Recruitment of intron-encoded and co-opted proteins in splicing of the bI3 group I intron RNA

Detectable splicing by the Saccharomyces cerevisiae mitochondrial b13 group I intron RNA in vitro is shown to require both an intron-encoded protein, the b13 maturase, and the nuclear-encoded protein, Mrs1. Both proteins bind independently to the b13 RNA. The b13 maturase binds as a monomer, whereas Mrs1 is a dinner in solution that assembles as two dimers, cooperatively, on the RNA. The active six-subunit complex has a molecular mass of 420 kDa, splices with a k(cat) of 0.3 min(-1), and binds the guanosine nucleophile with an affinity comparable to other group I introns. The functional b13 maturase domain is translated from within the RNA that encodes the intron, has evolved a high-affinity RNA-binding activity, and is a member of the LAGLIDADG family of DNA endonucleases, but appears to have lost DNA cleavage activity. Mrs1 is a divergent member of the RNase H fold superfamily of dimeric DNA junction-resolving enzymes that also appears to have lost its nuclease activity and now functions as a tetramer in RNA binding. Thus, the b13 ribonucleoprotein is the product of a process in which a once-catalytically active RNA now obligatorily requires two facilitating protein cofactors, both of which are compromised in their original functions.