Printed In Great Britain The Development of Cytochromes during the Cell Cycle of a Glucose - Repressed Fission Yeast , Schizosaccharomyces pombe 972 h -

1. Spectrophotometric analysis of intact cells of Schizosaccharomyces pombe, harvested from exponentially growing cultures during the phase of glucose repression, revealed thepresence ofcytochromesa+a3, cand at least two species of cytochrome b. 2. An absorption maximum at 554nm at 77°K, previously attributed to cytochrome cl, has been identified as a b-type cytochrome. 3. CO-difference spectra reveal the presence of cytochromes P-420 and P-450 in addition to cytochrome a3. 4. The cell cycle was analysed by separation of cells into classes representing successive stages in the cell cycle by isopycnic zonal centrifugation. 5. Cytochromes C548, b554 and b560 each exhibited a single broad maximum of synthesis during the cell cycle. 6. Amounts of cytochromes a+a3 and b563 (tentatively identified as cytochrome bT by its reaction on pulsing anaerobic cell suspensions with 02) oscillated in phase, and showed two maxima during the cycle; the second maximum of cytochromes a+a3 was coincident with a maximum of activity ofenzymically active cytochrome c oxidase. 7. The amount ofcytochromeP-420 decreased during the first three-quarters ofthe cell-cycle, whereas that ofcytochromeP-450 increased during this period. 8. The discrepancy between spectrophotometric and enzymic assay of cytochrome c oxidase, the changing ratio of cytochrome a3/cytochrome a and the relationship between changes in cellular content of cytochromes and previous observations on respiratory oscillations during the cell cycle are discussed.