Actively growing cells are particularly sensitive to air-drying and most die instantly upon exposure to the atmosphere. In contrast, the members of a small, diverse group of prokaryotic and eukaryotic organisms have the capacity to withstand the most acute of cell-water deficits. How they do so remains an intriguing problem in biology. One cyanobacterium, Nostoc Commune, has a marked capacity for desiccation tolerance and has been developed as a model for the anhydrobiotic prokaryotic cell. In response to air-drying cells of N. commune synthesize water stress polypeptides (Wsp) that constitute around 70% of the total soluble cell protein and approximately 2% of the dry weight of a desiccated colony. Wsp remain stable in cells stored for decades in the air-dried state. Wsp are secreted and accumulate within a complex extracellular polysaccharide (EPS) that completely envelops the cell filaments. Within the EPS the Wsp form multimeric complexes, through strong ionic interactions, with a secreted, water-soluble UV-A/B-absorbing pigment (UVP).
The principle objectives of this study are: 1) to isolate wsp from Nostoc, and to characterize the gene, its flanking regions and its control elements; 2) to understand how wsp transcription and Wsp synthesis are regulated by water stress; and 3) to crystallize Wsp. The secretion of EPS and UVP by N. commune represent two strategies that the cells use to achieve protection during long-term desiccation. The evidence found by Dr. Potts suggests that Wsp play a role in one or both of these strategies. The knowledge of how N. commune overcomes acute cell-water deficit will provide an insight into the underlying mechanisms of desiccation tolerance in all organisms.
