Cell refractive index has been used to monitor peroxisome behaviour in the yeast by means of flow cytometry. Peroxisomes are inducible organelles which may occupy a large fraction of the cell volume when yeast cells are growing in methanol media. These organelles harbour a catalase that decomposes the hydrogen peroxide produced in methanol oxidation by alcohol oxidase, a peroxisomal enzyme whose subunits are arranged to form a regular crystalloid. Peroxisomes undergo a degradation process mediated by vacuoles whenever they and their enzymes become metabolically redundant (e.g. during growth on glucose). Flow cytometric analyses of side scattered light (depending on cell volume, morphology and structure) and fluorescein isothiocyanate retention (due to the vacuole) were made on two wild-type strains of during exponential growth in glucose and methanol media and during nutritional shifts from one carbon source to the other. The same parameters were also analysed for a mutant strain only partially repressed by glucose. We show that both the parameters are substrate-dependent and appear to reflect peroxisome development in the cells. The data reported correlate well with the known cytological and biochemical data, showing the possibility of using flow cytometry, a fast and sensitive technique, to analyse the dynamics of peroxisome proliferation and degradation in response to environmental as well as genetic factors.


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