PARENT SESSION
Posters P6B Photosynthetic acclimation: Mechanisms and gene expression. Abstracts (531-578)

NblC - A novel cyanobacterial modulator of pigment level during nutrient limitation. Eleonara Sendersky, Roxane Lahmi¹, Judith Shaltiel¹, Alexander Perelman¹, Rakefet Schwarz^*,1, ¹ Faculty of Life Sciences, Ramat-Gan, Israel

ABSTRACT- Modulation of pigment level in response to environmental cues is an essential process that allows photosynthetic organisms to adjust light harvesting to the metabolic needs of the cell and minimizes the photo-oxidative damage resulting from surplus excitation. Mutants that, unlike wild type cells, do not degrade their light harvesting pigments under sulfur and nitrogen starvation have been used to identify components of the degradation pathway. Starved mutant cultures appear blue green rather than yellowish or bleached as starved wild type cultures, and therefore the phenotype was termed non-bleaching (nbl). Four components of the nbl-pathway have been previously identified: NblS and NblR were assigned a regulatory function whereas NblA and NblB appear to be involved in the degradation process. The specific role of the latter components is not clear, neither the mechanism of pigment degradation or its modulation by nutrient limitation. It was therefore desirable to isolate novel non-bleaching mutants to further elucidate the mechanism underlying modulation of pigment level. We have developed an efficient method, which employs Fluorescence Activated Cell Sorter (FACS) to screen for non-bleaching mutants. Characterization of one of these mutants uncovered a novel component of the degradation pathway designated NblC. Inactivation of nblC resulted in a non-bleaching phenotype during nitrogen, sulfur or phosphorus starvation. Therefore, NblC, similarly to NblR, belongs to a cascade of events resulting in a general acclimation response. Over expression of NblC by a foreign promoter resulted in pigment degradation under the inducing conditions. Interestingly, a strain of nblR-mutant in which NblC was over expressed retained its pigmentation, suggesting dependence of NblC on NblR. Transcription of nblC is induced during sulfur and nitrogen starvation. Furthermore, the NblC-mutant exhibited reduced viability under sulfur and nitrogen starvation as compared to wild type cells. The role of NblC during nutrient starvation will be discussed in light of transcription analysis of nblA and cpc operon (encoding for the subunits of phycocyanin, the major light harvesting pigment) in the wild type, in nblC-mutant and in wild type and mutant strains over expressing NblC or NblR.

KEY WORDS: modulation of transcription, phycobilisome, nutrient starvation