CHARACTERIZATION OF RBCS GENES IN THE FERN PTERIS-VITTATA AND THEIR PHOTOREGULATION

The fern Pteris vittata L. belongs to the evolutionarily highest group of vascular plants that still maintains a free-living gametophytic st age. The two-dimensional gametophytes developed under blue light exhib it higher CO2 fixation efficiency and different ribulose 1,5-bisphosph ate carboxylase/oxygenase (Rubisco) small subunit (SSU) composition wh en compared to the red-induced filamentous gametophytes (H. Eilenberg et al., 1991, Plant Physiol 95. 298-304). To unravel the correlation b etween SSU structural differences and right regulation, two rbcS genes and two additional partial cDNAs were characterized. Fern rbcS genes resemble those of higher plants in their promoter light-regulatory ele ments (LREs) and intron number and positions. However, the primary str ucture of the fern mature SSUs displays much higher divergency within the gene family. This structural variability was correlated with diffe rential steady-state mRNA levels under red and blue light. Genes rbcS- 1 and -4 show 4- to 6-fold higher transcript levels in red light while rbcS-2 and -3 contribute relatively more to the blue rbcS mRNA levels . Five of the 12 amino acids that differ between rbcS-2 and -4 affect hydrophobicity and might play a crucial role in determining the effici ency of CO2 fixation. Dendrograms of Rubisco SSUs and LSUs indicate ea rly divergence of the fern types from the rest of the vascular plants, However, prominent higher-plant-like Rubisco features such as high ca rboxylation efficiency, promoter LREs and exon-intron structure, sugge st that molecular specialization of the higher-plant Rubisco prototype occurred earlier than the emergence of ferns.