High levels of photosystem I subunit II (PsaD) mRNA result in the accumulation of the PsaD polypeptide only in the presence of light.

The light-regulated mRNA and polypeptide accumulation of the nuclear encoded subunit II (PsaD) of the photosystem I reaction center was studied during the greening of etiolated spinach seedlings. Upon exposure to continuous white light, the mRNA, detected at low levels in etiolated seedlings, accumulated in a specific pattern. In contrast, the PsaD subunit could not be detected in the etiolated seedlings; the polypeptide could first be detected in thylakoid membranes approximately 4 h after exposure to continuous light. A pulse of red light induced the expression of the PsaD mRNA, but the polypeptide could not be detected unless the seedlings were exposed to light. In the light (but not in the dark), the PsaD mRNA was found associated with the polysomal fraction. Taken together, the data suggest a dual regulatory mechanism in which both the level of mRNA and the presence of light control the accumulation of the PsaD polypeptide.