Vector-mediated overexpression of catalase A in the yeast Saccharomyces cerevisiae induces inclusion body formation.

To study the morphological effects of overexpression of catalase A in yeast, the gene coding for catalase A was introduced into Saccharomyces cerevisiae on a multicopy vector. After induction of microbody biogenesis and catalase A expression by growth on oleic acid as sole carbon source, cells were analyzed by immunofluorescence and immunoelectron microscopy. In addition, overexpression of catalase A was studied by quantitative immunoblotting and by activity measurement. Quantitative immunoblotting resulted in a 16-fold difference between immunoreactive material from transformed and non-transformed cells. An 18-fold increase of enzyme activity was measured in transformed cells due to overexpression of catalase A from plasmid pAH521. Immunofluorescent staining of semithin sections of Lowicryl HM20-embedded cells with anti-catalase localized peroxisomes and--at a low percentage--larger particles. By immunoelectron microscopy, these larger structures could be identified as agranular, electron-dense aggregates which are morphologically clearly distinct from the cytoplasm and not bounded by a membrane. These structures, which have been named inclusion bodies, contain catalase A but not other peroxisomal enzymes like thiolase. These findings suggest that cells are capable of compensating for overproduced proteins by formation of particular types of structures.