Procedures for the Generation of Mature Chlamydomonas reinhardtii Zygotes for Molecular and Biochemical Analyses

An Animal-Like Cryptochrome Controls the Chlamydomonas Sexual Cycle

Cryptochromes are known as flavin-binding blue light receptors in bacteria, fungi, plants, and insects. The animal-like cryptochrome (aCRY) of the green alga Chlamydomonas reinhardtii has extended our view on cryptochromes, because it responds also to other wavelengths of the visible spectrum, including red light. Here, we have investigated if aCRY is involved in the regulation of the sexual life cycle of C. reinhardtii, which is controlled by blue and red light at the steps of gametogenesis along with its restoration and germination. We show that aCRY is differentially expressed not only during the life cycle but also within the cell as part of the soluble and/or membrane-associated protein fraction. Moreover, localization of aCRY within the algal cell body varies between vegetative cells and the different cell types of gametogenesis. aCRY is significantly (early day) or to a small extent (late night) enriched in the nucleus in vegetative cells. In pregametes, gametes and dark-inactivated gametes, aCRY is localized over the cell body. aCRY plays an important role in the sexual life cycle of C. reinhardtii: It controls the germination of the alga, under which the zygote undergoes meiosis, in a positive manner, similar to the regulation by the blue light receptors phototropin and plant cryptochrome (pCRY). However, aCRY acts in combination with pCRY as a negative regulator for mating ability as well as for mating maintenance, opposite to the function of phototropin in these processes.

miRNAs control gene expression in the single-cell alga Chlamydomonas reinhardtii

MicroRNAs (miRNAs) in eukaryotes guide post-transcriptional regulation by means of targeted RNA degradation and translational arrest. They are released by a Dicer nuclease as a 21-24-nucleotide RNA duplex from a precursor in which an imperfectly matched inverted repeat forms a partly double-stranded region. One of the two strands is then recruited by an Argonaute nuclease that is the effector protein of the silencing mechanism. Short interfering RNAs (siRNAs), which are similar to miRNAs, are also produced by Dicer but the precursors are perfectly double-stranded RNA. These siRNAs guide post-transcriptional regulation, as with miRNAs, and epigenetic genome modification. Diverse eukaryotes including fungi, plants, protozoans and metazoans produce siRNAs but, until now, miRNAs have not been described in unicellular organisms and it has been suggested that they evolved together with multicellularity in separate plant and animal lineages. Here we show that the unicellular alga Chlamydomonas reinhardtii contains miRNAs, putative evolutionary precursors of miRNAs and species of siRNAs resembling those in higher plants. The common features of miRNAs and siRNAs in an alga and in higher plants indicate that complex RNA-silencing systems evolved before multicellularity and were a feature of primitive eukaryotic cells.

A complex system of small RNAs in the unicellular green alga Chlamydomonas reinhardtii

Endogenous small RNAs function in RNA interference (RNAi) pathways to control gene expression through mRNA cleavage, translational repression, or chromatin modification. Plants and animals contain many microRNAs (miRNAs) that play vital roles in development, including helping to specify cell type and tissue identity. To date, no miRNAs have been reported in unicellular organisms. Here we show that Chlamydomonas reinhardtii, a unicellular green alga, encodes many miRNAs. We also show that a Chlamydomonas miRNA can direct the cleavage of its target mRNA in vivo and in vitro. We further show that the expression of some miRNAs/Candidates increases or decreases during Chlamydomonas gametogenesis. In addition to miRNAs, Chlamydomonas harbors other types of small RNAs including phased small interfering RNAs (siRNAs) that are reminiscent of plant trans-acting siRNAs, as well as siRNAs originating from protein-coding genes and transposons. Our findings suggest that the miRNA pathway and some siRNA pathways are ancient mechanisms of gene regulation that evolved prior to the emergence of multicellularity.

Functional analysis and regulation of the malate synthase from Chlamydomonas reinhardtii

Malate synthase (EC 2.3.3.9, formerly EC 4.1.2.2) has been investigated in the unicellular green algae Chlamydomonas reinhardtii. The molecular characteristics and the regulation of gene expression have been investigated for the enzyme. A full-length malate synthase cDNA has been isolated, containing an open reading frame of 1,641 bp encoding a polypeptide of 546 amino acids. This protein shares the conserved signature of the malate synthase family, along with the catalytic residues essential for enzymatic activity and a C-terminal motif that matches the consensus for glyoxysome import. Functionality studies have been facilitated by heterologous expression of the malate synthase cDNA in Escherichia coli. The remarkable metabolic versatility of the alga has been used to analyse the metabolic control of malate synthase gene expression. The data strongly support the role of acetate and light as the main regulatory effectors, and the existence of cross-talk between the two signalling pathways.

Genes expressed during sexual differentiation of Chlamydomonas reinhardtii

Four genes specifically expressed during gametogenesis of Chlamydomonas reinhardtii have been cloned and their expression patterns analyzed. mRNAs encoded by these gamete-specific genes (gas) were absent or present only at very low levels in vegetative cells and mature zygotes. In young zygotes 2 h after gamete fusion, the mRNAs of three gas genes still persisted. The gas mRNAs accumulated during gametic differentiation. The temporal patterns of accumulation of individual mRNAs differed; some started to increase early during gametogenesis, others accumulated in the late phase. The accumulation of one of the late mRNAs (gas28) was strictly light-dependent. To illustrate the utility of the genes cloned in the analysis of sexual differentiation in Chlamydomonas reinhardtii we show that in a gametogenesis-defective mutant, the expression of late genes is prevented while that of early genes is normal.

Gametic Differentiation of Chlamydomonas reinhardtii: Control by Nitrogen and Light

Gametic differentiation of the unicellular green alga Chlamydomonas reinhardtii proceeds in two steps controlled by the extrinsic signals nitrogen deficiency and light. Nitrogen deprivation induces the differentiation of vegetative cells to sexually immature pregametes. A light signal is required to convert the pregametes to gametes. Both signals are also required for the maintenance of mating competence. Two converging signal transduction chains are proposed to control gamete formation. For the differentiation of pregametes to gametes, a fluence rate-dependent reaction, requiring continuous irradiation, is suggested by photobiological experiments.

Action Spectrum for the Light-Dependent Step in Gametic Differentiation of Chlamydomonas reinhardtii

Differentiation of Chlamydomonas reinhardtii vegetative cells to gametes requires two environmental signals: nitrogen starvation and light. Vegetative cells incubated without nitrogen differentiate into pregametes. Pregametes can be converted into sexually mature gametes by irradiation with light. The action spectrum for the light-dependent step in gamete formation showed two maxima at 370 and 450 nanometers. This is similar to the spectrum of other blue light/ultraviolet light-A-absorbing photoreceptors.

Identification of novel genes specifically expressed in Chlamydomonas reinhardtii zygotes

The maturation of zygotes formed by the fusion of two gametes is the essential part of the diploid phase of the Chlamydomonas reinhardtii sexual life cycle and results in mature zygotes competent to germinate. To understand the molecular mechanisms underlying zygote maturation and the attainment of competence for germination we isolated genomic clones representing three different genes that are specifically expressed in Chlamydomonas reinhardtii zygotes. Accumulation of the RNAs started more than 24 h after mating, setting these genes apart from genes expressed in young zygotes. Upon light-induced germination of zygotes, the mRNAs disappeared. The patterns of RNA accumulation and disappearance were gene-specific and suggested a function of these genes in maturation and/or in initial steps of germination.