Effect of the Prion [ PSI +] on Expression of COX5b Gene Encoding Subunit of Cytochrome Oxidase in Saccharomyces cerevisiae.
Agnieszka Jankowska (1), Isabelle Hatin (2), Olivier Namy (2), Jean-Pierre Rousset (2), Magdalena Boguta (1)
(1) Institute of Biochemistry and Biophysics, PAS, Pawinskiego 5a, 02-106 Warsaw, Poland; (2) Institute of Genetics and Microbiology, University Paris-Sud, 91405 Orsay, France
The yeast COX5a and COX5b nuclear genes encode two forms of Cox5 subunit of the mitochondrial cytochrome c oxidase. Both genes are inversely regulated by oxygen: transcription of COX5a is stimulated while transcription of COX5b gene in aerobic conditions is repressed. Repressor of COX5b is encoded by the IXR1 gene which contains a readthrough motif surrounding the termination codon followed by a 3' open reading frame. Readthrough of the IXR1 termination codon was measured in [ PSI + ] and [ psi - ] strains using the lacZ-luc dual gene reporter system. [ PSI + ] caused an 11-fold increase in IXR1 readthrough indicating a potential role of an elongated Ixr1 protein with altered activity. Consequently, expression of the COX5b gene in aerobic conditions should depend on [ PSI + ]. To assess this, we compared COX5b -dependent phenotypes in isogenic [ PSI + ] and [ psi - ] strains. In both strains the COX5a gene was deleted and COX5b dosage was increased by introducing a plasmid copy. That resulted in respiratory competence in [ psi - ], but not in [ PSI + ] cells. In [ PSI + ], however, the COX5b -dependent respiratory deficiency could be overcome by curing of [ PSI ] with guanidine hydrochloride or overproduction of Hsp104 chaperone. Finally, we have shown decreased levels of COX5b transcript in [ PSI + ] strains indicating stronger repression than in [ psi - ] strains. Our results suggest a novel mechanism of COX5b regulation mediated by [ PSI + ] which might be important under conditions of variable oxygen tension.