Kinetics of photoacclimation in response to a shift to high light of the red alga Rhodella violacea adapted to low irradiance.

The unicellular rhodophyte Rhodella violacea can adapt to a wide range of irradiances. To create a light stress, cells acclimated to low light were transferred to higher irradiance and the kinetics of various changes produced by the light shift were analyzed. The proton gradient generated by excess light led to a non-photochemical quenching of the chlorophyll fluorescence and some photoinhibition of photosystem II centers was also produced by the light stress. After the shift to higher irradiance, the mRNA levels of three chloroplast genes that encode phycoerythrin and phycocyanin apoproteins and heme oxygenase (the first enzyme specific to the bilin synthesis) were negatively regulated. A change in the amount of thylakoids and in the total pigment content of the cells occurred during light acclimation after a light stress. The change in the size of the phycobilisome was limited to dissapearance of the terminal phycoerythrin hexamers in some of the rods. The ability of R. violacea to photoacclimate depends both on large changes in thylakoid number and pigment content and on smaller changes in the antenna size of photosystem II.

Differential transcription of phycobiliprotein components in Rhodella violacea. Light and nitrogen effects on the 33-kilodalton phycoerythrin rod linker polypeptide, phycocyanin, and phycoerythrin transcripts.

In Rhodella violacea phycoerythrin (PE) has two transcripts, a premessenger and a mature messenger (the gene contains an intron). Phycocyanin, which is plastid-encoded, and the 33-kD PE rod linker polypeptide, which is nuclear-encoded, have only one transcript. The PE premessenger had a rapid turnover; mature transcripts were stable in the light and more stable in the dark. In the presence of rifampicin, cells that shifted from dark to light exhibited an active translation of preexisting transcripts. There are indications of a modulation of the nuclear genome expression by the chloroplast; it may involve an unstable, plastid-encoded translational activator. All transcripts disappeared rapidly during nitrogen starvation. If nitrogen addition was carried out in the dark, active transcription and translation resumed as in light conditions, but ceased after 2 d. Both nitrogen and light were required for a total recovery after nitrogen starvation. Compared with the transcripts of phycobilisome components studied so far in cyanobacteria and Rhodophyceae, the mature transcripts of R. violacea are very stable when nitrogen is not limiting. The unstable PE premessenger is a good indicator of active transcription. This organism is therefore an interesting model to study the regulation of gene expression and the interactions between chloroplastic and nuclear genomes.