Chloroplastic genomes of Ginkgo biloba and Chlamydomonas moewusii contain a chlB gene encoding one subunit of a light-independent protochlorophyllide reductase

Effects of sodium sulfate on the freshwater microalga Chlamydomonas moewusii: implications for the optimization of algal culture media

The study of the microalgal growth kinetics is an indispensable tool in all fields of phycology. Knowing the optimal nutrient concentration is an important issue that will help to develop efficient growth systems for these microorganisms. Although nitrogen and phosphorus are well studied for this purpose, sulfur seems to be less investigated. Sulfate is a primary sulfur source used by microalgae; moreover, the concentration of this compound is increasing in freshwater systems due to pollution. The aim of this study was to investigate the effects of different sodium sulfate concentrations in the culture medium on growth and growth kinetics of the freshwater microalga Chlamydomonas moewusii. Production of biomass, chl content, kinetic equations, and a mathematical model that describe the microalgal growth in relation with the concentration of sodium sulfate were obtained. The lowest concentration of sodium sulfate allowing optimal growth was 0.1 mM. Concentrations higher than 3 mM generated a toxic effect. This work demonstrates that this toxic effect was not directly due to the excess of sulfate ion but by the elevation of the ionic strength. An inhibition model was successfully used to simulate the relationship between specific growth rate and sodium sulfate in this microalga.

Light and temperature regulation of greening in dark-grown ginkgo (Ginkgo biloba)

The last steps of chlorophyll (Chl) biosynthesis were studied at different light intensities and temperatures in dark-germinated ginkgo (Ginkgo biloba L.) seedlings. Pigment contents and 77 K fluorescence emission spectra were measured and the plastid ultrastructure was analysed. All dark-grown organs contained protochlorophyllide (Pchlide) forms with similar spectral properties to those of dark-grown angiosperm seedlings, but the ratios of these forms to each other were different. The short-wavelength, monomeric Pchlide forms were always dominating. Etioplasts with small prolamellar bodies (PLBs) and few prothylakoids (PTs) differentiated in the dark-grown stems. Upon illumination with high light intensities (800 micromol m(-2) s(-1) photon flux density, PFD), photo-oxidation and bleaching occurred in the stems and the presence of (1)O(2) was detected. When Chl accumulated in plants illuminated with 15 micromol m(-2) s(-1) PFD it was significantly slower at 10 degrees C than at 20 degrees C. At room temperature, the transformation of etioplasts into young chloroplasts was observed at low light, while it was delayed at 10 degrees C. Grana did not appear in the plastids even after 48 h of greening at 20 degrees C. Reaccumulation of Pchlide forms and re-formation of PLBs occurred when etiolated samples were illuminated with 200 micromol m(-2) s(-1) PFD at room temperature for 24 h and were then re-etiolated for 5 days. The Pchlide forms appeared during re-etiolation had similar spectral properties to those of etiolated seedlings. These results show that ginkgo seedlings are very sensitive to temperature and light conditions during their greening, a fact that should be considered for ginkgo cultivation.

Expression Switching of Three Genes Encoding Light-independent Protochlorophyllide Oxidoreductase in Chlorella protothecoides

Three chloroplast genes, chlL, chlN and chlB, encoding the light-independent protochlorophyllide oxidoreductase (LIPOR) in Chlorella protothecoides CS-41 growing either photoautotrophically, mixotrophically or heterotrophically, were all transcribed constitutively independent of illumination and presence of glucose. Steady-state amounts of all three transcripts in the light-grown cells were, however, approximately two- to three-fold greater than those in the dark-grown cells. In addition, Western blotting demonstrated that approximately the same amount of protein was present in cultures grown mixtrophically or heterotrophically both containing glucose. However, much less protein was in photoautotrophic cells. These results suggest that LIPOR activity depends on post-transcriptional and post-translational regulation. Moreover, the fact that LIPOR accumulates in the light-grown cells indicates that LIPOR, which was thought to work only in darkness, may partially account for the protochlorophyllide photoreduction in light. Therefore, LIPOR switches on both in light and in darkness.

Relaxation of Functional Constraint on Light-Independent Protochlorophyllide Oxidoreductase in Thuja

The light-independent protochlorophyllide oxidoreductase (DPOR) plays a key role in the ability of nonflowering plants and algae to synthesize chlorophyll in darkness. This enzyme consists of three subunits encoded by the chlB, chlL, and chlN genes in the plastid genome. Previously, we found a high nonsynonymous substitution rate (dN) of the chlL gene in the lineage of Thuja standishii, a conifer belonging to the Cupressaceae. Here we revealed that the acceleration of dN in the chlL occurred as well in other species of Thuja, Thuja occidentalis and Thuja plicata. In addition, dark-grown seedlings of T. occidentalis were found to exhibit a pale yellowish color, and their chlorophyll concentration was much lower than that of other species of Cupressaceae. The results suggested that the species of Thuja have lost the ability to synthesize chlorophyll in darkness, and the functional constraint on the DPOR would thus be expected to be relaxed in this genus. Therefore, we expected to find that the evolutionary rates of all subunits of DPOR would in this case be accelerated. Sequence analyses of the chlN and chlB (encoding the other subunits of DPOR) in 18 species of Cupressaceae revealed that the dN of the chlN gene was accelerated in Thuja as was the dN of the chlL gene, but the dN of the chlB gene did not appear to differ significantly among the species of Cupressaceae. Sequencing of reverse transcription-polymerase chain reaction (RT-PCR) products of these genes showed that RNA editing was rare and unlikely to have contributed to the acceleration. Moreover, the RT-PCR analysis indicated that all chl genes were still transcriptionally active in T. occidentalis. Based on these results, it appears that species of Thuja still bear the DPOR protein, although the enzyme has lost its activity because of nonsynonymous mutations of some of the chl genes. The lack of acceleration of the dN of the chlB gene might be accounted for by various unknown functions of its gene product.

Protochlorophyllide reduction: mechanisms and evolutions

Protochlorophyllide (Pchlide) reductases are key enzymes in the process of chlorophyll biosynthesis. In this review, current knowledge on the molecular organization, substrate specificity and assembly of the light-dependent reduced nicotinamide adenine dinucleotide phosphate:Pchlide oxidoreductases are discussed. Characteristics of light-independent enzymes are also described briefly, and the possible reasons for the selection of light-dependent enzymes during the course of evolution are discussed.

Photosynthetic electron transport regulates the stability of the transcript for the protochlorophyllide oxidoreductase gene in the liverwort, Marchantia paleacea var. diptera

The transfer of Marchantia paleacea var. diptera cells to darkness caused a reversible repression in the accumulation of transcript for a gene, por, encoding the NADPH: protochlorophyllide oxidoreductase (EC 1.3.1.33). The photosynthetic inhibitor DCMU and DBMIB repressed the accumulation in light. In the presence of transcription inhibitor cordycepin, not only incubation in the dark but also addition of DCMU or DBMIB in light stimulated the degradation of the por transcript. These findings suggest that photosynthetic electron transport is involved in regulating the stability of the por transcript.

Genetic analysis of chlorophyll biosynthesis

During this decade, there have been major advancements in the understanding of genetic loci involved in synthesis of the family of Mg-tetrapyrroles known as chlorophylls and bacteriochlorophylls. Molecular genetic analysis of Mg-tetrapyrrole biosynthesis was initiated by the performance of detailed sequence and mutational analysis of the photosynthesis gene cluster from Rhodobacter capsulatus. These studies provided the first detailed understanding of genes involved in bacteriochlorophyll a biosynthesis. In the short time since these studies were initiated, most of the chlorophyll biosynthesis genes have been identified by virtue of their ability to complement bacteriochlorophyll a biosynthesis mutants as well as by sequence homology comparisons. This review is centered on a discussion of our current understanding of bacterial, algal, and plant genes that code for enzymes in the Mg-branch of the tetrapyrrole biosynthetic pathway that are responsible for synthesis of chlorophylls and bacteriochlorophylls.

Protochlorophyllide reductase in photosynthetic prokaryotes and its role in chlorophyll synthesis

1. DNA sequences hybridizing with the wheat protochlorophyllide reductase gene have been detected during genomic Southern blots of various cyanobacterial DNA samples. No such hybridization was observed with DNA from photosynthetic bacteria. 2. A fragment amplified from Phormidium laminosum DNA has been characterized and shown to be 73% similar to the corresponding wheat sequence. At the protein level the similarity is 91%. When used as a probe for Southern blotting the Phormidium DNA fragment confirmed the authenticity of some of the original signals obtained with the wheat probe. 3. Peptides of molecular mass 36, 30 and 60 kDa are immunodetected by a wheat reductase antibody during Western blotting of Phormidium preparations. These are purported to correspond to the cyanobacterial mature reductase protein, a stable proteolytic fragment and soluble dimeric forms of the latter respectively. 4. Adaptation of Phormidium to growth in red light (delta > 670 nm) or darkness led to no significant changes in the total level of immunodetected peptides or protochlorophyllide within the cells. 5. The specific activity of the reductase in Phormidium membranes has been tentatively estimated as 0.5 unit/mg of protein, a value comparable with that found in preparations from mature chloroplasts of higher plants.