SIGNS OF TRANSLATIONAL REGULATION WITHIN THE TRANSCRIPT LEADER OF A PLANT PLASMA-MEMBRANE H-ATPASE GENE()

Transcripts of most plant plasma membrane H+-ATPase genes possess a le ader (5' untranslated region) that is unusually long and that contains a short upstream open reading frame (uORF), two features which sugges t posttranscriptional regulation. To investigate the putative role of the transcript leader, we have placed the leader of pma3, one of the N icotiana plumbaginifolia H+-ATPase genes, between the CaMV 35S promote r and the sequence coding for the beta-glucuronidase (GUS) reporter ge ne. Transient expression of this chimeric gene and of derived mutants was analysed in electroporated tobacco protoplasts. The whole leader h ad a positive effect on translation, since deletion of most of its seq uence reduced GUS activity. Suppression of the uORF by point mutation of its initiating AUG increased GUS activity by about 55%. Analysis of various deletions and mutations suggested that the uORF is translated by at least two-thirds of scanning ribosomes, half of which subsequen tly reinitiate downstream translation under our experimental condition s. Reinitiation did not depend on the nucleotide sequence of the uORF, nor on that separating the uORF and the main open reading frame. We c onclude that the pma3 transcript possesses features of translational r egulation, whose mode of functioning has yet to be discovered.