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 premess enger and a mature messenger (the gene contains an intron). Phycocyani n, which is plastid-encoded, and the 33-kD PE rod linker polypeptide, which is nuclear-encoded, have only one transcript. The PE premessenge r had a rapid turnover; mature transcripts were stable in the light an d more stable in the dark. In the presence of rifampicin, cells that s hifted from dark to light exhibited an active translation of preexisti ng transcripts. There are indications of a modulation of the nuclear g enome expression by the chloroplast; it may involve an unstable, plast id-encoded translational activator. All transcripts disappeared rapidl y during nitrogen starvation. If nitrogen addition was carried out in the dark, active transcription and translation resumed as in light con ditions, but ceased after 2 d. Both nitrogen and light were required f or a total recovery after nitrogen starvation. Compared with the trans cripts of phycobilisome components studied so far in cyanobacteria and Rhodophyceae, the mature transcripts of R. violacea are very stable w hen nitrogen is not limiting. The unstable PE premessenger is a good i ndicator of active transcription. This organism is therefore an intere sting model to study the regulation of gene expression and the interac tions between chloroplastic and nuclear genomes.