Protein-dependent splicing of a group I intron in ribonucleoprotein particles and soluble fractions.

The group I intron in the Neurospora mitochondrial large rRNA gene is not self-splicing in vitro. Here, we show that this intron can be spliced from 35S pre-rRNA in RNPs or from deproteinized 35S pre-rRNA or in vitro transcripts by a soluble activity that is present in mitochondrial lysates and can be released from RNPs. Splicing occurs by the same guanosine-initiated transesterification mechanism characteristic of self-splicing group I introns, but is absolutely dependent upon proteins that are presumably required for correct folding of the pre-rRNA. The soluble splicing activity is not simply associated with large subunit ribosomal proteins. Nuclear mutant cyt18-1, which is defective in splicing a number of group I introns in vivo, is grossly deficient in the soluble splicing activity. Our results suggest that the cyt18 gene encodes or regulates a component of an activity that functions in splicing group I introns in Neurospora mitochondria.