What is the basis of autosplicing ?

Group I Introns Undertake Self-Splicing by Transesterification Splicing occurs by two transesterification reactions, without requiring input of energy. The only factors required for autosplicing (or self-splicing) in vitro by group I introns are two metal ions and a guanosine nucleotide. Self-splicing occurs by transesterification reactions in which bonds are exchanged directly. The excised intron can form circles.