Group I intron homing in Bacillus phages SPO1 and SP82:: a gene conversion event initiated by a nicking homing endonuclease

Many group I introns encode endonucleases that promote intron homing by initiating a double-stranded break-mediated homologous recombination event. In this work we describe intron homing in Bacillus subtilis phages SPO1 and SP82. The introns encode the DNA endonucleases I-HmuI and I-HmuII, respectively, which belong to the H-N-H endonuclease family and possess nicking activity in vitro. Coinfections of B. subtilis with intron-minus and intron-plus phages indicate that I-HmuI and I-HmuII are required for homing of the SPO1 and SP82 introns, respectively. The homing process is a gene conversion event that does not require the major B. subtilis recombination pathways, suggesting that the necessary functions are provided by phage-encoded factors. Our results provide the first examples of H-N-H endonuclease-mediated intron homing and the first demonstration of intron homing initiated by a nicking endonuclease.