Cloning and characterization of the actin-encoding gene of Chlamydomonas reinhardtii

Isolation and characterization of Ulva prolifera actin1 gene and function verification of the 5' flanking region as a strong promoter

Ulva prolifera is a green macroalgae with an extremely high growth rate that can accumulate biomass with considerable protein content. To set up an available seaweed expression system, a prior step is to isolate endogenous and strong constitutive promoters. For this reason, the full-length genomic actin1 gene from U. prolifera (Upactin1) was cloned and its 5′ flanking sequence was obtained by genome walking. The Upactin1 open reading frame consisted of 1134 nucleotides encoding 377 amino acid residues. Besides 4 exons and 3 introns in the coding region, an extra leader intron was identified in the 5′ untranslated region. According to quantitative GUS assays based on transient expression, the promoter activity of the Upactin1 5′ flanking region was found to be several times higher than that of the widely-used cauliflower mosaic virus 35S (CaMV35S) in all tested species of Ulva. In addition, precise deletion of the leader intron led to a significant decrease of promoter strength in U. prolifera, and almost entire loss of strength in U. linza and U. pertusa. To our knowledge, this is the first report to prove function of a leader intron in algae. The 5′ flanking region of Upactin1 was shown to be a much stronger promoter than the foreign CaMV35S, and its activity was highly dependent on the presence of the leader intron. We propose that the Upactin1 promoter could serve as an endogenous and strong constitutive element for genetic engineering of U. prolifera.

In silico cloning and characterization of the glycerol-3-phosphate dehydrogenase (GPDH) gene family in the green microalga Chlamydomonas reinhardtii

Glycerol-3-phosphate dehydrogenase (GPDH) catalyzes the conversion of dihydroxyacetone phosphate (DHAP) and NADH to glycerol-3-phosphate (G3P) and NAD(+). G3P is important as a precursor for glycerol and glycerolipid synthesis in microalgae. A GPDH enzyme has been previously purified from the green microalga Chlamydomonas reinhardtii, however, no genes coding for GPDH have been characterized before. In this study, we report the in silico characterization of three putative GPDH genes from C. reinhardtii: CrGPDH1, CrGPDH2, and CrGPDH3. These sequences showed a significant similarity to characterized GPDH genes from the microalgae Dunaliella salina and Dunaliella viridis. The prediction of the three-dimensional structure of the proteins showed the characteristic fold topology of GPDH enzymes. Furthermore, the phylogenetic analysis showed that the three CrGPDHs share the same clade with characterized GPDHs from Dunaliella suggesting a common evolutionary origin and a similar catalytic function. In addition, the K(a)/K(s) ratios of these sequences suggested that they are under purifying selection. Moreover, the expression analysis showed a constitutive expression of CrGPDH1, while CrGPDH2 and CrGPDH3 were induced in response to osmotic stress, suggesting a possible role for these two sequences in the synthesis of glycerol as a compatible solute in osmoregulation, and perhaps also in lipid synthesis in C. reinhardtii. This study has provided a foundation for further biochemical and genetic studies of the GPDH family in this model microalga, and also opportunities to assess the potential of these genes to enhance the synthesis of TAGs for biodiesel production.

Extremely low polymerizability of a highly-divergent Chlamydomonas actin (NAP)

Novel actin-like protein (NAP) is a highly divergent actin expressed in Chlamydomonas. With its low sequence similarity, it is uncertain whether NAP can polymerize into filaments. Here I assessed it by ectopically expressing enhanced green fluorescent protein-tagged NAP (EGFP-NAP) in cultured cells. EGFP-NAP was excluded from stress fibres but partially co-localized with endogenous actin in the cell periphery. In fluorescence recovery after photobleaching experiment, turnover rate of EGFP-NAP was similar to the estimated diffusion rate of monomeric actin. Therefore, EGFP-NAP likely accumulates by diffusion. These findings suggest that NAP has extremely poor ability to polymerize.

An axonemal PP2A B-subunit is required for PP2A localization and flagellar motility

Analysis of Chlamydomonas axonemes revealed that the protein phosphatase, PP2A, is localized to the outer doublet microtubules and is implicated in regulation of dynein-driven motility. We tested the hypothesis that PP2A is localized to the axoneme by a specialized, highly conserved 55-kDa B-type subunit identified in the Chlamydomonas flagellar proteome. The B-subunit gene is defective in the motility mutant pf4. Consistent with our hypothesis, both the B- and C- subunits of PP2A fail to assemble in pf4 axonemes, while the dyneins and other axonemal structures are fully assembled in pf4 axonemes. Two pf4 intragenic revertants were recovered that restore PP2A to the axonemes and re-establish nearly wild-type motility. The revertants confirmed that the slow-swimming Pf4 phenotype is a result of the defective PP2A B-subunit. These results demonstrate that the axonemal B-subunit is, in part, an anchor protein required for PP2A localization and that PP2A is required for normal ciliary motility.

Nucleotide diversity in the mitochondrial and nuclear compartments of Chlamydomonas reinhardtii: investigating the origins of genome architecture

Background

The magnitude of intronic and intergenic DNA can vary substantially both within and among evolutionary lineages; however, the forces responsible for this disparity in genome compactness are conjectural. One explanation, termed the mutational-burden hypothesis, posits that genome compactness is primarily driven by two nonadaptive processes: mutation and random genetic drift – the effects of which can be discerned by measuring the nucleotide diversity at silent sites (π_silent), defined as noncoding sites and the synonymous sites of protein-coding regions. The mutational-burden hypothesis holds that π_silent is negatively correlated to genome compactness. We used the model organism Chlamydomonas reinhardtii, which has a streamlined, coding-dense mitochondrial genome and an noncompact, intron-rich nuclear genome, to investigate the mutational-burden hypothesis. For measuring π_silent we sequenced the complete mitochondrial genome and portions of 7 nuclear genes from 7 geographical isolates of C. reinhardtii.

Results

We found significantly more nucleotide diversity in the nuclear compartment of C. reinhardtii than in the mitochondrial compartment: net values of π_silent for the nuclear and mitochondrial genomes were 32 × 10^-3 and 8.5 × 10^-3, respectively; and when insertions and deletions (indels) are factored in, these values become 49 × 10^-3 for the nuclear DNA and 11 × 10^-3 for the mitochondrial DNA (mtDNA). Furthermore, our investigations of C. reinhardtii revealed 4 previously undiscovered mitochondrial introns, one of which contains a fragment of the large-subunit (LSU) rRNA gene and another of which is found in a region of the LSU-rRNA gene not previously reported (for any taxon) to contain introns.

Conclusion

At first glance our results are in opposition to the mutational-burden hypothesis: π_silent was approximately 4 times greater in the nuclear compartment of C. reinhardtii relative to the mitochondrial compartment. However, when we consider the encumbrance of noncoding DNA in each of these C. reinhardtii compartments, we conclude that introns in the mtDNA impose a greater burden than those in the nuclear DNA and suggest that the same may be true for the intergenic regions. Overall, we cannot reject the mutational-burden hypothesis and feel that more data on nucleotide diversity from green algae and other protists are needed.

Quantitative analysis of cell-type specific gene expression in the green alga Volvox carteri

Background

The multicellular alga Volvox carteri possesses only two cell types: mortal, motile somatic cells and potentially immortal, immotile reproductive cells. It is therefore an attractive model system for studying how cell-autonomous cytodifferentiation is programmed within a genome. Moreover, there are ongoing genome projects both in Volvox carteri and in the closely related unicellular alga Chlamydomonas reinhardtii. However, gene sequencing is only the beginning. To identify cell-type specific expression and to determine relative expression rates, we evaluate the potential of real-time RT-PCR for quantifying gene transcript levels.

Results

Here we analyze a diversified pool of 39 target genes by real-time RT-PCR for each cell type. This gene pool contains previously known genes with unknown localization of cellular expression, 28 novel genes which are described in this study for the first time, and a few known, cell-type specific genes as a control. The respective gene products are, for instance, part of photosynthesis, cellular regulation, stress response, or transport processes. We provide expression data for all these genes.

Conclusion

The results show that quantitative real-time RT-PCR is a favorable approach to analyze cell-type specific gene expression in Volvox, which can be extended to a much larger number of genes or to developmental or metabolic mutants. Our expression data also provide a basis for a detailed analysis of individual, previously unknown, cell-type specifically expressed genes.

Impaired flagellar regeneration due to uncoordinated expression of two divergent actin genes in Chlamydomonas

Chlamydomonas has two actin genes: one encoding a conventional actin (90% amino acid identity with mammalian actin) and the other a highly divergent actin (NAP; 64% identity). The expression of the two genes is regulated in a mutually exclusive manner. Thus, ida5, a mutant that lacks the conventional actin (CrA) gene, expresses NAP abundantly, whereas wild-type cells express NAP only negligibly under normal conditions. To explore the physiological significance of the two actins, chimeric genes with the 5' upstream region of one gene replaced by that of the other were constructed and used to transform ida5. The transformant (TF5) with a chimeric clone comprising the 5'-untranslated region from the NAP gene and the CrA-encoding sequence recovered the dyneins missing in ida5 and showed almost normal motility. After deflagellation of this transformant, however, only about 30% of cells grew flagella, unlike wild-type cells, >80% of which displayed reflagellation. Transformant TF10, which contains the CrA upstream region and NAP coding region, underwent reflagellation normally, as did the parent strain, ida5. In TF5, the mRNA level of both CrA and NAP was increased greatly during reflagellation. In light of the recent finding that NAP mRNA is expressed transiently upon reflagellation in wild-type cells, the described results suggest that 1) the expression of NAP mRNA is indispensable for flagellation and 2) robust expression of CrA may inhibit proper flagellation by interfering with the function of NAP in the early stages of reflagellation.

Use of Physcomitrella patens actin 5' regions for high transgene expression: importance of 5' introns

We have isolated four actin (Act) genes from Physcomitrella patens and used their corresponding 5' regions for recombinant expression of the human vascular endothelial growth factor (rhVEGF121) in transiently transformed Physcomitrella protoplasts and in stable transformed lines. In the transient system, we found up to 11-fold activity of the corresponding 5' regions as compared with that of the plant constitutive 35S promoter. Moreover, the use of an optimised expression vector in which the human VEGF signal peptide was exchanged with a plant signal peptide resulted in an additional 7-fold increase in secreted rhVEGF. We found that the 5' introns of PpAct1, PpAct5 and PpAct7 are essential for high expression. The enhancing mechanisms of the introns, however, seem to be different since in the case of PpAct1, the expression level is stimulated only in the presence of the endogenous promoter, whereas the 5' introns of PpAct5 and PpAct7 stimulate expression also in combination with the 35S promoter. Beyond this, the isolated 5' regions are shown to be useful for high expression levels in transgenic moss lines with values of secreted rhVEGF up to 96 microg g(-1) dry weight.

The transcriptional program of synchronous gametogenesis in Chlamydomonas reinhardtii

Cells of Chlamydomonas reinhardtii undergo gametogenesis to produce sexually competent gametes under nitrogen-starved conditions. By using a synchronized system for gametogenesis of early G1 cells, several previously identified marker genes and 18 novel nitrogen-starved gametogenesis (NSG) genes isolated by macroarray analysis were placed into at least three temporal classes of expression. Early genes are induced transiently in the first 2 h after transfer to nitrogen-free medium. Middle genes are strongly induced between 3 h and 4 h after nitrogen removal, a time corresponding to the acquisition of mating competency, suggesting their involvement in the gamete program. Late genes are induced between 5 h and 8 h after nitrogen removal, a time after the completion of gametic differentiation, suggesting that they are not directly involved in the formation of sexually competent gametes. All of the 18 NSG genes examined are induced in both mating-type plus and minus gametes and about two-thirds of the genes are also expressed in the mitotic cell cycle, especially at S/M phases.

Turnover of actin in Chlamydomonas flagella detected by fluorescence recovery after photobleaching (FRAP)

Recent indirect observations have suggested that various axonemal proteins in cilia and flagella of live cells undergo turnover independently of shortening or elongation of the axoneme. To gain direct evidence, here we examined using a FRAP (fluorescence recovery after photobleaching) technique whether actin, a subunit of inner arm dynein, is being turned over in Chlamydomonas flagella. Fluorescently labeled rabbit actin was introduced by electroporation into the cells of ida5oda1, a double mutant between oda1 lacking outer arm dynein and ida5 lacking several species of inner arm dyneins due to the absence of a conventional-type actin. In actin-loaded cells, flagella became motile and fluorescent due to incorporation of inner-arm dyneins containing the labeled actin. Cells were sandwiched between an agar layer and a coverslip so as to restrict flagellar movement. After a small portion of a flagellum was photobleached, the fluorescence intensity in the bleached area was monitored with a sensitive video camera. The fluorescence intensity in the photobleached region was found to recover 10-40% of the original level over several tens of minutes without changing its position. The time course and extent of the recovery varied greatly from one cell to another, suggesting that the turnover depends on cellular conditions. Western blot analysis indicated that 70-80% of flagellar actin was associated with the axoneme. Hence this experiment provides direct evidence that an axonemal component undergoes dynamic exchange in stationary flagella.

Isolation and characterization of a single-copy actin gene from a sterile mutant of Ulva pertusa (Ulvales, Chlorophyta)

We constructed a cDNA library from sterile Ulva pertusa (Ulvales, Chlorophyta), and isolated and characterized a full-length cDNA clone encoding actin. The actin (ACT) cDNA consisted of 1487 nucleotides (nt) and had an open reading frame (ORF) encoding a polypeptide of 377 amino acid (AA) residues. The ACT gene had one intron in the 5'-untranslated region and three introns in the coding region. Transcription started 26 nt downstream of the putative TATA box. A potential polyadenylation signal, TGTAG, was located 100 nt downstream of the terminator codon, TAG. Amino acid alignment with actins from various algae and land plants showed that sterile U. pertusa actin was more similar to actins from Chlorophyta, Phaeophyta, Euglenophyta, and higher plants (over 76.9%) than to actins from Rhodophyta. Southern blot analysis indicated that the sterile U. pertusa genome has only a single actin-encoding gene. Thalli grown on a 12D/12L photoperiod increased in surface area some two-fold over 24 h regardless of the nutritional conditions. The growth rate of thalli during the light period was significantly higher than that during the dark period. Northern hybridization indicated that the expression of actin mRNA was induced and repressed by the light and dark treatments, respectively. These results suggest that the U. pertusa cell division cycle has a periodicity of 24 h and that the ACT gene is highly transcribed during cell growth and development in the light period.

A novel family of unconventional actins in volvocalean algae

The unicellular green alga Chlamydomonas reinhardtii has two actin genes, one encoding a conventional actin (90% amino acid identity with mammalian actin), the other a highly divergent actin (64% identity) named novel actin-like protein (NAP). To see whether the presence of conventional and unconventional actins in a single organism is unique to C. reinhardtii, we searched for genomic sequences related to the NAP sequence in several other species of volvocalean algae. Here we show that Chlamydomonas moewusii and Volvox carteri also have, in addition to a conventional actin, an unconventional actin similar to the C. reinhardtii NAP. Analyses of the deduced protein sequences indicated that the NAP homologues form a distinct group derived from conventional actin.

Expression of conventional and unconventional actins in Chlamydomonas reinhardtii upon deflagellation and sexual adhesion

Chlamydomonas has two actin genes, one coding for a conventional actin and the other coding for a highly divergent actin. The divergent actin NAP (for "novel actin-like protein") is expressed only negligibly in wild-type cells but abundantly in a null mutant of conventional actin, the ida5 mutant. The presence of the dormant NAP gene suggests that NAP may also have its own function in wild-type cells under some conditions. However, no specific functions have been suggested. In this study, we examined the expression of actin and NAP in wild-type and ida5 cells under conditions where actin function has been shown to be important. We found that deflagellation induces the expression of NAP as well as that of actin in wild-type cells. The expressed NAP becomes localized to the regrown flagella, apparently without being associated with dynein. Mating of gametes also increased the expression of actin in wild-type cells and that of NAP in ida5 cells, resulting in accumulation of these proteins in flagella (in both wild-type and ida5 cells) and the fertilization tubule (only in wild-type cells). However, it did not induce significant NAP expression in wild-type cells. These and other observations suggest that the expression of actin and NAP mRNAs is controlled by two discrete mechanisms and that NAP plays a role in flagellar formation in wild-type cells.

Molecular map of the Chlamydomonas reinhardtii nuclear genome

We have prepared a molecular map of the Chlamydomonas reinhardtii genome anchored to the genetic map. The map consists of 264 markers, including sequence-tagged sites (STS), scored by use of PCR and agarose gel electrophoresis, and restriction fragment length polymorphism markers, scored by use of Southern blot hybridization. All molecular markers tested map to one of the 17 known linkage groups of C. reinhardtii. The map covers approximately 1,000 centimorgans (cM). Any position on the C. reinhardtii genetic map is, on average, within 2 cM of a mapped molecular marker. This molecular map, in combination with the ongoing mapping of bacterial artificial chromosome (BAC) clones and the forthcoming sequence of the C. reinhardtii nuclear genome, should greatly facilitate isolation of genes of interest by using positional cloning methods. In addition, the presence of easily assayed STS markers on each arm of each linkage group should be very useful in mapping new mutations in preparation for positional cloning.

Dynein motors of the Chlamydomonas flagellum

Chlamydomonas is a biflagellate unicellular green alga that has proven especially amenable for the analysis of microtubule (MT)-based molecular motors, notably dyneins. These enzymes form the inner and outer arms of the flagellum and are also required for intraflagellar transport. Dyneins have masses of approximately 1-2 MDa and consist of up to 15 different polypeptides. Nucleotide binding/hydrolysis and MT motor activity are associated with the heavy chains, and we detail here our current model for the substructural organization of these approximately 520-kDa proteins. The remaining polypeptides play a variety of roles in dynein function, including attachment of the motor to cargo, regulation of motor activity in response to specific inputs, and their necessity for the assembly and/or stability of the entire complex. The combination of genetic, physiological, structural, and biochemical approaches has made the Chlamydomonas flagellum a very powerful model system in which to dissect the function of these fascinating molecular motors.

Chlamydomonas reinhardtiiproduces a profilin with unusual biochemical properties

We report the characterization of a profilin orthologue from Chlamydomonas reinhardtii. CrPRF, probably the only profilin isoform, is present in both the cell body and flagella. Examination of vegetative and gametic cells by immunofluorescence microscopy using multiple fixation procedures also revealed enrichment of CrPRF at the anterior of the cell near the base of flagella and near the base of the fertilization tubule in mating type plus gametes. Purified, recombinant CrPRF binds to actin with a Kd value ∼10–7 and displaces nuclei in a live cell ‘nuclear displacement’ assay, consistent with profilin’s ability to bind G-actin in vivo. However, when compared with other profilin isoforms, CrPRF has a relatively low affinity for poly-L-proline and for phosphatidylinositol (4,5) bisphosphate micelles. Furthermore, and surprisingly, CrPRF inhibits exchange of adenine nucleotide on G-actin in a manner similar to human ADF or DNase I. Thus, we postulate that a primary role for CrPRF is to sequester actin in Chlamydomonas. The unusual biochemical properties of CrPRF offer a new opportunity to distinguish specific functions for profilin isoforms.

Association between actin and light chains in Chlamydomonas flagellar inner-arm dyneins

Inner dynein arms in cilia and flagella contain actin as a subunit; however, the function of this actin is totally unknown. Here we performed chemical crosslinking experiments to examine the interaction of actin with other subunits. Six of the seven Chlamydomonas inner-arm dynein species separated by anion-exchange chromatography contain actin and either one of the two previously identified light chains, p28 and centrin, in a mutually exclusive manner. Western blotting of chemically crosslinked dyneins indicated that actin is directly associated with p28 and centrin but not with the dynein heavy chains (HCs). In contrast, p28 and centrin both appeared to interact directly with the N-terminal half of the HCs. Thus it is likely that actin is associated with the heavy chains through p28/centrin. These light chains may well function in the assembly or targeting of the inner arm to the correct axonemal location.

Characterization of the EYE2 gene required for eyespot assembly in Chlamydomonas reinhardtii

The unicellular biflagellate green alga Chlamydomonas reinhardtii can perceive light and respond by altering its swimming behavior. The eyespot is a specialized structure for sensing light, which is assembled de novo at every cell division from components located in two different cellular compartments. Photoreceptors and associated signal transduction components are localized in a discrete patch of the plasma membrane. This patch is tightly packed against an underlying sandwich of chloroplast membranes and carotenoid-filled lipid granules, which aids the cell in distinguishing light direction. In a prior screen for mutant strains with eyespot defects, the EYE2 locus was defined by the single eye2-1 allele. The mutant strain has no eyespot by light microscopy and has no organized carotenoid granule layers as judged by electron microscopy. Here we demonstrate that the eye2-1 mutant is capable of responding to light, although the strain is far less sensitive than wild type to low light intensities and orients imprecisely. Therefore, pigment granule layer assembly in the chloroplast is not required for photoreceptor localization in the plasma membrane. A plasmid-insertion mutagenesis screen yielded the eye2-2 allele, which allowed the isolation and characterization of the EYE2 gene. The EYE2 protein is a member of the thioredoxin superfamily. Site-directed mutagenesis of the active site cysteines demonstrated that EYE2 function in eyespot assembly is redox independent, similar to the auxiliary functions of other thioredoxin family members in protein folding and complex assembly.

Highly divergent actin expressed in a Chlamydomonas mutant lacking the conventional actin gene

The Chlamydomonas mutant ida5 is deficient in the conventional actin gene and its axoneme lacks a subset of inner dynein arms that contain actin as a subunit. However, this mutant retains some other inner dynein arms because a novel protein (NAP) is expressed as a substitute for actin. In this study, we show by sequence analysis that NAP is identical to a putative actin-related protein, the cDNA sequence of which has recently been reported and shown to have 64% amino acid identity with conventional actin. A polyclonal antibody raised against a synthetic polypeptide corresponding to the NH2-terminal sequence of this protein specifically reacted with the spot corresponding to NAP in two-dimensional electrophoresis patterns. NAP apparently can substitute for conventional actin in some, but not all, cellular functions, and therefore can be regarded as a highly divergent actin. This unconventional actin appears to be expressed only when conventional actin is absent.

Cloning and characterization of a gene encoding an actin-related protein in Chlamydomonas

The genomic sequence of an actin-related gene in Chlamydomonas reinhardtii has been determined. The deduced amino acid sequence of this gene shares a 63.9% identity with that of a recently reported conventional actin-encoding gene in C. reinhardtii. Phylogenetic analysis shows that the product of this actin-related gene does not fit into conventional actin or any major actin-related protein categories. The actin-related gene in C. reinhardtii contains seven introns in the coding region and, as described for the conventional actin gene, it contains several sequences similar to the 'tub box' sequence motif in its 5'-upstream region. Southern blot analysis of the gene shows a hybridization pattern different from that of the conventional actin gene, indicating that these genes are distinct from one another. Northern blot analysis of poly(A)+RNA shows the messages of the two genes to be very similar in size, yet the message level of the actin-related gene is significantly lower than that of the conventional actin gene.

Immunochemical characterization of a human sperm fibrous sheath protein, its developmental expression pattern, and morphogenetic relationships with actin

Among the monoclonal antibodies (MAbs) prepared against human sperm extracts, MAb 4F7 was found to be specific to the human and Macaca fascicularis sperm cytoskeletal fibrous sheath (FS). In Western blotting, MAb 4F7 stains a doublet of polypeptides of about M(r) 95 x 10(3) in extracts of human sperm cells. These polypeptides are not recognized by the KL1 anti-cytokeratin MAb, nor by the MAbs known to bind to the carboxy terminal (IFA) and to the amino terminal (ME101) rod domain of intermediate filaments. Sequential extraction procedures shows that the FS polypeptides recognized by MAb 4F7 are exposed after treatment with 8 M urea 4F7 immunoreactivity is lost after treatment with high ionic solutions (NaCl; KCl, Kl). Immunogold electron microscopy reveals that this protein is present throughout the FS. This FS antigenic determinant first accumulates in an FS proximal body in late spermatids, then in granules extending distally along the flagellum. Staining of spermatozoa with flagellar dysgenesis reveals that this FS protein colocalizes with actin no matter what the location of their abnormal assembly. These data suggest that the transient microtubule-like spindle-shaped body of as yet unknown function could be involved in FS protein deposition and that the assembly of the FS and actin could be under the control of some common morphogenetical factor(s). MAb 4F7 should allow further investigations of this peri-axonemal structure in both normal and pathological conditions.

Chlamydomonas inner-arm dynein mutant, ida5, has a mutation in an actin-encoding gene

Chlamydomonas flagellar inner-arm dynein consists of seven subspecies (a-g), of which all but f contain actin as subunits. The mutant ida5 and a new strain, ida5-t, lack four subspecies (a, c, d, and e). These mutants were found to have mutations in the conventional actin gene, such that its product is totally lost; ida5 has a single-base deletion that results in a stop codon at a position about two-thirds from the 5' end of the coding region, and ida5-t lacks a large portion of the entire actin gene. Two-dimensional gel electrophoresis patterns of the axonemes and inner-arm subspecies b and g of ida5 lacked the spot of actin (isoelectric point [pI] = approximately 5.3) but had two novel spots with pIs of approximately 5.6 and approximately 5.7 instead. Western blot with different kinds of anti-actin antibodies suggested that the proteins responsible for the two novel spots and conventional actin are different but share some antigenicity. Since Chlamydomonas has been shown to have only a single copy of the conventional actin gene, it is likely that the novel spots in ida5 and ida5-t originated from another gene(s) that codes for a novel actin-like protein(s) (NAP), which has hitherto been undetected in wild-type cells. These mutants retain the two inner-arm subspecies b and g, in addition to f, possibly because NAP can functionally substitute for the actin in these subspecies while they cannot in other subspecies. The net growth rate of ida5 and ida5-t cells did not differ from that of wild type, but the mating efficiency was greatly reduced. This defect was apparently caused by deficient growth of the fertilization tubule. These results suggest that NAP can carry out some, but not all, functions performed by conventional actin in the cytoplasm and raise the possibility that Chlamydomonas can live without ordinary actin.