Regulation of Phycobilisome Structure and Gene Expression by Light Intensity

Abstract

The cyanobacterium Agmenellum quadruplicatum PR-6 (Synechococcus sp PCC 7002) was grown turbidostatically in white light at three levels of irradiance: 20, 200, and 1260 microeinsteins per square meter per second. Phycobilisomes were isolated from each culture and analyzed by absorbance, gel electrophoresis, and electron microscopy. The ratio of phycocyanin to allophycocyanin decreased 1.8-fold from the lowest to highest irradiance. This change was due entirely to an approximately 2.5-fold decrease in one structural unit of rod domains, the complex of phycocyanin, and a 33-kilodalton linker polypeptide (LR33). For a given irradiance, phycobilisomes from cells grown on ammonium as the nitrogen source had 10 to 20% more phycocyanin than those from nitrate cultures. Total RNA was isolated from all cultures and probed with gene fragments specific to phycocyanin and allophycocyanin subunits and LR33. The relative level of RNAs encoding phycocyanin and allophycocyanin was found to vary with light intensity in parallel with the phycobiliprotein ratio. Hence, the light-harvesting capacity of phycobilisomes is directly regulated by relative levels of phycobiliprotein mRNA. The LR33 transcript occurs as a 3' extension on about 10% of phycocyanin transcripts. The ratio of RNA encoding LR33 to that encoding phycocyanin did not vary with irradiance, although the protein ratio changed 1.7-to twofold between extremes. Based on these and other observations, we propose that the LR33 protein is constitutively synthesized at a rate higher than that required to complex with available phycocyanin.The cyanobacterium Agmenellum quadruplicatum (Synechococcus sp PCC 7002) was chosen for this study. The phycobilisomes of this organism are well described in terms ofoverall structure and the function ofindividual polypeptide components (reviewed in refs. 2 and 6). The major components of this complex are the pigmented proteins, PC' and AP. The former is localized in rod substructures, where it is bound to linker polypeptides called LRC29, LR33, and LR9. Rods are linked to a central core domain by LRC29. The core contains AP and its associated linker polypeptides.The genes encoding the a and 13 subunits of PC (cpcA and cpcB, respectively) are linked in tandem with those encoding LR33 (cpcC) and LR9 (cpcD) (7-9, 20) (Fig. 4). Genes encoding AP a and ,B subunits (apcA and apcB, respectively) and one linker polypeptide (LC8.5) are similarly clustered (4). Transcripts encoding these cpc and apc loci have been mapped (12). The major transcript of each locus encodes only the a and ,B subunits of the respective phycobiliproteins. Minor transcripts extend further in the 3' direction to encode linker polypeptides. Having this knowledge of phycobilisome structure and gene organization in A. quadruplicatum, we could readily investigate responses to changes in light intensity.In addition to irradiance, we simultaneously examined the effect of nitrogen source on phycobilisome structure. This is of interest because phycobiliproteins are strongly...

Evidence for a Transient Association of New Proteins with the Spirulina maxima Phycobilisome in Relation to Light Intensity

“…The discussion of these results will lead us to attribute a putative role of a chaperone protein to a particular polypeptide of 47 kD, which until now was considered a contaminant de Lorimier et al, 1992). In HL, we observed the appearance of two additional polypeptides, P 30 and P 29 , which had never been detected before, and the appearance was concomitant with the loss of the PC distal disk and the disappearance of the L R 33 polypeptide.…”

“…Thus, L R 33 must be regulated by the light intensity at a posttranslational level, and more precisely at the level of its association with PBS. A similar type of regulation has been considered by de Lorimier et al (1992) in Synechococcus sp. PCC 7002.…”

“…Conceming the origin and role of the 47-kD polypeptide, our results led us to consider that this polypeptide is playing an important role in the PBS elaboration and cannot be a contaminant as proposed by Bryant et al (1990) andde Lorimier et al (1992) for an analogous 45-kD polypeptide found in Synechococcus sp. PCC 7002.…”

Biogenesis of Phycobiliproteins

“…The authors hypothesized that this form of PC was a connector between rods and the core, but this assertion was based only on the fact that this form of PC copurifies with some AP, and stoichiometric competence for this assertion was not demonstrated. If this type of "connector" were to be required to attach rods onto cores, then the extensive electron microscopic analyses performed on isolated PBS would probably have revealed this extra rod segment, and demethyl-PC should have accounted for a major fraction of the total PC (41)(42)(43)(44)(45)(46)(47)(48)(49). We have also observed small amounts (ϳ1-3%) of unmethylated PC subunits in Synechococcus sp.…”

Disruption of a gene encoding a novel thioredoxin-like protein alters the cyanobacterial photosynthetic apparatus

“…Within these basic functional constraints, the structure of the photosynthetic apparatus is dynamic. For example, while the PBS-to-Chl ratio generally reflects the PSII-to-PSI ratio, under some conditions cyanobacteria can modulate the light-harvesting capacity of PBS relative to Chl independently of the PSII-to-PSI ratio by changing the size of the PBS (11,20,40) or the number of PBS relative to PSII (42). Cyanobacteria grown in light that is harvested primarily by PBS show a decline in the PSII-to-PSI ratio, while those grown in light harvested primarily by PSI show the opposite response (1,26,41,44).…”