DNA microarrays bearing nearly all of the genes of the unicellular cyanobacterium Synechocystis sp PCC 6803 were used to examine the temporal program of gene expression during acclimation from low to high light intensity. A complete pattern is provided of gene expression during acclimation of a photosynthetic organism to changing light intensity. More than 160 responsive genes were identified and classified into distinct sets. Genes involved in light absorption and photochemical reactions were downregulated within 15 min of exposure to high light intensity, whereas those associated with CO 2 fixation and protection from photoinhibition were upregulated. Changes in the expression of genes involved in replication, transcription, and translation, which were induced to support cellular proliferation, occurred later. Several unidentified open reading frames were induced or repressed. The possible involvement of these genes in the acclimation to high light conditions is discussed. INTRODUCTIONPhotosynthetic organisms must acclimate to changing light intensity in their environment. The acclimation process includes changes in the photosynthetic apparatus, presumably to balance energy input and consumption (Boardman, 1977). If energy supply (light harvesting and electron transport) exceeds its dissipation (by CO 2 fixation and other energy-demanding processes), particularly under high light (HL) conditions, the photosynthetic electron transport components could become relatively reduced. This may result in excess production of reactive oxygen species (ROS) leading to severe damage to many cellular processes (Asada, 1994). Absorption of excess light energy therefore must be avoided by reduction of both antenna size and photosystem content. Furthermore, under HL conditions the capacity for CO 2 fixation increases (Björkman, 1981;Anderson, 1986) and protection from ROS is enhanced (Grace and Logan, 1996).Acclimation from low light (LL) to HL conditions can be divided into short-term and long-term processes (Anderson, 1986;Anderson et al., 1995). Short-term acclimation includes state transitions, protective energy dissipation (Campbell et al., 1998;Niyogi, 1999), changes in the efficiency of energy transfer from the harvesting complex to photosystem II (PSII; Hassidim et al., 1997), and the formation of nonfunctional PSII reaction centers (Chow, 1994;Anderson et al., 1997). These responses occur rather rapidly and usually are completed within several minutes. Conversely, the long-term acclimation to HL is much slower because it involves changes in the composition, function, and structure of the photosynthetic apparatus as well as other photosynthesis-related components. Completion of the long-term processes may take hours or even days.Physiological responses to changing light intensity have been examined extensively Gibson, 1982a, 1982b;Anderson, 1986;Neale and Melis, 1986), and some of the relevant molecular mechanisms have been described (Anderson et al., 1995;Hihara, 1999;Niyogi, 1999). Nevertheless, the mechanisms that ...

DNA microarrays bearing nearly all of the genes of the unicellular cyanobacterium Synechocystis sp PCC 6803 were used to examine the temporal program of gene expression during acclimation from low to high light intensity. A complete pattern is provided of gene expression during acclimation of a photosynthetic organism to changing light intensity. More than 160 responsive genes were identified and classified into distinct sets. Genes involved in light absorption and photochemical reactions were downregulated within 15 min of exposure to high light intensity, whereas those associated with CO(2) fixation and protection from photoinhibition were upregulated. Changes in the expression of genes involved in replication, transcription, and translation, which were induced to support cellular proliferation, occurred later. Several unidentified open reading frames were induced or repressed. The possible involvement of these genes in the acclimation to high light conditions is discussed.

Every cyanobacterial species possesses multiple genes encoding AbrB-like transcriptional regulators (cyAbrBs) distinct from those conserved among other bacterial species. In this study, two genes encoding cyAbrBs in Synechocystis sp. PCC 6803, sll0359 and sll0822, were insertionally disrupted in order to examine their physiological roles. A fully segregated disrupted mutant of sll0822 (Dsll0822 mutant) but not of sll0359 was obtained, although both mutants exhibited similar phenotypes (i.e. decreases in growth rate and pigment content). The growth rate of the Dsll0822 mutant was low under any condition, but the low pigment content could be partially recovered by nitrate supplementation of the medium. DNA microarray and RNA-blot analyses revealed that the level of expression of a part of the NtcA regulon, such as urtA, amt1, glnB, sigE, and the nrt operon, was significantly decreased in the Dsll0822 mutant, although the induction of these genes upon nitrogen depletion was still observed to some extent. Sll0822 seems to work in parallel with NtcA to achieve flexible regulation of the nitrogen uptake system. The Sll0822 protein exists mainly in a dimeric form in vivo, and the amount of the protein was not affected by nitrogen availability. This observation, together with the low binding specificity of the purified histidine-tagged Sll0822 protein, implies that the activity of Sll0822 may be posttranslationally modulated in Synechocystis cells.

Whole-genome DNA microarrays were used to evaluate the effect of the redox state of the photosynthetic electron transport chain on gene expression in Synechocystis sp. strain PCC 6803. Two specific inhibitors of electron transport, 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and 2,5-dibromo-3-methyl-6-isopropylp-benzoquinone (DBMIB), were added to the cultures, and changes in accumulation of transcripts were examined. About 140 genes were highlighted as reproducibly affected by the change in the redox state of the photosynthetic electron transport chain. It was shown that some stress-responsive genes but not photosynthetic genes were under the control of the redox state of the plastoquinone pool in Synechocystis sp. strain PCC 6803.Photosynthetic organisms must cope with changes in their light environment by various acclimation responses. In order to acclimate to light regimens, organisms need sensors to monitor changing light regimen, signal transduction systems, and output mechanisms for rearrangement of their photosynthetic machinery. The sensing mechanism for changing light conditions and subsequent signal transduction are poorly understood (14). However, it has recently become clear that light-induced electron transport plays a very important role in both transcriptional and posttranscriptional regulations in various photosynthetic organisms. It has been reported that in green algae and higher plants, transcription (9, 17, 25), mRNA stability (2), splicing (8), translation (7, 18), and protein phosphorylation (29) are regulated by the redox state of the photosynthetic electron transport chain. In cyanobacteria, the main targets for redox regulation are transcription and stability of mRNA. mRNA levels of several photosynthesis-related genes, such as psbA, which encodes the reaction center D1 polypeptide of photosystem II (PSII), psaE, which codes for a subunit of photosystem I (PSI), cpcBA, which encodes ␤ and ␣ subunits of phycocyanin, and rbcLS, which encodes subunits of ribulose 1,5-bisphosphate carboxylase, were shown to be affected by the redox state of the photosynthetic electron transport chain (3,4,6,21). Also, the thioredoxin gene (23), genes for fatty acid desaturases (19), some nitrogen-regulated genes (5, 10, 28), and heat shock genes (12) were reported to be under the control of photosynthetic electron transport. This raised the question of how many genes in the whole genome are regulated by photosynthetic electron transport. To answer this question, we investigated the entire profile of accumulated transcripts upon the change in redox state of intersystem electron transfer components in the cyanobacterium Synechocystis sp. strain PCC 6803 with a whole-genome DNA microarray. The microarray was recently used to examine the temporal program of gene expression during acclimation from low to high light intensity (15).Effects of DCMU and DBMIB on the redox state of the photosynthetic electron transport chain. In the present study, we employed two electron transport inhibitors, 3-(3,4-dichloropheny...

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