REGULATION OF NUCLEAR GENE-EXPRESSION FOR PLASTIDOGENESIS AS AFFECTED BY DEVELOPMENTAL STAGE OF PLASTIDS

A controlled coordination in expression between nuclear genome and plastid genome regulates the biogenesis of plastids. A regulatory control from the developing plastids is recently discovered as a prerequisite for the expression of nuclear genes involved in the plastidogenesis. The developmental stage of plastids selectively regulates recognition, import and accomodation of cytoplasmically synthesized plastidic proteins. Induced inhibition of these processes affects accumulation of the plastidic polypeptides as determined at the transcript and the product level. Such a regulatory control has been regarded as "Plastidic signal". The plastidic signal was demonstrated to first come into effect during the onset of development of plastids and was shown to be continuously operative during the entire course of plastidogenesis. Further studies have also characterized that the plastidic signal was at least not a translational product in the plastids. Various possible mechanisms of this phenomenon are critically analyzed and caveats are noted. It is suggested possible that an accumulation of the import impeded precursor plastidic polypeptides in the cytosol beyond a physiologically permitted basal level may result in feed back control of their own over production. The development stage of plastids may thus determine repression or derepression of the nuclear gene expression destined for plastidogenesis. Specific testable predictions could also be generated from the thus postulated hypothesis, several of which have already been stated earlier and are now critically evaluated in favor of this hypothesis. Further feasible experiments to test the proposed hypothesis are discussed in perspective.