Cyanobacteria are unique in the sense that they are prokaryotes that carry out oxygenic photosynthesis equivalent to the one found in higher plants. Thus, they comprise an interesting model system for studies of the evolution, regulation and function of photo-synthesis. They also possess a light antenna, the phycobilisome, that is unique and that may offer interesting possibilities to study photon transport, light regulation and the bio-genesis of soluble, multi-protein complexes. Being bacteria, they also present experimental advantages compared to higher plants. They are easy to grow on plates and in liquid culture, DNA, RNA and protein are easily isolated and they are easily amenable to genetic engineering. We are working with the photoautotrophic cyanobacterium, Synechococccus . It offers unique features because it is easier to grow, easier to manipulate genetically, its photo-synthesis is easier to study, the light antenna is better known and we have more experience in studying its photosystem II. During the years, we have also isolated a number of mutants that present a valuable resource. Working with Synechococccus ism greatly facilitated by the fact that the complete genome is available from the cyanobacterium Synechocystis .

Light Antenna Biogenesis
The photosynthetic light antenna harvests photons in pigment molecules bound to comp-licated protein complexes. The captured energy is transferred to the photosynthetic reac-tion centers. The light antenna of cyanobacteria, the phycobilisome, sits on the surface of the thylakoid membrane and attaches to photo-system II. Thus, it is water-soluble and consists of a highly ordered core structure from which highly organized rods fan out. The rods change in length depending upon the growth conditions thereby regulating the light energy harvested by PSII. Our studies and others show that the phycobilisome is regulated both at the trans-criptional and post-transcriptional levels. Our data also shows that the presence and levels of certain key com-ponents, the structural rod linker polypeptides, regulates the biogenesis of the rod.

 

We have previously studied the structure and energy transfer of the phycobilisome in detail but not the biogenesis. We have found that the ordered presence of linkers make the rod change in length. The biogenesis of the phycobilisome has been considered an auto-assembly process due to results from in vitro experiments 15 years ago. However, looking from the all data collected from experiments centered around chaperones this can be questioned. In collaboration with dr. Adrian K. Clarke at our Dept., we are studying the novel concept that chaperones are involved in phycobilisome biogenesis and maturation.

 

mRNA maturation
We have previously found that the phycobilisome mRNAs are subjective to specific mRNA processing and that different parts of the mRNA have different half-lives. In the regulation of the phycobilisome rod, we also find an interesting inverse relationship between the levels of the corresponding mRNA and amount of rod associated proteins. During the last year, we have been successful in cloning different genes coding for components of the degradosome from Synechococccus . The degradosome from Escherichia coli contains the proteins RnaseE, PNPase, enolase and helicase. We have cloned most of these genes from Synechococccus . We are using these genes in combination with E. coli mutants as well as defined mRNA species to monitor the action of the cyanobacterial degradosome.

 

Transcript profiling
In our Poplar EST project, the Umeő Plant Science Center will have established the DNA Chip Array Technology together with researchers at KTH. The Chip technology might not work for prokaryotes because of the short half-lives of bacterial mRNAs. However, cyanobacteria have typical mRNA half-lives of 10 - 20 minutes. Thus, we want to use a limited number of PCR amplified genes to see if the Chip technology can be used in cyanobacteria to study multiplex gene expression patterns, especially of stress regulated genes.

Selected publications

 

Kalla, S.R., Bhalerao, R.P. and Gustafsson, P. () Regulation of the phycobilisome rod proteins and mRNA at different light intensities in the cyanobacterium Synechococccus . Gene 126: 77-83.

 

Bhalerao, R.P., and Gustafsson, P. () Factors influencing the phycobilisome rod composition of the cyanobacterium Synechococccus sp. PCC : Effects of reduced phycocyanin content, lack of rod-linkers and over-expression of the rod-terminating linker. Physiologia Plantarum 90: 187-197.

 

Bhalerao, R.P., Gillbro, T. and Gustafsson, P. () Functional phycobilisome core in a phycocyanin-less mutant of cyanobacterium Synechococccus sp. PCC . Photosynthetic Research 45: 61-70.

 

Bhalerao, R.P., Collier, J.L., Gustafsson, P. and Grossman, A. R. () The structure of phycobilisomes in mutants of Synechococcus sp. PCC devoid of specific linker polypeptides. Photochemistry and Photobiology 61: 298-302.

 

Campbell, D., Zhou, G., Gustafsson, P., ÷quist, G. and Clarke, A.K. () Electron transport regulates exchange of two forms of Photosystem II D1 protein in the cyanobacterium Synechococccus sp. PCC . EMBO Journal 14: .

 

÷quist, G., Campbell, D., Clarke, A.K. and Gustafsson, P. () The cyanobacterium Synechococcus modulates PS II function in response to excitation stress through D1 exchange. Photosynthesis Research, 46: 151-158.