Homing sweet homing: mobile introns in bacterial viruses

Several bacteriophage group I introns are self-splicing at the RNA level, while they behave as mobile genetic elements at the DNA level. Contained within the catalytic RNAs are open reading frames which encode endonucleases that initiate the intron mobility reaction. This process, called homing, is a gene conversion event whereby an intron transfers from an intron-plus allele to the cognate intron-minus allele. Such mobile introns are phylogenetically diverse, being found in both prokaryotic and eukaryotic genomes. In this context, the accessibility of the phage systems and the unusual nature of the phage endonuclease:target site interactions make them a rewarding focus of study. We will discuss the mobility process and the hierarchy of relationships between the bacteriophage, its group I introns and the intron-encoded endonucleases.