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Conditional tolerance of temperate phages via transcription-dependent CRISPR-Cas targeting.

Goldberg GW, Jiang W, Bikard D, Marraffini LA - Nature (2014)

Bottom Line: Conditional tolerance is achieved through transcription-dependent DNA targeting, and ensures that targeting is resumed upon induction of the prophage lytic cycle.Our results provide evidence for the functional divergence of CRISPR-Cas systems and highlight the importance of targeting mechanism diversity.In addition, they extend the concept of 'tolerance to non-self' to the prokaryotic branch of adaptive immunity.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Bacteriology, The Rockefeller University, New York, New York 10065, USA.

ABSTRACT
A fundamental feature of immune systems is the ability to distinguish pathogenic from self and commensal elements, and to attack the former but tolerate the latter. Prokaryotic CRISPR-Cas immune systems defend against phage infection by using Cas nucleases and small RNA guides that specify one or more target sites for cleavage of the viral genome. Temperate phages include viruses that can integrate into the bacterial chromosome, and they can carry genes that provide a fitness advantage to the lysogenic host. However, CRISPR-Cas targeting that relies strictly on DNA sequence recognition provides indiscriminate immunity both to lytic and lysogenic infection by temperate phages-compromising the genetic stability of these potentially beneficial elements altogether. Here we show that the Staphylococcus epidermidis CRISPR-Cas system can prevent lytic infection but tolerate lysogenization by temperate phages. Conditional tolerance is achieved through transcription-dependent DNA targeting, and ensures that targeting is resumed upon induction of the prophage lytic cycle. Our results provide evidence for the functional divergence of CRISPR-Cas systems and highlight the importance of targeting mechanism diversity. In addition, they extend the concept of 'tolerance to non-self' to the prokaryotic branch of adaptive immunity.

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Type III-A CRISPR immunity can block lytic infection but tolerate lysogenizationa, Base pairing interaction between crRNA 32T and its target in the ΦNM1 genome (highlighted in gray). The crRNA tag is a sequence transcribed from the CRISPR repeat that needs to be unpaired with the flanking region of the target to license immunity. The target gene is transcribed from left to right. b, CRISPR immunity against ΦNM1 infection provided by spacers 32T and 32T* (similar to 32T but without mismatches), measured as a decrease in the number of plaque forming units (pfu) with respect to the non-targeting control pGG3 (C). c, Lysogenization with ΦNM1-ErmR in the presence of spacers 32T and 32T* or the pGG3 control (C), measured as the number of chloramphenicol- and erythromycin-resistant colony forming units (cfu) per ml obtained after infection. Control cells lysogenized with ΦNM1 (C’) lack the ermC insertion and do not yield erythromycin-resistant cfu. Error bars: mean ± s.d. (n=3).
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Figure 1: Type III-A CRISPR immunity can block lytic infection but tolerate lysogenizationa, Base pairing interaction between crRNA 32T and its target in the ΦNM1 genome (highlighted in gray). The crRNA tag is a sequence transcribed from the CRISPR repeat that needs to be unpaired with the flanking region of the target to license immunity. The target gene is transcribed from left to right. b, CRISPR immunity against ΦNM1 infection provided by spacers 32T and 32T* (similar to 32T but without mismatches), measured as a decrease in the number of plaque forming units (pfu) with respect to the non-targeting control pGG3 (C). c, Lysogenization with ΦNM1-ErmR in the presence of spacers 32T and 32T* or the pGG3 control (C), measured as the number of chloramphenicol- and erythromycin-resistant colony forming units (cfu) per ml obtained after infection. Control cells lysogenized with ΦNM1 (C’) lack the ermC insertion and do not yield erythromycin-resistant cfu. Error bars: mean ± s.d. (n=3).

Mentions: In order to investigate the behavior of type III CRISPR immunity during temperate phage infection, we introduced pGG3, a plasmid carrying the type III-A CRISPR-Cas system of Staphylococcus epidermidis RP62a (ref. 17), into Staphylococcus aureus RN4220 (ref. 18). This strain is sensitive to the lambda-like temperate phages of S. aureus Newman, a clinical isolate harboring four heteroimmune prophages (ΦNM1-4) which carry genes that enhance the pathogenicity of their host19. We also identified a spacer in one of the CRISPR loci of S. aureus MSHR1132 (ref. 20) with near-perfect identity to a conserved target sequence present in ΦNM1 (Fig. 1a), ΦNM2, and ΦNM4. This spacer, referred to as 32T (Supplementary Table 1), was added to the CRISPR locus of pGG3. Using ΦNM1, we first established that this spacer prevents lytic infection by showing that plaquing efficiency is reduced approximately seven orders of magnitude when compared to a strain carrying the pGG3 plasmid without the ΦNM1-targeting spacer (Fig. 1b). We then introduced an erythromycin resistance gene (ermC) into ΦNM1 to facilitate quantification of lysogens which have stably integrated a chromosomal prophage (creating ΦNM1-ErmR). Using this system, we expected to find results consistent with a report describing CRISPR-mediated immunity to lysogenization by phage lambda in E. coli6. Surprisingly, we obtained the same efficiency of lysogenization compared to the control strain lacking spacer 32T (Fig. 1c). To test whether the presence of mismatches between the 32T crRNA and its target was influencing this phenomenon, we engineered spacer 32T* with a perfect match to its target, but obtained the same results (Fig. 1b and c). We next sought to determine whether genetic CRISPR-Cas inactivation is responsible for the apparent tolerance of these lysogens by testing them for sensitivity to ΦNM2. All 14 clones maintained resistance to ΦNM2 mediated by spacer 32T (Extended Data Fig. 1a, d). Finally, we demonstrated that spacer 32T tolerance does not result from genetic alteration of the target phage (Extended Data Fig. 1b, c). Tolerance was also observed for ΦNM4 (Extended Data Fig. 2), demonstrating that the tolerance phenomenon is not specific for the ΦNM1-ErmR phage or its integration locus. These results demonstrate that type III-A CRISPR immunity can block lytic infection but tolerate lysogenization without concomitant genetic CRISPR-Cas inactivation or alteration of the phage genome.


Conditional tolerance of temperate phages via transcription-dependent CRISPR-Cas targeting.

Goldberg GW, Jiang W, Bikard D, Marraffini LA - Nature (2014)

Type III-A CRISPR immunity can block lytic infection but tolerate lysogenizationa, Base pairing interaction between crRNA 32T and its target in the ΦNM1 genome (highlighted in gray). The crRNA tag is a sequence transcribed from the CRISPR repeat that needs to be unpaired with the flanking region of the target to license immunity. The target gene is transcribed from left to right. b, CRISPR immunity against ΦNM1 infection provided by spacers 32T and 32T* (similar to 32T but without mismatches), measured as a decrease in the number of plaque forming units (pfu) with respect to the non-targeting control pGG3 (C). c, Lysogenization with ΦNM1-ErmR in the presence of spacers 32T and 32T* or the pGG3 control (C), measured as the number of chloramphenicol- and erythromycin-resistant colony forming units (cfu) per ml obtained after infection. Control cells lysogenized with ΦNM1 (C’) lack the ermC insertion and do not yield erythromycin-resistant cfu. Error bars: mean ± s.d. (n=3).
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Related In: Results  -  Collection

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Figure 1: Type III-A CRISPR immunity can block lytic infection but tolerate lysogenizationa, Base pairing interaction between crRNA 32T and its target in the ΦNM1 genome (highlighted in gray). The crRNA tag is a sequence transcribed from the CRISPR repeat that needs to be unpaired with the flanking region of the target to license immunity. The target gene is transcribed from left to right. b, CRISPR immunity against ΦNM1 infection provided by spacers 32T and 32T* (similar to 32T but without mismatches), measured as a decrease in the number of plaque forming units (pfu) with respect to the non-targeting control pGG3 (C). c, Lysogenization with ΦNM1-ErmR in the presence of spacers 32T and 32T* or the pGG3 control (C), measured as the number of chloramphenicol- and erythromycin-resistant colony forming units (cfu) per ml obtained after infection. Control cells lysogenized with ΦNM1 (C’) lack the ermC insertion and do not yield erythromycin-resistant cfu. Error bars: mean ± s.d. (n=3).
Mentions: In order to investigate the behavior of type III CRISPR immunity during temperate phage infection, we introduced pGG3, a plasmid carrying the type III-A CRISPR-Cas system of Staphylococcus epidermidis RP62a (ref. 17), into Staphylococcus aureus RN4220 (ref. 18). This strain is sensitive to the lambda-like temperate phages of S. aureus Newman, a clinical isolate harboring four heteroimmune prophages (ΦNM1-4) which carry genes that enhance the pathogenicity of their host19. We also identified a spacer in one of the CRISPR loci of S. aureus MSHR1132 (ref. 20) with near-perfect identity to a conserved target sequence present in ΦNM1 (Fig. 1a), ΦNM2, and ΦNM4. This spacer, referred to as 32T (Supplementary Table 1), was added to the CRISPR locus of pGG3. Using ΦNM1, we first established that this spacer prevents lytic infection by showing that plaquing efficiency is reduced approximately seven orders of magnitude when compared to a strain carrying the pGG3 plasmid without the ΦNM1-targeting spacer (Fig. 1b). We then introduced an erythromycin resistance gene (ermC) into ΦNM1 to facilitate quantification of lysogens which have stably integrated a chromosomal prophage (creating ΦNM1-ErmR). Using this system, we expected to find results consistent with a report describing CRISPR-mediated immunity to lysogenization by phage lambda in E. coli6. Surprisingly, we obtained the same efficiency of lysogenization compared to the control strain lacking spacer 32T (Fig. 1c). To test whether the presence of mismatches between the 32T crRNA and its target was influencing this phenomenon, we engineered spacer 32T* with a perfect match to its target, but obtained the same results (Fig. 1b and c). We next sought to determine whether genetic CRISPR-Cas inactivation is responsible for the apparent tolerance of these lysogens by testing them for sensitivity to ΦNM2. All 14 clones maintained resistance to ΦNM2 mediated by spacer 32T (Extended Data Fig. 1a, d). Finally, we demonstrated that spacer 32T tolerance does not result from genetic alteration of the target phage (Extended Data Fig. 1b, c). Tolerance was also observed for ΦNM4 (Extended Data Fig. 2), demonstrating that the tolerance phenomenon is not specific for the ΦNM1-ErmR phage or its integration locus. These results demonstrate that type III-A CRISPR immunity can block lytic infection but tolerate lysogenization without concomitant genetic CRISPR-Cas inactivation or alteration of the phage genome.

Bottom Line: Conditional tolerance is achieved through transcription-dependent DNA targeting, and ensures that targeting is resumed upon induction of the prophage lytic cycle.Our results provide evidence for the functional divergence of CRISPR-Cas systems and highlight the importance of targeting mechanism diversity.In addition, they extend the concept of 'tolerance to non-self' to the prokaryotic branch of adaptive immunity.

View Article: PubMed Central - PubMed

Affiliation: Laboratory of Bacteriology, The Rockefeller University, New York, New York 10065, USA.

ABSTRACT
A fundamental feature of immune systems is the ability to distinguish pathogenic from self and commensal elements, and to attack the former but tolerate the latter. Prokaryotic CRISPR-Cas immune systems defend against phage infection by using Cas nucleases and small RNA guides that specify one or more target sites for cleavage of the viral genome. Temperate phages include viruses that can integrate into the bacterial chromosome, and they can carry genes that provide a fitness advantage to the lysogenic host. However, CRISPR-Cas targeting that relies strictly on DNA sequence recognition provides indiscriminate immunity both to lytic and lysogenic infection by temperate phages-compromising the genetic stability of these potentially beneficial elements altogether. Here we show that the Staphylococcus epidermidis CRISPR-Cas system can prevent lytic infection but tolerate lysogenization by temperate phages. Conditional tolerance is achieved through transcription-dependent DNA targeting, and ensures that targeting is resumed upon induction of the prophage lytic cycle. Our results provide evidence for the functional divergence of CRISPR-Cas systems and highlight the importance of targeting mechanism diversity. In addition, they extend the concept of 'tolerance to non-self' to the prokaryotic branch of adaptive immunity.

Show MeSH
Related in: MedlinePlus