Arabidopsis thaliana plants have been transformed with an antisense gene to the psbW of photosystem II (PSII). Eight transgenic lines containing low levels of psbW mRNA have been obtained. Transgenic seedlings with low contents of PsbW protein (more than 96% reduced) were selected by Western blotting and used for photosynthetic functional studies. There were no distinct differences in phenotype between the antisense and wild type plants during vegetative period under normal growth light intensities. However, a sucrose gradient separation of briefly solubilized thylakoid membranes revealed that no dimeric PSII supracomplex could be detected in the transgenic plants lacking the PsbW protein. Furthermore, analysis of isolated thylakoids demonstrated that the oxygen-evolving rate in antisense plants decreased by 50% compared with the wild type. This was found to be due to up to 40% of D1 and D2 reaction center proteins of PSII disappearing in the transgenic plants. The absence of the PsbW protein also altered the contents of other PSII proteins to differing extents. These results show that in the absence of the PsbW protein, the stability of the dimeric PSII is diminished and consequently the total number of PSII complexes is greatly reduced. Thus the nuclear encoded PsbW protein may play a crucial role in the biogenesis and regulation of the photosynthetic apparatus.