Abstract
The plastid psbA mRNA is present in all tissues, while the encoded 32 kDa D1 protein of photosystem II accumulates tissue-specifically and in response to light. To study the regulation of D1 accumulation, a chimeric uidA gene encoding beta-glucuronidase (GUS) under control of the psbA 5'- and 3'-regulatory regions (224 and 393 bp, respectively), was integrated into the tobacco plastid genome. A high level of GUS accumulation in leaves and the lack of GUS in roots, with uidA mRNA present in both tissues, indicated tissue-specific accumulation of the chimeric gene product. Light-regulated accumulation of GUS in seedlings was shown. (i) Light-induced accumulation (100-fold) of GUS in etiolated cotyledons was accompanied by only a modest increase in mRNA levels. (ii) Inhibition of GUS synthesis was observed in cotyledons when light-grown seedlings were transferred to the dark, with no reduction in mRNA levels. Tissue-specific and light-regulated accumulation of GUS indicates that D1 accumulation is controlled via cis-acting regulatory elements in the untranslated region of the psbA mRNA. We propose that in tobacco, control of translation initiation is the primary mechanism regulating D1 protein accumulation.
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