The PsbS gene product (PSII-S) which is an integral subunit of photosystem II has recently been reported to be a new type of pigment-binding protein [11]. The chlorophylls of the PSII-S protein exhibit weak excitonic coupling and this protein is stable also in the absence of pigments. Here we investigated the expression of the PsbS gene in etiolated spinach seedlings grown either in complete darkness or exposed to light of various qualities. The results obtained reveal that the PsbS gene expression in etiolated spinach plants is subjected to endogenous control. This developmental control occurs at different levels of gene expression and results in transient accumulation of the PsbS transcripts with progressing etiolation. During the first two days after emerging of the cotyledons from the seed coat the steady-state level of the PsbS transcripts is regulated mainly through increased transcriptional activity of the PsbS gene. Prolonged growth of the seedlings in the dark resulted in additional post-transcriptional control of the PsbS transcript level. Translational activity of PsbS mRNA estimated by an integration of mRNA into polysomal complexes shows that the translation rate of PsbS mRNA is less influenced by seedling age. The maximal rate of translation is reached at the first day after cotyledons emergence but the translational activity of PsbS mRNA remains still high (50% of maximum) in 8-day old etiolated spinach plants. In addition to the light-independent developmental control, the PsbS gene expression is positively regulated by phytochrome in etiolated seedlings exposed to light. Red light, however, negatively influences the abundance of PsbS transcripts at post-transcriptional level. Studies on blue or far-red light effects reveal that the accumulation of PsbS transcripts exhibits the characteristics of very-low-fluence responses of the phytochrome receptor.
Abbreviations: Cab, genes coding for chlorophyll a/b-binding proteins; ELIPs, early light-inducible proteins; Elip, gene coding for early light-inducible protein; Pcr, gene coding for protochlorophyllide reductase; PhyA, gene coding for phytochrome type I protein; PS II, photosystem II; PSII-S, 22 kDa protein of PSII, PsbS, gene coding for 22 kDa protein of PSII; rbcL, gene coding for large subunit of ribulose-bisphosphate carboxylase; RbcS, gene coding for small subunit of ribulose-bisphosphate carboxylase; TubB, gene coding for 3-tubulin