Structural and functional analysis of YopR and identification of an additional key component of the SPβ phage lysis-lysogeny management system

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 of Bacillus subtilis 168 was recently shown to possess an arbitrium system defining its replication stage. Using an historic B. subtilis strain 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 the yopR gene, rendering the encoded YopRG136E protein 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 a B. subtilis strain carrying SPβ c2 identified YosL as a novel component of the lysis-lysogeny management system, as the presence of yosL is crucial for the induction of the lytic cycle of SPβ.