Yeast phosphorylase is phosphorylated and activated by a cyclic AMP‐independent protein kinase (called phosphorylase kinase) and a cyclic AMP‐dependent protein kinase. Only in the presence of both kinases is phosphorylase fully activated and phosphorylated. No evidence was found for the presence of two phosphorylation sites as an identical phosphopeptide pattern of phosphorylase is obtained after phosphorylation by either one or both kinases. The kinases probably phosphorylate identical sites but recognize different subunits of phosphorylase. Phosphorylase kinase phosphorylates the high‐M r subunit while cAMP‐dependent protein kinase phosphorylates the low‐M r subunit.

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Phosphorylase was purified from young and senescent potato tubers. Antibodies raised against the enzyme from young tubers crossreacted with phosphorylase from old tissue, although the latter exhibited different physico-chemical properties. In polyacrylamide gel electrophoresis it migrated with higher mobility, its subunit molecular weight was determined in the range of 40,000 in contrast to 100,000 of the phosphorylase in young tubers. The enzyme of senescent tubers displayed an isoelectric point of 5.4 different from the one of young tubers with 5.0, and the diffusion coefficients of the two enzymes varied. The appearance of the phosphorylase form typical for senescent tissue is connected with changes in the intracellular localization as revealed by immunofluorescence. Before massive starch accumulation is initiated, non-vacuolated subepidermal cells contain antigenically active material in their cytoplasm. During starch accumulation in fully differentiated storage parenchyma, only amyloplasts fluoresce, indicating the presence of adsorbed phosphorylase protein. Cytoplasmic phosphorylase can be detected in the continuance of senescence and, finally, after 16 months of tuber storage, the particle-bound enzyme had mostly disappeared. Simultaneously, we observed membrane destruction and decomposition on the ultrastructural level. The phosphorylase from senescent potatoes is a converted molecule and seems to be formed by proteolytic cleavage. The location of phosphorylase in the amyloplasts during starch synthesis indicates that it also plays a role in starch synthesis and not only in its degradation.

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