CHARACTERIZATION OF GENES EXPRESSED IN MESOPHYLL PROTOPLASTS OF NICOTIANA-SYLVESTRIS BEFORE THE REINITIATION OF THE DNA REPLICATIONAL ACTIVITY

To decypher the early events preceding the re-entry of somatic cells into the cell cycle, we constructed a cDNA library from 6-h-old protoplasts of Nicotiana sylvestris. We characterized three mRNAs, via their cDNAs, that accumulate at very high levels 6 h after the beginning of the culture. Two of them could be identified by comparison of the deduced amino acid sequence to databanks. 6P10 is a novel type I trypsin inhibitor, which has the peculiarity of being devoid of the pro-sequence peptide described to be essential for transport to the vacuole. 6P73 is a novel, moderately anionic peroxidase. 6P50 belongs to a gene family not yet identified. These genes are highly expressed in protoplasts at the beginning of the culture and moderately in roots, but are neither expressed in response to chemical treatment, heat shock, pathogen attacks nor during tumor induction. These findings suggest that the activation of these genes corresponds not only to a specific adaptation of protoplasts to the new environment but also, since their level of expression decreases at the onset of division, to a sequence of events connected with the establishment of the new program of gene expression of the dividing cell.