DIFFERENT SEQUENCES FOR 5'-UNTRANSLATED LEADERS OF NUCLEAR GENES FOR PLASTID PROTEINS AFFECT THE EXPRESSION OF THE BETA-GLUCURONIDASE GENE

Expression of chimeric uidA gene fusions (for bacterial beta-glucuroni dase) with 5'-flanking sequences of the spinach AtpC and PetE genes (e ncoding the subunit gamma of the chloroplast ATP synthase and plastocy anin, respectively) requires sequences for the 5'-untranslated leaders . The sequence for the PetE leader does not exhibit significant simila rities to those of other leader sequences. Closer inspection of PetE u ncovered that the crucial region is located in the vicinity of the tra nscription start site (+5/+15, TTGTCATTTCT). In contrast, 3' deletions of sequences for the AtpC leader revealed that the region in the vici nity of the translation initiation codon is essential for uidA gene ex pression (+103/+176). This segment contains a CT-rich sequence (TTCTCT CTCCT), which is found identically or in a slightly modified form in s equences for 85 plant leaders deposited in the EMBL data bank. Site-di rected mutagenesis of the CT-rich sequence resulted in a three-fold re duction of the transcription of the transgene. It is concluded (1) tha t different elements in the sequences for the spinach PetE and AtpC le aders 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 trans cription of the AtpC::uidA gene fusion.