Functionally distinct nucleic acid binding sites for a group I intron encoded RNA maturase/DNA homing endonuclease

A large number of group I introns encode a family of homologous proteins that either promote intron splicing (maturases) or are site-specific DNA endonucleases that function in intron mobility (a process called homing). Genetic studies have shown that some of these proteins have both activities, yet how a single protein carries out both functions remains obscure. The similarity between respective DNA-binding sites and the RNA structure. near the 5' and 3' splice sites has fueled speculation that, such proteins may use analogous interactions to perform both functions.,, The Aspergillus nidulans mitochondrial COB group I intron encodes a bifunctional protein, I-Ani I, that has both RNA maturaso and site-specific DNA endonuclease activities in vitro. Here, we show that I-Ani I shows distinctive features of the endonuclease family to which it belongs, including highly specific, tight binding and sequential DNA strand cleavage. Competition experiments demonstrate that I-Ani I binds the COB intron RNA even in saturating concentrations of its DNA target site substrate, suggesting that the protein has a separate binding site for RNA. hi addition, we provide evidence that two different DNA-binding site mutants of I-Ani I have little effect on the protein's RNA maturation activity. Since RNA splicing is likely a secondary adaptation of the protein, these observations support a model in which homing endonucleases may have developed maturase function by utilizing a hitherto non-functional protein surface. (C) 2003 Elsevier Science Ltd. All rights reserved.