Intronic GIY-YIG endonuclease gene in the mitochondrial genome of Podospora curvicolla: evidence for mobility

Endonuclease genes encoded in invasive introns are themselves supposed to b e mobile elements which, during evolution, have colonized pre-existing intr ons converting them into invasive elements, This hypothesis is supported by numerous data concerning the LAGLI-DADG subclass of intronic endonucleases . Less is known about the GIY-YIG ORFs which constitute another family of e ndonucleases. In this paper we describe the presence of one optional GIY-YI G ORF in the second intron of the mitochondrial cytochrome b gene in the fu ngus Podospora curvicolla, We show that this GIY-YIG ORF is efficiently tra nsferred from an ORF-containing intron to an ORF-less allele. We also show that the products of both the GIY-YIG ORF and the noncanonical LAGLI-DADG-G IY-YIG ORF, which is generated by its integration, have endonuclease activi ties which recognize and cut the insertion site of the optional sequence. T his constitutes the first direct evidence for potential mobility of an intr onic GIY-YIG endonuclease, We discuss the role that such a mobile sequence could have played during evolution.