All group I intron maturases contain two copies of a conserved amino acid motif designated LAGLIDADG. Over 200 other proteins also contain one or two copies of this motif and most of these proteins are site-specific DNA endonucleases that function in genetic mobility (a process called "homing": Dujon 1989; Chevalier and Stoddard 2001). Genes coding for these proteins are found in self-splicing polypeptide inteins, and group I and II introns as well as free standing ORFs (Chevalier and Stoddard 2001; Toor and Zimmer-ly 2002). Group I introns that code for LAGLIDADG endonucleases are mobile; they spread through populations by a gene conversion process that cop ies them to cognate, intron-less alleles. Insertion of a group I intron/endonu-clease gene into a specific site is initiated by a double-stranded cut in the recipient allele catalyzed by the homing endonuclease. Cleavage stimulates the double-strand break repair response that uses the donor allele as a template for DNA synthesis and ultimately results in a copy of the intron inserted into the recipient allele. Members of the LAGLIDADG endonuclease family recognize long target sites (>18 bp) and make base-specific contacts in the major groove (Chevalier et al., this Vol.).
Many observations suggest that maturase function arose from homing en-donucleases that had invaded pre-existing group I introns. First, most homing endonucleases are not required for intron splicing. Second, four of the eight confirmed RNA maturases are also homing endonucleases (Table 1). In addition, the S. cerevisiae bI2 maturase can gain homing endonuclease activity by mutation of just two non-adjacent amino acid residues (Szczepanek and La-zowska 1996). Third, the finding that identical or closely related introns contain different homing endonuclease genes supports the hypothesis that the proteins are ancestral elements that colonized group I introns. Given these observations, it seems likely that RNA maturase activity is a secondary adaptation of LAGLIDADG homing endonucleases (for further discussion, see Lam-bowitz and Belfort 1993).
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