The cyanobacterium Synechococcus resists UV-B by exchanging photosystem II reaction-center D1 proteins

Abstract

Current ambient UV-B levels can significantly depress productivity in aquatic habitats, largely because UV-B inhibits several steps of photosynthesis, including the photooxidation of water catalyzed by photosystem II. We show that upon UV-B exposure the cyanobacterium Synechococcus sp. PCC 7942 rapidly changes the expression of a family of three psbA genes encoding photosystem II D1 proteins. In wild-type cells the psbAI gene is expressed constitutively, but strong accumulations of psbAII and psbAIII transcripts are induced within 15 min of moderate UV-B exposure (0.4 W/m2). This transcriptional response causes an exchange of two distinct photosystem II D1 proteins. D1:1 is encoded by psbAI, but on UV-B exposure, it is largely replaced by the alternate D1:2 form, encoded by both psbAII and psbAIII. The total content of D1 and other photosystem II reaction center protein, D2, remained unchanged throughout the UV exposure, as did the content and composition of the phycobilisome. Wild-type cells suffered only slight transient inhibition of photosystem II function under UV-B exposure. In marked contrast, under the same UV-B treatment, a mutant strain expressing only psbAI suffered severe (40%) and sustained inhibition of photosystem II function. Another mutant strain with constitutive expression of psbAII and psbAIII was almost completely resistant to the UV-B treatment, showing no inhibition of photosystem II function and only a slight drop in electron transport. In Synechococcus the rapid exchange of alternate D1 forms, therefore, accounts for much of the cellular resistance to UV-B inhibition of photosystem II activity and photosynthetic electron transport. This molecular plasticity may be an important element in community-level responses to UV-B, where susceptibility to UV-B inhibition of photosynthesis changes diurnally.

Figure 1

Relative spectral irradiance profiles of UV-B and blue light sources. UV-B, solid curve; blue light, dashed curve. The culture flasks blocked wavelengths less than 280 nm and thus the UV-C range was excluded. In all treatments, an incandescent growth light of 50 μmol of photons per m² per s was maintained. UV-B treatments added UV-B at culture surface intensities of 0.2, 0.4, or 0.8 W/m² (about 0.5, 1, and 2 μmol of UV-B per m² per s, respectively). Blue light treatments added culture surface intensities of 5 μmol of photons per m² per s (about 1.3 W/m²).

Figure 2

Differential expression of psbA transcripts after moderate UV-B irradiation supplemented to the standard growth light. Synechococcus sp. PCC 7942 wild-type cells grown at 50 μmol of photons per m² per s were exposed to supplementary UV-B at 0.4 W/m² for 2 h and then allowed to recover without UV-B for 30 min. Cell samples were taken at the indicated times for RNA isolations. The levels of psbAI, psbAII, and psbAIII mRNA were detected by hybridization with 100-bp DNA probes specific for the unique 5′ untranslated region of each psbA transcript. Representative autoradiograms from one of two replicates are shown. Molecular size markers in kb are indicated on the left.

Figure 3

Quantification of psbA transcripts under control, UV-B, and postexposure recovery treatments. (A) Levels of full-length 1.3-kb transcripts. psbAI, •; psbAII, □; psbAIII, ▵. Values plotted are the average of two independent replicates, expressed relative to the content at time zero. (B) Ratio of psbA mRNA species (1.3/0.25 kb). psbAI, •; psbAII, □; psbAIII, ▵. Values plotted are the average of two replicates.

Figure 4

Rapid exchange between D1:1 and D1:2 with increasing levels of supplemented UV-B irradiation and subsequent recovery. Synechococcus sp. PCC 7942 wild-type cells grown at 37°C with white light at 50 μmol of photons per m² per s and bubbled with 5% CO₂ were supplemented with UV-B at 0.2, 0.4, or 0.8 W/m² for 2 h and then allowed to recover for 1 h without UV-B. Protein extracts were taken at the indicated times, and the composition of D1 protein was determined by immunoblotting with polyclonal antibodies specific for either D1:1 or D1:2. The figure shows results representative of three replicates.

Figure 5

D1, D2, and phycobilisome protein content during moderate UV-B exposure and recovery. Wild-type cells were supplemented with UV-B at 0.4 W/m² for 2 h and then allowed to recover for 1 h without UV-B as described in Fig. 3. Total D1 (D1:1 and D1:2), D2, and phycobilisome were detected by using specific polyclonal antibodies. The figure shows results representative of three replicates.

Figure 6

D1:1 and D1:2 content in cells exposed to 50 μmol of white light per m² per s (control growth conditions) supplemented with 6 μmol of blue light per m² per s. The figure shows results representative of three replicates.

Figure 7

Photosynthetic activity under moderate UV-B exposure. Wild-type cells (○), initially containing D1:1 but switching to D1:2 within 30 min, R2S2C3 mutant cells containing only D1:1 protein (▪), and R2K1 mutant cells containing only D1:2 protein (♦). (A) PSII function measured by using the Chl fluorescence parameter F_V/F_M. (B) Photosynthetic electron transport measured as gross oxygen evolution. Values are expressed as a percentage of the 0-h control (100%) and represent the mean ± SEM (n = 3).