Changes in Accumulation and Synthesis of Transcripts Encoding Phycobilisome Components during Acclimation of Fremyella diplosiphon to Different Light Qualities.

We have used gene-specific DNA fragments as hybridization probes to quantitate the levels of transcripts encoding several phycobilisome polypeptides in the cyanobacterium Fremyella diplosiphon in response to changes in the light environment. While the levels of transcripts encoding allophycocyanin, the core linker polypeptide, and the constitutive phycocyanin subunits are similar in F. diplosiphon grown either in red or green light, the levels of other transcripts change dramatically. Transcripts encoding the inducible phycocyanin subunits are barely detected in green light-grown cells and very abundant in red light-grown cells, while the level of phycoerythrin mRNA is approximately 10-fold more in green than red light-grown cells. Quantitation of the phycoerythrin and inducible phycocyanin transcripts after transfer of cultures from green to red light and red to green light demonstrate that both increase rapidly upon exposure of cells to inductive illumination. The decrease in the phycoerythrin mRNA level in red light is much slower than the decline in the levels of the inducible phycocyanin transcripts in green light. Since the half-lives of the inducible phycocyanin and phycoerythrin transcripts do not change when F. diplosiphon is exposed to red or green illumination, the steady state levels of these mRNAs are primarily controlled by the rate of transcription. Therefore, the high level of phycoerythrin mRNA maintained for several hours after cultures are transferred from green to red illumination must result from continued transcription of the phycoerythrin gene set. Differences in expression from the phycoerythrin and inducible phycocyanin gene sets in response to light quality are discussed in terms of possible mechanisms involved in their regulation.