Abstract The mitochondrial gene coding for the large rRNA contains a self-splicing optional group-I intron (Sc-LSU · 1) in some Saccharomyces cerevisiae strains. Al- though the mechanisms of splicing have been extensively studied, little is known about the possible interactions of this intron with other mitochondrial molecules such as pro- teins. Using glycerol gradients, we have compared the sed- imentation coefficients of mitochondrial transcripts con- taining the Sc-LSU · 1 intron in native yeast extracts and in purified RNA preparations. By comparing extracts from ρ + and ρ – cells we have found that at least three RNA spe- cies containing the Sc-LSU · 1 intron (4.5 kb, 2.7 kb and 1.2 kb respectively) are associated in vivo with a multi- molecular complex of sedimentation coefficient 50S made up of nuclearly encoded proteins. Another RNA species of 2.7 kb, which may correspond to a cleavage at the dode- camer sequence of the intron, is not associated with the same particle. The possibility that the 50S particle corre- sponds to the mitochondrial ribosome or its precursor form(s) is discussed. Key words Yeast · Mitochondria · Group-I intron · Sedimentation coefficient Introduction In the yeast Saccharomyces cerevisiae the mitochondrial gene for the large rRNA subunit contains an optional group-I intron (Sc-LSU · 1, Dujon 1980). This intron con- tains an internal ORF that encodes a highly specific dou- ble-strand endonuclease, I-SceI (Jacquier and Dujon 1985; Colleaux et al. 1986), which determines intron homing, a DNA-mediated event, initiated by a double-strand break in the intron-less gene (Dujon et al. 1985; Zinn and Butow 1985; reviewed in Dujon 1989). The Sc-LSU · 1 intron RNA undergoes self-splicing in vitro (Van der Horst and Tabak 1985) by a mechanism common with that of the more extensively studied intron of Tetrahymena (Cech 1990). An interesting feature of this process is the destiny of the excised intron which is found in two forms: linear and cir- cularized (Tabak et al. 1984). In mitochondria, transcripts of different sizes contain- ing the Sc-LSU · 1 intron have been described (Bos et al. 1980; Zhu et al. 1987). A 4.5-kb transcript, containing both exonic and intronic sequences, must correspond to the pre- cursor RNA prior to splicing, while two transcripts of about 1.2 kb represent the excised intron. It has been shown that at least one transcript containing the Sc-LSU · 1 intron is processed at a dodecamer sequence located at the end of the I-SceI ORF (Osinga et al. 1984; Hofmann et al. 1993). In addition, an unexpected transcript of approximately 2.7 kb hybridizes with both the intron sequence and the up- stream exon sequence. It has been suggested by Zhu et al. (1987) that the 2.7-kb transcript could correspond to an RNA molecule processed at the dodecamer and at an un- determined position within exon 1. Because the integrity of the dodecamer sequence is required for intron homing, it has also been suggested that the 2.7-kb transcript could be the mRNA for the I-SceI endonuclease (Zhu et al. 1989). A precise molecular characterization of these transcripts has not been carried out but at least two of them (1.2 and 2.7 kb) do not correspond to the products of a classical RNA splicing reaction but rather to an alternative path of maturation. Little is known of the possible interactions of mitochon- drial transcripts containing group-I introns with other mac- romolecules in vivo. In Neurospora crassa, a soluble splic- ing activity has been found associated with mitochondrial ribonucleoparticles. This activity is responsible for the Curr Genet (1997) 32: 175–181 © Springer-Verlag 1997 Received: 28 April / 24 June 1997 Guy-Franck Richard · Bernard Dujon Association of transcripts from a group-I intron-containing gene with high sedimentation coefficient particles ORIGINAL PAPER G.-F. Richard 1 · B. Dujon () Unité de Génétique Moléculaire des Levures, Département des Biotechnologies (URA 1300 du CNRS and UFR927 Université P. M. Curie), Institut Pasteur, 25 rue du Docteur Roux, F-75724 Paris-Cedex 15, France Present address: 1 Rosenstiel Center and Department of Biology, Brandeis University, Waltham, Massachusetts 02254-9110, USA Communicated by K. Wolf