Segments encoding 5'-untranslated leaders of genes for thylakoid proteins contain cis-elements essential for transcription.

The promoter region -118/-29 of the spinach PetH gene encoding the ferredoxin-NADP(+)-oxidoreductase contains crucial cis-elements for the regulated expression, while sequences for the 5'-untranslated leader determine the quantitative expression of chimeric GUS gene fusions in transgenic tobacco. Deletion of leader sequences in chimeric GUS gene fusions of the spinach PetE and PsaF genes (for plastocyanin and the subunit III of photosystem I, respectively) results also in a decline in the GUS activity. Appropriate gene constructs and run-on transcription assays demonstrate unambiguously that the leaders of all three genes are involved in transcription rather than in post-transcriptional processes. They appear to contain gene-specific control elements rather than cis-determinants for general initiation factors. Expression-relevant segments in the PsaF and PetH leaders contain two CT-rich sequences, designated CT-LB and CT-B, of which at least the former binds to a protein factor in gel mobility shift assays. These motifs are not found in the PetE leader. The findings imply that leader sequences may contain cis-elements that are essential for the transcription, that they influence GUS gene expression quantitatively rather than qualitatively, and that these elements, as those of promoters, can be quite variable in sequence.