Analysis of a herbicide resistant mutant obtained by transformation of the Chlamydomonas chloroplast

Silica Hydrogels as Entrapment Material for Microalgae

Despite being a promising feedstock for food, feed, chemicals, and biofuels, microalgal production processes are still uneconomical due to slow growth rates, costly media, problematic downstreaming processes, and rather low cell densities. Immobilization via entrapment constitutes a promising tool to overcome these drawbacks of microalgal production and enables continuous processes with protection against shear forces and contaminations. In contrast to biopolymer gels, inorganic silica hydrogels are highly transparent and chemically, mechanically, thermally, and biologically stable. Since the first report on entrapment of living cells in silica hydrogels in 1989, efforts were made to increase the biocompatibility by omitting organic solvents during hydrolysis, removing toxic by-products, and replacing detrimental mineral acids or bases for pH adjustment. Furthermore, methods were developed to decrease the stiffness in order to enable proliferation of entrapped cells. This review aims to provide an overview of studied entrapment methods in silica hydrogels, specifically for rather sensitive microalgae.

Rapid, ATP-dependent degradation of a truncated D1 protein in the chloroplast

The D1 protein constitutes one of the reaction center subunits of photosystem II and turns over rapidly due to photooxidative damage. Here, we studied the degradation of a truncated D1 protein. A plasmid with a precise deletion in the reading frame of the psbA gene encoding D1 was introduced into the chloroplast of Chlamydomonas reinhardtii. A homoplasmic mutant containing the desired gene was able to synthesize the truncated form of the polypeptide, but could not accumulate significant levels of it. As a consequence, other central photosystem II subunits did not assemble within the thylakoid membrane. In vivo pulse-chase experiments showed that the abnormal D1 protein is rapidly degraded in the light. Degradation was delayed in the light in the presence of an uncoupler, or when cells were incubated in the dark. Pulse-chase experiments performed in vitro indicate that an ATP and metal-dependent protease is responsible for the breakdown process. The paper describes the first in vivo and in vitro functional test for ATP-dependent degradation of a defect polypeptide in chloroplasts. The possible involvement of proteases similar to those removing abnormal proteins in prokaryotic organisms is discussed on the basis of proteases recently identified in chloroplasts.

Assembly of the Photosystem II oxygen-evolving complex is inhibited in psbA site-directed mutants of Chlamydomonas reinhardtii. Aspartate 170 of the D1 polypeptide

Photosystem II catalyzes the photooxidation of water to molecular oxygen, providing electrons to the photosynthetic electron transfer chain. The D1 and D2 chloroplast-encoded reaction center polypeptides bind cofactors essential for Photosystem II function. Transformation of the chloroplast genome of the eukaryotic green alga Chlamydomonas reinhardtii has allowed us to engineer site-directed mutants in which aspartate residue 170 of D1 is replaced by histidine (D170H), asparagine (D170N), threonine (D170T), or proline (D170P). Mutants D170T and D170P are completely deficient in oxygen evolution, but retain normal (D170T) or 50% (D170P) levels of Photosystem II reaction centers. D170H and D170N accumulate wild-type levels of PSII centers, yet evolve oxygen at rates approximately 45% and 15% those of control cells, respectively. Kinetic analysis of chlorophyll fluorescence in the mutants reveals a specific defect in electron donation to the reaction center. Measurements of oxygen flash yields in D170H show, however, that those reaction centers capable of evolving oxygen function normally. We conclude that aspartate residue 170 of the D1 polypeptide plays a critical role in the initial binding of manganese as the functional chloroplast oxygen-evolving complex is assembled.

A robust protocol for site-directed mutagenesis of the D1 protein inChlamydomonas reinhardtii: A PCR-splicedpsbA gene in a plasmid conferring spectinomycin resistance was introduced into apsbA deletion strain

In this paper, we describe a protocol to obtain a site-directed mutants in thepsbA gene ofChlamydomonas reinhardtii, which overcomes several drawbacks of previous protocols, and makes it possible to generate a mutant within a month. Since the large size of the gene, and the presence of four large introns has made molecular genetics of thepsbA gene rather unwieldy, we have spliced all of the exons of thepsbA gene by PCR to facilitate genetic manipulation and sequencing of the gene. The resultant construct (plasmid pBA153, with several unique restriction sites introduced at exon boundaries) carried 1.2 and 1.8 kb intact sequences from the 5'- and 3'-flanking regions, respectively. The plasmid was used to transform a D1-deletion mutant and was found to complement the deletion and restore photosynthetic activity. In addition, a bacterialaadA gene conferring spectinomycin resistance (spe (r)) was inserted downstream of the intron-freepsbA gene, to give construct pBA155. This allowed selection of mutant strains deficient in photosynthesis by using spectinomycin resistance, and eliminated the possibility of selection for revertant strains which is a consequence of having to use photosynthetic activity as a selection pressure. Finally, pBA155 was used to construct pBA157, in which additional restriction sites were inserted to facilitate cassette mutagenesis for generation of mutations in spans thought to be involved in donor-side interactions. AllpsbA deletion strains transformed with intron-freepsbA-aadA constructs encoding the wild-type D1 sequence, and screened on spectinomycin plates for thespe (r) phenotype, were able to grow photosynthetically, and all showed identical kinetics for electron transfer from primary (QA) to secondary quinone (QB) in Photosystem II, as assayed by the decay of the high fluorescence yield on oxidation of the reduced primary acceptor (QA (-)).

Construction of a Chlamydomonas reinhardtii mutant with an intronless psbA gene

Efficient chloroplast transformation systems now available allow the manipulation of the evolutionarily highly conserved psbA gene in the eucaryotic organism Chlamydomonas reinhardtii. Two copies of this gene in the inverted repeat region of the chloroplast genome contain four large group I introns. To analyse possible functions of these introns and to generate a mutant for simplified psbA gene manipulations, a psbA cDNA fragment was introduced into a psbA deletion mutant using the biolistic transformation method. A transformant with no introns in the psbA gene has been obtained and represents the first example of the removal of a complete set of introns from a chloroplast gene. The newly generated strain is photosynthetically competent and contains no detectable recipient genome copies. The loss of all four introns appears to be phenotypically silent.