Pervasive transcription of the mitochondrial genome in Candida albicans is revealed in mutants lacking the mtEXO RNase complex

The mitochondrial genome of the pathogenic yeast Candida albicans displays a typical organization of several (eight) primary transcription units separated by noncoding regions. Presence of genes encoding Complex I subunits and the stability of its mtDNA sequence make it an attractive model to study organellar genome expression using transcriptomic approaches. The main activity responsible for RNA degradation in mitochondria is a two-component complex (mtEXO) consisting of a 3'-5' exoribonuclease, in yeasts encoded by the DSS1 gene, and a conserved Suv3p helicase. In C. albicans, deletion of either DSS1 or SUV3 gene results in multiple defects in mitochondrial genome expression leading to the loss of respiratory competence. Transcriptomic analysis reveals pervasive transcription in mutants lacking the mtEXO activity, with evidence of the entire genome being transcribed, whereas in wild-type strains no RNAs corresponding to a significant fraction of the noncoding genome can be detected. Antisense ('mirror') transcripts, absent from normal mitochondria are also prominent in the mutants. The expression of multiple mature transcripts, particularly those translated from bicistronic mRNAs, as well as those that contain introns is affected in the mutants, resulting in a decreased level of proteins and reduced respiratory complex activity. The phenotype is most severe in the case of Complex IV, where a decrease of mature COX1 mRNA level to similar to 5% results in a complete loss of activity. These results show that RNA degradation by mtEXO is essential for shaping the mitochondrial transcriptome and is required to maintain the functional demarcation between transcription units and non-coding genome segments.