Abstract The intronic mat-r ORF encodes a protein with significant homology to retroviral reverse transcriptases. Here, we describe the nucleotide sequence of potato mat-r and study the editing status of mat-r transcripts in two systems, potato and wheat, where the mat-r ORF is part of the trans-introns but in two different configurations relative to nad1 exons d and e. In potato and wheat, 13 and 15 C-to-U transitions respectively were observed. Most transcripts were partially edited, but potato transcripts were edited more efficiently than wheat transcripts. As in functional mitochondrial genes, RNA editing increased the similarity between plant mat-r proteins and their ho- mologous non-plant counterparts. Interestingly, editing of mat-r was clustered in the reverse-transcriptase (RT) and the maturase (X) domains, two well defined regions hav- ing known functions in other systems. These results, to- gether with the integrity and sequence conservation of mat-r, strongly suggest that the encoded protein plays a functional role in plant mitochondria. Key words RNA editing · Plant mitochondria · Maturase · Reverse transcriptase Introduction Sequences similar to the reverse transcriptase (RT) of ret- roviruses are widely distributed among organisms and have been found in mitochondrial group-II introns, certain DNA viruses and a wide variety of transposable elements (Xiong and Eickbush 1988). The group-II intron-encoded RT-like proteins may function in intron mobility and RNA splic- ing. Thus, the proteins encoded by yeast cox1 introns 1 and 2 and a bacterial intron have been shown to function as maturases in splicing the intron in which they are en- coded, and to have an RT activity that plays a role in in- tron mobility (Carignani et al. 1983; Matsuura et al. 1997 and references therein). In plants, RT-like sequences are also found in mitochondrial group-II introns and are re- lated to the fungal maturases, being designated as matu- rase-related (mat-r) ORFs. There are nine different mat-r ORFs in mitochondria of the bryophyte Marchantia polymorpha (Oda et al. 1992) but only one such ORF in higher-plant mitochondria, lo- cated between nad1 exons d and e (Wahleithner et al. 1990; Chapdelaine and Bonen 1991; Conklin et al. 1991; Wis- singer et al. 1991; Thomson et al. 1994). Interestingly, nad1 exons are scattered in higher-plant mitochondria, being located in three or four different genomic regions. This organization probably originated from recombination events (Londsdale 1984); consequently, maturation of nad1 transcripts is a complex process involving both cis- and trans-splicing events. The organization of mat-r and nad1 exons d and e differs among higher plants. In Oenoth- era, broad bean and soybean, mat-r is present in the cis- intron linking exons d and e. In wheat, maize and petunia, mat-r is found in the trans-introns. In the latter species, exons d and e are not covalently linked and the intron is removed by a trans-splicing event. In fact, mat-r can be found in two different configurations: being physically linked to exon e in monocots and to exon d in petunia. In addition to splicing, the coding sequences of mito- chondrial mRNAs are modified by RNA editing in plants. This process occurs essentially by C-to-U conversion Curr Genet (1998) 33: 420–428 © Springer-Verlag 1998 Received: 15 February / 25 March 1998 Dominique Bégu · Ana Mercado · Jean-Claude Farré Alejandra Moenne · Loreto Holuigue Alejandro Araya · Xavier Jordana Editing status of mat-r transcripts in mitochondria from two plant species: C-to-U changes occur in putative functional RT and maturase domains D. Bégu · J. C. Farré · A. Araya () Laboratoire de Réplication et Expression des Gènes Eucaryotes et Rétroviraux, EP-630, C.N.R.S.-Université de Bordeaux II, 1, rue Camille Saint-Saëns, F-33077 Bordeaux Cedex, France e-mail: Alexandre.Araya@ibgc.u-bordeaux2.fr Tel.: 33-56-99 90 30 Fax: 33-56-99 90 57 A. Mercado · L. Holuigue · X. Jordana Departamento de Genética Molecular y Microbiologia, Facultad de Ciencias Biológicas, P. Universidad Católica de Chile, Casilla 114-D, Santiago, Chile A. Moenne Departamento de Ciencias Biológicas, Facultad de Química y Biología, Universidad de Santiago, Casilla, 307-Correo 2, Santiago, Chile D. Bégu and A. Mercado contributed equally to this work Communicated by P. P. Slonimski ORIGINAL PAPER