Summary The activities of ribulose-1,5-bisphosphate carboxylase (RuBPCase 2 )), plastid and cytoplasmic leucyl- and valyl-tRNA synthetases have been compared during etioplast and chloroplast development in excised Cucurbita pepo cotyledons under the influence of benzylaminopurine action. Our results demonstrate that the activities of the plastid-associated enzymes are stimulated by the hormone in etiolated and greening cotyydons. Cytoplasmic leucyl-tRNA synthetase activity is not :specifically increased in light or upon hormone addition. The rate of RuBPCase stimulation in light or after cytokinin treatment differs from that of plastid leucyl-tRNA synthetase, especially in later stages of cotyledon development. There is a preponderance of cytokinin control over light control of chloroplast enzyme formation under our experimental conditions. It is suggested that benzylaminopurine specifically stimulates protein biosynthesis in plastids, in addition to a less promoted synthesis of proteins of cytoplasmic origin. The variation in hormone- stimulated activities observed for particular plastid enzymes may be due to different rates ,of formation at their sites of synthesis.
