A number of cyanobacteria have been shown to respond to conditions of macronutrient deprivation (phosphorus, sulfur and nitrogen) by degrading the light harvesting phycobilisome complex, eliminating most of the photosynthetic membranes and synthesizing proteins involved in nutrient acquisition and assimilation. If nutrients are withheld for long periods of time the organism can maintain viability by entering a quiescent or dormant phase. A detailed molecular examination of the responses of the unicellular cyanobacterium Anacystis nidulans, strain R2, to sulfur deprivation has been initiated. Many genes that are activated when sulfur is eliminated from the growth medium have been isolated and at least partially sequenced. Some of these genes encode proteins involved in sulfate uptake (polypeptides of the four components sulfate transport system) while others may be important for growth on sulfur-containing compounds such as thiosulfate or glutathione. To help elucidate biochemical or genetic changes that accompany the acclimation of Anacystis to sulfur stress, proteins that accumulate and genes that are activated when sulfur is eliminated from the growth medium are being characterized. Molecules involved in sensing the sulfur status of the environment and elements required for gene regulation will be established. Most organisms have the capacity to modulate specific physiological processes to accomodate changes in their environment. This research addresses the manner in which external information is perceived by cells and the events involved in eliciting alterations in the physiological state through changes in gene expression. Attention will be focused on the response of a cyanobacterium to sulfur deprivation. The results of this work will provide an increased understanding of the structural and functional components required by the cells to withstand nutrient limiting conditions, the interrelationship between photosynthetic carbon fixation and the sulfur status of the cell, and the mechanism by which photosynthetic organisms sense and respond to their environment.
