Different sequences for 5?-untranslated leaders of nuclear genes for plastid proteins affect the expression of the ?-glucuronidase gene

Expression of chimeric uidA gene fusions (for bacterial beta-glucuronidase) with 5'-flanking sequences of the spinach AtpC and PetE genes (encoding the subunit gamma of the chloroplast ATP synthase and plastocyanin, respectively) requires sequences for the 5'-untranslated leaders. The sequence for the PetE leader does not exhibit significant similarities to those of other leader sequences. Closer inspection of PetE uncovered that the crucial region is located in the vicinity of the transcription start site (+5/+15, TTGTCATTTCT). In contrast, 3' deletions of sequences for the AtpC leader revealed that the region in the vicinity of the translation initiation codon is essential for uidA gene expression (+103/+176). This segment contains a CT-rich sequence (TTCTCTCTCCT), which is found identically or in a slightly modified form in sequences for 85 plant leaders deposited in the EMBL data bank. Site-directed mutagenesis of the CT-rich sequence resulted in a three-fold reduction of the transcription of the transgene. It is concluded (1) that different elements in the sequences for the spinach PetE and AtpC leaders control the expression of the uidA gene, (2) that these elements operate transcriptionally rather than post-transcriptionally and (3) that a CT-rich sequence represents a crucial cis element for the transcription of the AtpC::uidA gene fusion.