Curr Genet (1993)24:122-125 Current Genetics © Springer-Verlag 1993 Symmetrical transcription in the tRNA region of the mitochondrial genome of Saccharomyces cerevisiae P. Grisanti, S. Francisci, P. Tataseo, C. Palleschi Department of Developmental and Ceil Biology,Universityof Rome, "La Sapienza", Piazza Aldo Moro 5, 1-00185 Rome, Italy Received: 15 August 1992 / Accepted: 25 January, 1992 Abstract. The occurrence of discrete transcripts originat- ing from the non-coding strand of the yeast mitochondri- al genome is described. The region under investigation is localized in the large tRNA gene cluster between the LSU ribosomal RNA and OXI 1 genes. The transcripts origi- nating from the non-coding strand were detected in a wild-type strain and in a rho- mutant. Their size range includes transcripts of about 2000 nucleotides able to accommodate more than one "anti-tRNA". In some cases their extremities can be mapped near highly-con- served nonanucleotides that could function as origins of transcription. The involvement of the tRNA-processing machinery in the cleavage of these transcripts is also hy- pothesized. Key words: Symmetrical transcription - Mitochondria - S. cerevisiae Introduction The transcription of both strands of a specific DNA seg- ment is not a rare event among prokaryotes. The tran- scripts originating from the non-coding strand are usual- ly referred to as "antisense RNA". In most cases the antisense RNAs are involved in the repression (or nega- tive regulation) of various cellular functions by mecha- nisms in which the crucial step is the annealing of an antisense-RNA with its respective sense-RNA. Several examples have been extensively studied and reviewed, e.g., the replication and incompatibility of the FI plasmid group are controlled via antisense-RNA and the same mechanism is involved in the regulated expression of some bacterial and viral genes (Green et al. 1986; Eguchi et al. 1991). Some results have been published concerning the occurrence of symmetrical transcription among eu- karyotes: the GART locus of Drosophila melanogaster (Henikoff etal. 1986), the murine GnRH-SH locus Correspondence to: C. Palleschi (Adelman et al. 1987) and the Xenopus locus coding for the fibroblast growth factor (Volk et al. 1989) represent striking examples. The mammalian mitochondrial (rot) genome has its genetic information distribution along both strands of the organellar DNA: the majority of the mitochondrial genes are coded for by the heavy strand while some tRNAs are coded for by the light strand (Anderson et al. 1981). A detailed analysis of the symmetrical transcrip- tion in the D-loop region of rat mtDNA has been per- formed (Sbisfi et al. 1990). As far as the yeast mitochondrial genome is con- cerned, indications about the possible occurrence of sym- metrical transcripts have been reported occasionally but no detailed study has been carried out. The first hint of the occurrence of symmetrical transcription in yeast mi- tochondria came from the report that the hybridization of labelled DNA with homologous RNA at saturation levels could be competed out to some extent by auto- annealing the mtRNA (Jakovicic et al. 1979). An electron microscope analysis revealed that a certain fraction of the mtRNA was represented by double-stranded linear molecules resistant to DNase I but sensitive to RNase III (Beilhartz et al. 1982). A discrete transcript of 1500 nucle- otides (nt) was observed in the OXI1 gene region by hy- bridization with a single-stranded "non-coding" DNA probe (Coruzzi et al. 1981). To study the occurrence of symmetrical transcription in the yeast mitochondrial genome, we have analyzed the DNA region containing the major tRNA cluster whose transcription has been previously well characterized (Francisci et al. 1987; Frontali et al. 1982). The data re- ported here demonstrate the presence of discrete tran- scripts from the non-coding strand of this mtDNA region in a wild-type strain and in a mitochondrial deletion mu- tant retaining only a portion of the tRNA gene cluster. Materials and methods Strains and growth conditions. The wild-typestrain Saccharornyces cerevisiae D237-10B and its rho-derivative DS200/A5 (kindly pro-