In principle, the
mutational saturation of all nucleus-encoded plastid
proteins is feasible, but such a large-scale effort has not
yet been launched. Current studies focus on the identification
and characterization of nuclear mutations for a
limited number of genes, although large-scale projects
have recently begun. One example of a reverse genetics
screen involved the knockout of more than a dozen genes
for components of PSI in A. thaliana (Fig. 2). The PSI
complex of this plant is a mosaic of plastid- and nucleusencoded
protein subunits; of the latter, three (PSI-D, -E
and -H) are each encoded by two functional gene copies.
Cross-species comparison of PSI raises the question of why
the subunits D, E and F in higher plants contain Nterminal
extensions not present in their cyanobacterial
orthologues. A second question concerns the function of
PSI-G, -H, -N and -O,