AbstractSinceEuglena gracilisKlebs var.bacillarisPringsheim contains a species of DNA unique to the chloroplast, an important question concerns the extent to which light unblocks the reading of the organelle's template to provide the informational RNA's necessary to construct the plastid proteins. Experiments with32Pilabeling of chloroplast and nonchloroplast RNA's during light‐induced chloroplast development show that both the RNA of the chloroplast and of the rest of the cell become labeled during this process, with the chloroplast RNA's displaying the higher specific activity. The fact that chloroplast RNA is not uniquely labeled indicates that process other than a simple reading of the chloroplast DNA are involved. If we are to preserve the concept of a reasonable degree of chloroplast informational autonomy, we may assume, from this and other data, that the light induction of chloroplast development involves not only the unblocking of chloroplast DNA to make information available, but also a concomitant unblocking of other sites of informational RNA synthesis (e.g., nuclear and mitochondrial DNA's). Such sites external to the developing chloroplast may be concerned with making available the building blocks and energy necessary for the synthesis of chloroplast constituents coded for by the chloroplast DNA. This model leads to the prediction that photosynthesis could be gratuitous for chloroplast development if these nonchloroplast sites were providing most of the building blocks and energy. Experiments are reported which show that chloroplast formation and the acquisition of photosynthetic competence can be achieved under conditions where photosynthesis is completely inhibited for the entire span of development by using the highly selective inhibitor 3, (3,4‐dichlorophenyl) 1, 1‐dimethyl urea (DCMU), in agreement with the proposed model. The fact that more than just the chloroplast responds to the inducing signals for chloroplast differentiation raises the problem of experimental measurement of interaction among cellular organelles. Since chloroplast development is usually carried out in resting cells to avoid complications due to cell division, we discuss the limitations imposed by turnover in such nondividing systems and present evidence that most of the RNA labeling observed, although actinomycin‐D‐sensitive, is due to turnover and/or the utilization of preexisting pools. Evidence obtained with mutants ofEuglenathat form only partial chloroplasts or that lack plastid DNA and plastid‐related structures is reported. Such evidence indicates that the functional proplastid restrains overall RNA labeling in the uninduced cells and suggests that the proplastid might be the source of regulatory metabolic signals in the normal plastid‐con
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