Virulent bacterial viruses, also known as phages or bacteriophages, are considered as a potential option to fight antibiotic-resistant bacteria. However, their biology is still poorly understood, and only a fraction of phage genes is assigned with a function. To enable the first classification, we explored new options to test phage genes for their requirement on viral replication. As a model, we used the smallest known Bacillus subtilis phage Goe1, and the Cas9-based mutagenesis vector pRH030 as a genetic tool. All phage genes were specifically disrupted, and individual survival rates and mutant genotypes were investigated. Surviving phages relied on the genome integrity through host intrinsic non-homologues end joining system or a natural alteration of the Cas9 target sequence. Quantification of phage survivors and verifying the underlying genetic situation enables the classification of genes in essential or non-essential sets for viral replication. We also observed structural genes to hold more natural mutations than genes of the genome replication machinery.
Prophages need to tightly control their lifestyle to either be maintained within the host genome or enter the lytic cycle. The SPβ prophage present in the genome ofBacillus subtilis168 was recently shown to possess anarbitriumsystem defining its replication stage. Using an historicB. subtilisstrain harboring the heat-sensitive SPβ c2 mutant, we analyzed a key component of the lysis-lysogeny decision system called YopR, which is crucial for maintenance of lysogeny. Here, we demonstrate that the heat-sensitive SPβ c2 phenotype is due to a single nucleotide exchange in theyopRgene, rendering the encoded YopRG136Eprotein temperature sensitive. Structural characterization of YopR revealed that the protein is a DNA-binding protein with an overall fold like tyrosine recombinases. Biochemical and functional analyses indicate that YopR has lost the recombinase function and the G136E exchange impairs its higher order structure and DNA binding activity. We also show that the serine recombinase SprA and its accessory factor SprB are not required for the heat-dependent induction of the lytic cycle of the SPβ c2 prophage. Finally, an evolution experiment with aB. subtilisstrain carrying SPβ c2 identified YosL as a novel component of the lysis-lysogeny management system, as the presence ofyosLis crucial for the induction of the lytic cycle of SPβ.
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