THE ROLE OF PLASTIDS IN THE EXPRESSION OF NUCLEAR GENES FOR THYLAKOIDPROTEINS STUDIED WITH CHIMERIC BETA-GLUCURONIDASE GENE FUSIONS

We have analyzed plastid and nuclear gene expression in tobacco seedli ngs using the carotenoid biosynthesis inhibitor norflurazon. mRNA leve ls for three nuclear-encoded chlorophyll-binding proteins of photosyst em I and photosystem II (CAB I and Il and the CP 24 apoprotein) are no longer detectable in photobleached seedlings, whereas those for other components of the thylakoid membrane (the 33- and 23-kD polypeptides and Rieske Fe/S polypeptide) accumulate to some extent. Transgenic tob acco seedlings with promoter fusions from genes for thylakoid membrane proteins exhibit a similar expression behavior: a CAB-beta-glucuronid ase (GUS) gene fusion is not expressed in herbicide-treated seedlings, whereas PC-, FNR-, PSAF-, and ATPC-promoter fusions are expressed, al though at reduced levels. All identified segments in nuclear promoters analyzed that have been shown to respond to light also respond to pho todamage to the plastids. Thus, the regulatory signal pathways either merge prior to gene regulation or interact with closely neighboring ci s elements. These results indicate that plastids control nuclear gene expression via different and gene-specific cis-regulatory elements and that CAB gene expression is different from the expression of the othe r genes tested. Finally, a plastid-directing import sequence from the maize Waxy gene is capable of directing the GUS protein into the photo damaged organelle. Therefore, plastid import seems to be functional in photobleached organelles.