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Last summer many of us were interested by the publication of Nature papers describing a photosystem I antenna ring formed under conditions of iron limitation. Jim Barber (Imperial College, London, UK) described in the opening lecture a series of events that lead to this remarkable structure. Iron limitation causes reduced synthesis of iron-rich photosystem I. In response to iron limitation, up-regulation of isiA and isiB genes leads to the synthesis of CP43’ (IsiA) protein and flavodoxin, respectively (flavodoxin functions as an alternative electron acceptor in place of ferredoxin). A ring of 18 chlorophyll-binding CP43’ proteins around photosystem I dramatically raises its light-absorbing capacity. Sucrose density gradient separation of photosystem I from iron-limited Synechocystis cells yields a heavy band consisting of CP43’-photosystem I supercomplexes. Analysis of the supercomplexes revealed the existence of chlorophyll molecules bound to some accessory photosystem I subunits such as PsaJ. These may act as a bridge for excitation energy between the antenna ring and the photosystem.
The pcb genes of Prochlorophytes are homologous to isiA. The supercomplex band was observed in gradients from Prochlorococcus marinus SS120, a deep-sea strain. It resembled the Synechocystis antenna but contained both chlorophyll a and b. A ring was sometimes found around photosystem II dimers in Prochlorococcus species; full 14 subunit rings were rarely observed but asymmetric 11 subunit rings were often encountered. Rings were also found in Acaryochloris marinus containing chlorophyll d.
In the lab, cyanobacteria grow contentedly in media with iron supplements and use phycobilisomes for light-harvesting. These studies show that ‘out in the wild’, where iron limitation is likely to be a common challenge, these CP43’ rings may be the norm. In his talk on marine cyanobacteria, Wolfgang Hess (Humboldt University, Berlin, Germany) mentioned the loss of phycobilisome-related genes from oceanic Prochlorococcus, and perhaps this is because the CP43’ antenna is more useful in these environments. A poster by Ulrike Geiß (University of Greifswald, Germany) described profiling of isiA gene expression to look directly at iron starvation in aquatic habitats. The gene was detected in various estuary samples and three gene families were described.
Those groups working on iron limitation were probably interested in B. Gemmer’s poster (University of Greifswald, Germany), which outlined the difficulties in producing thoroughly iron-depleted media. They explained their use of the siderophore desferrioxamine-B (DFOB). An iron stress response (monitored using GFP-tagged isiAB promoter in Synechocystis) was seen when DFOB was added even to iron-rich BG11 medium.
Salt stress was another hot topic, with Martin Hagemann (University Rostock, Germany) describing it as “a paradigm for acclimation to environmental stresses”. When salt concentrations are elevated, cells face multiple stresses – ionic, osmotic, oxidative etc. Cyanobacteria fight back with both specific and general responses. Cells accumulate compatible solutes such as glucosylglycerol. Transport mechanisms, energy production and other systems are affected. Of the salt-induced proteins identified, most were general stress proteins (chaperones, repair systems etc). Salt sensing is not well understood. Functional genomics revealed the involvement of alternative sigma factor SigF. DNA microarrays identified several histidine kinases involved in sensing and gene activation.
N. Yeremenko and V. Krasikov (University of Amsterdam, the Netherlands) described the use of DNA arrays to investigate patterns of gene expression under different growth conditions. They use an optimised mRNA purification and hybridisation to an array in which all Synechocystis genes are represented. Stress conditions investigated included phosphate or nitrate limitation and various levels of salt stress. Specific and general responses were detected.
Despite all the stress-related presentations and posters,
the only problems faced by this meeting’s attendees were heat stress and
football deprivation. Fortunately, all five workshop sessions benefited
from interesting enthusiastic speakers who easily held our attention even
in the unanticipated sleepy warm weather! Our excellent hosts provided
world cup updates where necessary. This enjoyable workshop was a huge success
and those of us who were lucky enough to spend a little time exploring
Stockholm have arrived home completely stress-free.