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cDNA fragments corresponding to an apple (Malus domestica[LI Borkh) calmodulin-binding polypeptide have been isolated and characterized. l h e protein encoded by this messenger contains a serine/threonine protein kinase catalytic domain followed by a calcium/calmodulin-binding regulatory domain, both exhibiting significant sequence similarities to the corresponding regions of the mammalian calcium/calmodulin-dependent protein kinase II subunits. These results confirm a potential regulatory role for calmodulin in phosphorylation-mediated signal transduction events.

An apple cDNA encoding the precursor of ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) activase has been characterized. Using this cDNA as a probe, leaf-specific and light-regulated accumulation of corresponding transcripts was detected. Rubisco activase transcripts also turned out to accumulate at growing levels during apple leaf development, to reach a maximum in fully expanded leaves. In contrast, chlorophyll a/b-binding protein (Cab) and rubisco large subunit mRNA levels reach a maximum earlier in the course of leaf development. Moreover, the accumulation of rubisco activase messengers appeared to follow an oscillating circadian rhythm qualitatively similar to that observed for Cab mRNA levels.

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