auxotrophic markers carried by one of the phi105c4 lysogens. Differences in inducing activity also were observed for different forms of phi105 DNA. The DNAs isolated from phi105 phage particles and phi105c4 lysogens were inactive, whereas DNA from cells lysogenized by wild-type phi105 induced a burst of phage. When tested for transforming activity, however, both phi105c4 and phi105 lysogen DNAs were equally effective. An induction mechanism which involves recombination at the prophage insertion site is proposed to explain these differences.
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Functional organization of a prophage of the temperate bacteriophage ZF40 of Erwinia carotovora subsp. carotovora which includes its immunity and inducibility as well as its effect on the host phenotype. It was established that the prophage ZF40 forms several different states in E. carotovora which are distinguished by the indices of spontaneous and lysogenic induction. In contrast to other prophages, including the lambdoid ones, the prophage ZF40 is capable to establish cytoplasmic overimmunity which protects the lysogenic system from superinfection by virulent mutants or other homoimmune bacteriophages. An increase of sensitivity of ZF40-lysogens to killing activity of colicino-like carotovoricin (CCTV) and destabilization of defective lysogeny, or resistant MCTV-prophages are related to the phenomenon of the phage lysogenic conversion of E. carotovora.
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ABSTRACT Eleven Bacillus isolates from the surface and subsurface waters of the Gulf of Mexico were examined for their capacity to sporulate and harbor prophages. Occurrence of sporulation in each isolate was assessed through decoyinine induction, and putative lysogens were identified by prophage induction by mitomycin C treatment. No obvious correlation between ability to sporulate and prophage induction was found. Four strains that contained inducible virus-like particles (VLPs) were shown to sporulate. Four strains did not produce spores upon induction by decoyinine but contained inducible VLPs. Two of the strains did not produce virus-like particles or sporulate significantly upon induction. Isolate B14905 had a high level of virus-like particle production and a high occurrence of sporulation and was further examined by genomic sequencing in an attempt to shed light on the relationship between sporulation and lysogeny. In silico analysis of the B14905 genome revealed four prophage-like regions, one of which was independently sequenced from a mitomycin C-induced lysate. Based on PCR and transmission electron microscopy (TEM) analysis of an induced phage lysate, one is a noninducible phage remnant, one may be a defective phage-like bacteriocin, and two were inducible prophages. One of the inducible phages contained four putative transcriptional regulators, one of which was a SinR-like regulator that may be involved in the regulation of host sporulation. Isolates that both possess the capacity to sporulate and contain temperate phage may be well adapted for survival in the oligotrophic ocean.
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Cocultures of Salmonella strains carrying or lacking specific prophages undergo swift composition changes as a result of phage-mediated killing of sensitive bacteria and lysogenic conversion of survivors. Thus, spontaneous prophage induction in a few lysogenic cells enhances the competitive fitness of the lysogen population as a whole, setting a selection regime that forces maintenance and spread of viral DNA. This is likely to account for the profusion of prophage sequences in bacterial genomes and may contribute to the evolutionary success of certain phylogenetic lineages.
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