Coordinated High-Light Response of Genes Encoding Subunits of Photosystem I Is Achieved by AT-Rich Upstream Sequences in the Cyanobacterium Synechocystis sp. Strain PCC 6803

Abstract

Genes encoding subunits of photosystem I (PSI genes) in the cyanobacterium Synechocystis sp. strain PCC 6803 are actively transcribed under low-light conditions, whereas their transcription is coordinately and rapidly down-regulated upon the shift to high-light conditions. In order to identify the molecular mechanism of the coordinated high-light response, we searched for common light-responsive elements in the promoter region of PSI genes. First, the precise architecture of the psaD promoter was determined and compared with the previously identified structure of the psaAB promoter. One of two promoters of the psaAB genes (P1) and of the psaD gene (P2) possessed an AT-rich light-responsive element located just upstream of the basal promoter region. These sequences enhanced the basal promoter activity under low-light conditions, and their activity was transiently suppressed upon the shift to high-light conditions. Subsequent analysis of psaC, psaE, psaK1, and psaLI promoters revealed that their light response was also achieved by AT-rich sequences located at the ؊70 to ؊46 region. These results clearly show that AT-rich upstream elements are responsible for the coordinated high-light response of PSI genes dispersed throughout Synechocystis genome.Photosynthetic organisms have ability to cope with the changes in light environment by modulating both the structure and the function of the photosynthetic machinery (31, 59). A typical example is the flexible control of the amounts of photosystem (PS) and light-harvesting antenna complexes depending on the availability of light energy (4, 27, 38). Under lightlimiting conditions, the amount of these complexes is maintained at high level, because maximal capture of light energy is required to fulfill the energy demand of cells. Under light-saturating conditions, on the other hand, they are largely downregulated since absorption of excess light energy tends to cause the generation of harmful reactive oxygen species (6).The dynamics of reaction center complexes during the process of high-light (HL) acclimation have been well characterized in cyanobacteria. Amount of PSI is more strictly downregulated than that of PSII upon the exposure to HL (28,40). The analysis of the pmgA mutant deficient in down-regulation of PSI content revealed that the selective repression of PSI is essential for growth under continuous HL conditions (28, 54). Although the primary determinant of PSI content under HL conditions has not been identified, transcriptional regulation is likely to be one of the important factors. The cyanobacterial PSI complex is comprised of about 11 subunits, with some exceptions (23), and genes encoding these subunits (PSI genes) are dispersed throughout the genome. In Synechocystis sp. strain PCC 6803, PSI genes are actively transcribed under low-light (LL) conditions, whereas their transcription is coordinately and rapidly down-regulated upon the shift to HL conditions (26,29,30,42,57), except for the psaK2 gene encoding an HL-inducible isoform of the PsaK subunit (19)...

Role of Multiple HLR1 Sequences in the Regulation of the Dual Promoters of thepsaABGenes inSynechocystissp. PCC 6803

“…In the P1 series, the luxAB transcript levels showed the typical pattern of HL response of PSI genes in strains P1A, P1Amt, P1B, and P1Bmt; the amount of the luxAB transcripts drastically decreased upon the shift to HL conditions and gradually increased with further incubation under HL (14)(15)(16). This indicates that HLR1A and HLR1B are not involved in the HL response of P1.…”

“…PCC 6803, genes encoding PSI subunits (PSI genes) are actively transcribed under low-light (LL) conditions, whereas their transcription is coordinately and strictly downregulated upon the shift to HL conditions preceding the decrease in protein level (5,8,9,14,21,23). We analyzed the architecture of PSI promoters and found that a high light regulatory 1 (HLR1) sequence located just upstream of the core promoter region is responsible for the coordinated HL response (15,16,19). The HLR1 sequence was initially identified by Eriksson et al (2) as a common sequence located upstream of HL-inducible genes such as psbA2, psbA3, hliA, and nblA in Synechocystis sp.…”

The Response Regulator RpaB Binds to the Upstream Element of Photosystem I Genes To Work for Positive Regulation under Low-Light Conditions in Synechocystis sp. Strain PCC 6803

“…strain PCC 6803 (the downstream region of the ndhB gene) and the spectinomycin resistance cassette (1,24). Reporter construct E1 containing the region from Ϫ70 to ϩ 90 of psaE, E2 containing the region from Ϫ45 to ϩ90 of psaE, and A61 containing the region from Ϫ69 to ϩ2 of psaA were generated as described in Muramatsu and Hihara (25). The base-substituted constructs, E1sub1 and E1sub2, were generated by using a KOD-Plus mutagenesis kit (Toyobo) according to the manufacturer's instruction.…”

“…PSI genes are actively transcribed under low-light (LL) conditions, whereas their transcription is coordinately and strictly downregulated upon the shift to HL conditions preceding the decrease in protein level (13,15,16,23,31). We conducted the deletion analysis of PSI promoters and found that an AT-rich upstream region from Ϫ70 to Ϫ46, relative to the transcription start site, is involved in upregulation of the promoter activity in every PSI gene (24,25). The addition of an AT-rich upstream element to the core promoter region stimulated the promoter activity 5-to 100-fold under LL conditions, whereas this positive regulation was suppressed within 1 h after the shift to HL.…”

Simultaneous Inactivation of Sigma Factors B and D Interferes with Light Acclimation of the Cyanobacterium Synechocystis sp. Strain PCC 6803

“…strain PCC 6803 (referred to hereafter strain 6803) is a unicellular cyanobacterium commonly used as a model organism in photosynthesis studies because of its suitability for genetic engineering (16). Cyanobacteria acclimate to changing light conditions by adjustments of antenna composition (23), by state transitions (19,55), and by varying the expression of genes coding for components of the photosynthetic apparatus (11,27). In addition to photosynthetic genes, many other genes involved in cellular processes are light regulated (13,14).…”