We demonstrate that photosynthetic adjustment at the leve1 of the light-harvesting complex associated with photosystem I1 (LCHII) in Dunaliella salina is a response to changes in the redox state of intersystem electron transport as estimated by photosystem II (PSII) excitation pressure. To elucidate the molecular basis of this phenomenon, LHCll apoprotein accumulation and cab mRNA abundance were examined. Crowth regimes that induced low, but equivalent, excitation pressures (either 13"C/20 pmol m-* s-' or 30°C/ 150 pmol m-' s-l) resulted in increased LHCll apoprotein and cab mRNA accumulation relative to alga1 cultures grown under high excitation pressures (either 13"C/150 pmol m-' s-' or 3O0C/25O0 pmol m-' s-l). Thermodynamic relaxation of high excitation pressures, accomplished by shifting cultures from a 13 to a 30°C growth regime at constant irradiance for 12 h, resulted in a 6-and 8-fold increase i n LHCll apoprotein and cab mRNA abundance, respectively. Similarly, photodynamic relaxation of high excitation pressure, accomplished by a shift from a light to a dark growth regime at constant temperature, resulted i n a 2.4-to 4-fold increase in LHCll apoprotein and cab mRNA levels, respectively. We conclude that photosynthetic adjustment to temperature mimics adjustment to high irradiance through a common redox sensinglsignaling mechanism. 60th temperature and light modulate the redox state of the first, stable quinone electron acceptor of PSII, which reflects the redox poise of intersystem electron transport. Changes in redox poise signal the nucleus to regulate cab mRNA abundance, which, in turn, determines the accumulation of light-harvesting apoprotein. This redox mechanism may represent a general acclimation mechanism for photosynthetic adjustment to environmental stimuli.In green algae and higher plants, the light-harvesting complexes are located in the chloroplast thylakoid membrane and are responsible for the capture and transfer of light energy to the photosynthetic reaction centers. The apoproteins of the major Chl a / b LHCII are encoded in the nucleus by the cab multigene family (Buetow et al., 1988;Jansson, 1994). Thus, regulatory signals must be exchanged between the nucleus and chloroplast to maintain optimal levels of the LHCII pigment-protein complexes (Simpson

Recently we have reported that the flavodoxin gene from the cyanobacterium Anacystis nidulans R2 is transcribed as part of an iron stress-induced operon containing multiple mRNA species' (D. E. Laudenbach, M. E. Reith, and N. A. Straus, J. Bacteriol. 170:258-265, 1988

A sulfur-regulated gene (cysA) that encodes the membrane-associated ATP-binding protein of the sulfate transport system of the cyanobacterium Synechococcus sp. strain PCC 7942 was recently isolated and sequenced. Adjacent to cysA and transcribed in the opposite direction is a gene encoding the sulfate-binding protein (sbpA Cyanobacteria are obligate photoautotrophs that undergo a complex set of distinct morphological and physiological changes when deprived of macronutrients such as sulfur and nitrogen. A visually dramatic response to nutrient deprivation is the degradation of the light-harvesting complex, the phycobilisome, and the concomitant decline in the levels of phycocyanin and allophycocyanin. Additionally, there is a decrease in chlorophyll levels and attenuation of the photosynthetic membranes (44, 60). Other changes that accompany nutrient deprivation include cell wall thickening (25) and the accumulation of electron-opaque inclusion bodies in the cytoplasm of the cell (31, 60).In addition to the general responses described above, cyanobacteria exhibit specific responses when deprived of a given nutrient. During growth in sulfur-deficient medium, the unicellular cyanobacterium Anacystis nidulans and the closely related Synechococcus sp. strain PCC 7942 display an increased rate of sulfate transport (15,24 In the enteric bacteria Salmonella typhimurium and Escherichia coli, sulfate transport is accomplished by a single active uptake system. This permease complex is composed of three cytoplasmic membrane components and a substratespecific binding protein located in the periplasmic space (3, 28). The sulfate-binding protein of S. typhimurium has been isolated and sequenced and its crystal structure has been resolved (23,36,40). It is present in the periplasmic space at very high levels and binds one sulfate molecule per molecule of protein. Two of the cytoplasmic membrane proteins appear to span the lipid bilayer and may form a channel for the passage of the substrate. A third membrane protein hydrolyzes ATP to provide the energy required for concentrating the substrate inside the cell (4).Much of the information on sulfate uptake in enteric bacteria has accrued from studies of cysteine auxotrophs. Mutations that block sulfate transport map to the cysA locus; all three cistrons in this locus are required for normal uptake (34,35 In this communication, we present a characterization of the region of DNA adjacent to cysA and conclude that it encodes the remaining components of the sulfate permease. These conclusions are based on homology to analogous polypeptides from both the sulfate permease system of S. typhimurium and E. coli and the phenotype of mutant strains

The nonheme, iron-sulfur protein ferredoxin is the terminal constituent of the photosynthetic electron transport chain. Under conditions of iron stress, mnany cyanobacteria and eucaryotic algae replace ferredoxin with the flavoprotein flavodoxin. The gene for flavodoxin was cloned from the cyanobacterium Anacyslis nidulans R2 by using three mixed oligonudeotide probes derived from the partial Synechococcus sp. strain PCC 6301 amino acid sequence. Nucleotide sequence analysis revealed a 513sbase-pair open reading frame with a deduced amino acid sequence having homology to other long-chain flavodoxins. Assuming proteolytic cleavage of the initial methionine residue, the molecular weight of the A. nidulans R2 flavodoxin is 18,609. Southern blot hybridization under conditions of reduced stringency detected only one copy of the flavodoxin sequence in the A. nidulans R2 genome. Northern (RNA) blot hybridization analyses by using cloned flavodoxin gene probes indicated that no transcripts are detectable under conditions of iron saturation. However, under iron-deficient growth conditions the flavodoxin gene appeared to be transcribed as piwt of a larger operon. The operon yielded at least three transcripts.