A quantitative assay is described for the measurement of cytochrome in intact cells of . The procedure involves flash photolysis of the CO-liganded, reduced enzyme in the absence of O at temperatures (approx. −100°C) at which the rate of recombination of CO is immeasurably slow. Other CO-binding pigments known to be present, particularly cytochrome , are excluded from the photodissociation spectrum under these conditions. Measurement of the content of cytochrome in bacteria separated into size (and thus age) classes by zonal centrifugation shows that the cytochrome accumulates continuously, probably exponentially, throughout the cell cycle and thus constitutes a constant proportion of cell protein during the cycle. The velocity of recombination of CO with cytochrome at −65°C is invariant over the cell cycle.


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