We have previously reported the finding of a yeast homologue, MRS6, of the choroideraemia gene and its function as component A of the yeast RabGGTase. The MRS6 gene is essential and its overexpression suppresses respiratory defect of a mrs2-1/wo mutant as well as phenotypic defects of a ypt1-ts mutant and an ira1 mutant giving a surprisingly wide spectrum of functions in yeast cells (e.g. mitochondria biogenesis, Ras-mediated signaling, secretion pathway). In order to clarify the function of the Mrs6p in mitochondrial biogenesis we have undertaken a morphological and functional study on the effects of Mrs6p depletion on mitochondrial development. Our preliminary studies showed that mitochondria from cells subjected to regulated depletion of the Mrs6p in a fermentable carbon source appear under the electromicroscope as globular entities with markedly developed cristae. Analysis of these cells showed typical profile of cytochrome spectra, indicating that cytochrome complex assembly was normal in these mitochondria. In addition, oxygen uptake was found to be normal with respect to the control cells. To further investigate the origins of the morphological alterations of mitochondria in Mrs6p-depleted cells, the appearance of mitochondria in well characterized secretion mutants blocked at different stages of the secretion pathway was studied.