nblS, a Gene Involved in Controlling Photosynthesis-Related Gene Expression during High Light and Nutrient Stress inSynechococcus elongatusPCC 7942

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Abstract
The HliA protein of the cyanobacteriumSynechococcus elongatusPCC 7942 is a small, thylakoid-associated protein that appears to play a role in photoprotection; its transcript rapidly accumulates in response to high-intensity light (HL) and thehligene family is required for survival of cells in high light. In order to discover regulatory factors involved in HL acclimation in cyanobacteria, a screen was performed for chemically generated mutants unable to properly control expression of thehliAgene in response to HL. One such mutant was identified, and complementation analysis led to the identification of the affected gene, designatednblS. Based on its deduced protein sequence, NblS appears to be a membrane-bound, PAS-domain-bearing, sensor histidine kinase of two-component regulatory systems in bacteria. ThenblSmutant was unable to properly control light intensity-mediated expression of several other photosynthesis-related genes, including all threepsbAgenes and thecpcBAgenes. The mutant was also unable to control expression of thehliAandpsbAgenes in response to low-intensity blue/UV-A light, a response that may be related to the HL-mediated regulation of the genes. Additionally, in response to nutrient deprivation, thenblSmutant was unable to properly control accumulation of thenblAtranscript and associated degradation of the light-harvesting phycobilisomes. ThenblSmutant dies more rapidly than wild-type cells following exposure to HL or nutrient deprivation, likely due to its inability to properly acclimate to these stress conditions. Thus, the NblS protein is involved in the control of a number of processes critical for altering the photosynthetic apparatus in response to both HL and nutrient stress conditions.