Two calmodulins in Naegleria flagellates: characterization, intracellular segregation, and programmed regulation of mRNA abundance during differentiation

Free-living amoebae and squatters in the wild: ecological and molecular features

Free-living amoebae are protists frequently found in water and soils. They feed on other microorganisms, mainly bacteria, and digest them through phagocytosis. It is accepted that these amoebae play an important role in the microbial ecology of these environments. There is a renewed interest for the free-living amoebae since the discovery of pathogenic bacteria that can resist phagocytosis and of giant viruses, underlying that amoebae might play a role in the evolution of other microorganisms, including several human pathogens. Recent advances, using molecular methods, allow to bring together new information about free-living amoebae. This review aims to provide a comprehensive overview of the newly gathered insights into (1) the free-living amoeba diversity, assessed with molecular tools, (2) the gene functions described to decipher the biology of the amoebae and (3) their interactions with other microorganisms in the environment.

Centrin is synthesized and assembled into basal bodies during Naegleria differentiation

During differentiation of Naegleria from vegetative amoebae to temporary flagellates, the microtubular cytoskeleton, including two basal bodies and flagella, is assembled de novo. Centrin is an integral component of these basal bodies [Levy et al., 1996, Cell Motil. Cytoskeleton 33: 298-323]. In many organisms, centrin appears to be a constitutive protein, but in Naegleria centrin gene expression occurs only during differentiation. Centrin mRNA, which has not been detected in amoebae, appears and disappears earlier in differentiation than a coordinately regulated set of differentiation-specific mRNAs encoding flagellar tubulin and calmodulin. Centrin antigen accumulates during differentiation, and then decreases in abundance as the flagellates mature and revert to amoebae. No localization of centrin has been detected in amoebae. During differentiation, centrin becomes localized to the basal bodies as soon as these structures are detected with anti-tubulin antibodies, first as a single dot and finally as two basal bodies. During reversion of flagellates to amoebae, centrin remains localized to the basal bodies for as long as they are present. When assembly of tubulin-containing structures during differentiation is prevented using oryzalin, centrin localization is prevented as well, yet inhibition of assembly does not affect accumulation of centrin antigen. Apparently in Naegleria, the role of centrin is primarily for a differentiation- or flagellate-specific function. The temporary presence of centrin is concurrent with the presence of centriolar basal bodies, which supports the conjecture that in Naegleria centrin may be needed only when these organelles are present.

Androcam, a Drosophila calmodulin-related protein, is expressed specifically in the testis and decorates loop kl-3 of the Y chromosome

The Drosophila genome encodes a protein that is 68% identical to Drosophila calmodulin (Cam). We show here that this Cam-related gene is specifically expressed in the germ-line of the testis, leading to the name Androcam (Acam). Early in spermatogenesis Acam accumulates on one of the chromatin loops of the Y chromosome, kl-3. This association with kl-3 may indicate an RNA processing-related role for Acam and/or could reflect an unusual storage/assembly function hypothesized for the Y loops. After meiosis Acam is detectable in developing sperm tail cytoplasm, where at least some of the protein is not tightly associated with tubulin. Late in spermiogenesis, some Acam staining overlaps the periphery of the investment cones, actin-containing structures hypothesized to support the motor function for cytoplasmic stripping of the tail. Acam cannot be detected in mature sperm by immunolocalization, but immunoblotting established that Acam is present in sperm stored in mated females, suggesting epitope masking during final maturation. Proteins more related to Acam than Cam are present in the testes of other Drosophila species and a mammalian species, the mouse.

mRNAs for microtubule proteins are specifically colocalized during the sequential formation of basal body, flagella, and cytoskeletal microtubules in the differentiation of Naegleria gruberi

We have examined the distribution of four mRNAs-alpha-tubulin, beta-tubulin, flagellar calmodulin, and Class I mRNA-during differentiation of Naegleria gruberi amebas into flagellates by in situ hybridization. Three of the four mRNAs-alpha-tubulin, beta-tubulin, and Class I mRNA-began to be colocalized at the periphery of the cells as soon as transcription of the respective genes was activated and before any microtubular structures were observable. At 70 min after the initiation of differentiation, these mRNAs were relocalized to the base of the growing flagella, adjacent to the basal bodies and microtubule organizing center for the cytoskeletal microtubules. Within an additional 15 min, the mRNAs were translocated to the posterior of the flagellated cells, and by the end of differentiation (120 min), very low levels of the mRNAs were observed. Cytochalasin D inhibited stage-specific localization of the mRNAs, demonstrating that RNA localization was actin dependent. Since cytochalasin D also blocked differentiation, this raises the possibility that actin-dependent RNA movement is an essential process for differentiation.

A flagellar calmodulin gene of Naegleria, coexpressed during differentiation with flagellar tubulin genes, shares DNA, RNA, and encoded protein sequence elements

Two calmodulins are synthesized during differentiation of Naegleria gruberi from amoebae to flagellates; one remains in the cell body and the other becomes localized in the flagella. The single, intronless, expressed gene for flagellar calmodulin has been cloned and sequenced. The encoded protein is a typical calmodulin with four putative calcium-binding domains, but it has an amino-terminal extension of 10 divergent amino acids preceding conserved calmodulin residue 4. The transcripts encoding flagellar calmodulin and flagellate cell body calmodulin are clearly divergent. Expression of the flagellar calmodulin gene is differentiation-specific; its mRNA appears and then disappears concurrently with those encoding flagellar alpha- and beta-tubulin. Three provocative sequence elements are shared among these unrelated coexpressed genes: (i) a palindromic DNA sequence element is found in duplicate or triplicate upstream to each transcribed region; (ii) a perfect 12-nucleotide match is found near the AUG start codon of flagellar calmodulin and alpha-tubulin; and (iii) the novel amino-terminal extension of flagellar calmodulin contains a 5-amino-acid element similar to the amino terminus of flagellar alpha-tubulin. These shared sequence elements are proposed to have roles in differentiation, possibly in regulation of transcription, mRNA stability, and localization of these proteins to flagella.

A calcineurin-B-encoding gene expressed during differentiation of the amoeboflagellate Naegleria gruberi contains two introns

One of two similar genes in the unicellular eukaryote Naegleria gruberi is shown to encode calcineurin B (CnB), the regulatory subunit of calcium-calmodulin-regulated protein phosphatase 2B. Over a span of 156 amino acids, excluding divergent N-termini, the encoded sequence shows 62% identity with vertebrate CnB, and also shows sequence elements specific, among calcium-binding proteins, to CnB. In contrast, the sequence shows only 23% identity with N. gruberi flagellar calmodulin. CNB mRNA is readily detected in amoebae; its abundance increases fourfold during differentiation to flagellates, reaches a peak at 50-70 min, when flagella are forming, and then declines. A genomic clone matches an expressed cDNA, except that it is interrupted by two phase I introns. The position of one intron, which separates the divergent N-terminal domain from the four calcium-binding domains (EF hands), is shared with a yeast CNB gene; the other is located in the central helix between the two pairs of calcium-binding loops; features that support an ancient origin. These introns, the first found in protein-coding genes of Naegleria, are flanked by characteristic splice junction sequences. N. gruberi CnB also shares similarities with recoverins. The finding in a protist of a CNB gene that contains two introns separating functional domains, shares similarities to recoverins and shows increased expression during differentiation is provocative. If the phylogeny of major groups derived from ribosomal RNA is accepted, Naegleria is among the earliest branching eukaryotes known to contain canonical pre-mRNA introns.

In vivo effect of calcitriol on calcium transport and calcium binding proteins in the spontaneously hypertensive rat

The abnormal intestinal Ca2+ transport reported in spontaneously hypertensive rats (SHR) has been attributed to decreased responsiveness to calcitriol. We reexamined this hypothesis by studying the calcitriol regulation of SHR duodenal calbindin-D9K and calmodulin and the relation of calcitriol to Ca2+ uptake by isolated enterocytes. SHR and normotensive Wistar-Kyoto (WKY) rats were injected with either 50 ng/d calcitriol (vit-D) or vehicle alone (control) for 3 days. Decreased calbindin-D9K (P < .001) and cellular Ca2+ flux (P < .001) were observed in control SHR. Calcitriol increased total cell and brush border calbindin-D9K (P < .0001); this variation paralleled plasma calcitriol levels in both strains. In contrast, Ca2+ flux, which increased in vit-D animals, remained lower in SHR for plasma calcitriol levels similar to those in WKY rats. Immunoreactive calmodulin was similar in both strains whether assayed in total cell or brush border membranes. In contrast, when measured by ligand blotting (45Ca), calmodulin was lower in SHR than in WKY rats (P < .01), suggesting the existence of a calmodulin pool with reduced Ca2+ binding capacity in the hypertensive strain. Calcitriol had no effect on calmodulin in either strain. In conclusion, Ca2+ binding protein regulation by calcitriol is normal in the SHR, and decreased hormone responsiveness cannot account for the defective duodenal calcium transport of this experimental model of hypertension.

Calmodulin isoforms in Arabidopsis encoded by multiple divergent mRNAs

Three new, unique cDNA sequences encoding isoforms of calmodulin (CaM) were isolated from an Arabidopsis cDNA library cloned in lambda gt10. These sequences (ACaM-4, -5, and -6) represent members of the Arabidopsis CaM gene family distinct from the three DNA sequences previously reported. ACaM-4 and -6 encode full-length copies of CaM mRNAs of ca. 0.75 kb. The ACaM-5 sequence encodes a partial length copy of CaM mRNA that is lacking sequences encoding the amino-terminal 10 amino acids of mature CaM and the initiator methionine. The derived amino acid sequence of ACaM-5 is identical to the sequences encoded by two of the previously characterized ACaM cDNAs, and is identical to TCH-1 mRNA, whose accumulation was increased by touch stimulation. The polypeptides encoded by ACaM-4 and -6 differ from that encoded by ACaM-5 by six and two amino acid substitutions, respectively. Most of the deduced amino acid sequence substitutions in the Arabidopsis CaM isoforms occurred in the fourth Ca(2+)-binding domain. Polymerase chain reaction amplification assays of ACaM-4, -5 and -6 mRNA sequences indicated that each accumulated in Arabidopsis leaf RNA fractions, but only ACaM-4 and -5 mRNAs were detected in silique total RNA. The six different CaM cDNA sequences each hybridize with unique EcoRI restriction fragments in genomic Southern blots of Arabidopsis DNA, indicating that these sequences were derived from distinct structural genes. Our results suggest that CaM isoforms in Arabidopsis may have evolved to optimize the interaction of this Ca(2+)-receptor protein with specific subsets of response elements.

Characterization of the human calmodulin-like protein expressed in Escherichia coli

The protein-coding region of an intronless human calmodulin-like gene [Koller, M., & Strehler, E. E. (1988) FEBS Lett. 239, 121-128] has been inserted into a pKK233-2 expression vector, and the 148-residue, M(r) = 16,800 human protein was purified to apparent homogeneity by phenyl-Sepharose affinity chromatography from cultures of Escherichia coli JM105 transformed with the recombinant vector. Several milligrams of the purified protein were obtained from 1 L of bacterial culture. A number of properties of human CLP were compared to those of bacterially expressed human calmodulin (CaM) and of bovine brain CaM. CLP showed a characteristic Ca(2+)-dependent electrophoretic mobility shift on SDS-polyacrylamide gels, although the magnitude of this shift was smaller than that observed with CaM. CLP was able to activate the 3',5'-cyclic nucleotide phosphodiesterase to the same Vmax as normal CaM, albeit with a 7-fold higher Kact. In contrast, the erythrocyte plasma membrane Ca(2+)-ATPase could only be stimulated to 62% of its maximal CaM-dependent activity by CLP. CLP was found to contain four Ca(2+)-binding sites with a mean affinity constant of 10(5) M-1, a value about 10-fold lower than that for CaM under comparable conditions. The highly tissue-specifically-expressed CLP represents a novel human Ca(2+)-binding protein showing characteristics of a CaM isoform.

Structure and expression of the Arabidopsis CaM-3 calmodulin gene

Genomic and cDNA sequences encoding a calmodulin (CaM) gene from Arabidopsis (ACaM-3) have been isolated and characterized. ACaM-3 represents a sequence distinct from two previously isolated Arabidopsis CaM cDNA clones. A 2.3 kb Eco RI restriction fragment was sequenced and found to encode a complete CaM-coding sequence interrupted by a single 491 bp intron, together with 750 bp and 600 bp of 5' and 3' flanking sequences, respectively. The polypeptide encoded by ACaM-3 is identical to that encoded by ACaM-2 and it differs from the one encoded by ACaM-1 by four of 148 residues. The putative promoter of ACaM-3 was atypical of CaM genes previously isolated from animals in that it contained consensus TATA and CAAT box sequences and lacked GC-rich regions. Two DNA sequence elements closely resembling cyclic AMP regulatory elements, which have been identified in animal CaM genes, were located in the 5' flanking region of ACaM-3. Northern blot and polymerase chain reaction amplification assays confirmed that each of the three ACaM mRNAs were expressed in similar but distinct patterns in different organs. ACaM-1 mRNA was the only species detectable in root RNA fractions, and ACaM-3 mRNA could not be detected in floral stalks. Accumulation of the three CaM mRNAs in leaves was induced by a touch stimulus, but the kinetics and extent of the induction varied among the three mRNA species. Run-on transcription assays indicated that a portion of the differences in accumulation of ACaM-1, 2, and 3 mRNAs in leaves and siliques was attributable to differences in their net rates of transcription.

Structural organization of the human CaMIII calmodulin gene

The complete structural organization of the human calmodulin III gene has been determined. This gene specifies the mRNA represented by the previously reported cDNA ht6. The gene contains six exons spread over a total of approx. 10 kb of DNA. Its exon-intron organization is identical to that of the only known chicken calmodulin gene and to that of two of the three characterized rat calmodulin genes. As in many other genes encoding Ca2+ binding proteins, intron 1 separates the ATG initiation codon from the remainder of the coding region. The major and two minor sites of transcription initiation have been determined by primer extension and ribonuclease protection assays. The DNA sequence in the promoter and 5' untranslated region is extremely GC-rich. No typical TATA and CAAT boxes are present upstream of the major transcriptional start site; however, a consensus CAAT box sequence is found further upstream and may play a role in transcriptional initiation from the minor start sites. Six sequence elements with high similarity to monkey SV40-like Sp1-binding regions are present in the putative promoter region, two of which contain perfect GGGCGG core sequences. The structure of the human calmodulin III gene promoter indicates that this gene belongs to a class of 'house-keeping' genes but that its level of expression may also be specifically regulated.

Electrophoretic karyotype and linkage groups of the amoeboflagellate Naegleria gruberi

We have constructed a molecular karyotype for two strains of Naegleria gruberi using pulsed field gel electrophoresis. Each strain has about 23 chromosomes, considerably more than any previous estimate. These chromosomes range in size from 400 kilobasepairs to over 2,000 kilobasepairs. In Naegleria, construction of the DNA karyotype depends on assessment of the anomalous electrophoretic mobility of the circular ribosomal RNA genes. We have determined the chromosomal locations of an identified unique gene (flagellar calmodulin) and four identified multigene families (alpha- and beta-tubulin, actin, ubiquitin), as well as three differentially expressed genes of unknown functions. The ca. 12 actin genes are dispersed over at least seven chromosomes, whereas the majority of the more than eight alpha-tubulin genes are confined to a single chromosome. The ubiquitin genes are found on five chromosomes in one strain and seven in the other and the beta-tubulin genes are on three or four. Our observations provide a foundation for molecular genetic studies in this organism.

Cyclical differentiation of Trypanosoma brucei involves changes in the cellular complement of calmodulin-binding proteins

The present study was undertaken to evaluate changes in the complement of calmodulin-binding proteins which accompany cyclical differentiation in Trypanosoma brucei. An [125I]trypanosome calmodulin overlay procedure was used to detect calmodulin-binding proteins with Mr of 126,000 and 106,000 that were present in homogenates of slender bloodstream froms but were absent in procyclic culture forms. Competition assays with unlabeled bovine brain or trypanosome calmodulins indicated that the developmentally regulated proteins associated with calmodulins from either source. Moreover, [125I]bovine brain calmodulin associated with the same proteins as trypanosome calmodulin. Homogenates of T. evansi exhibited the same pattern of calmodulin-binding activity as T. brucei slender bloodstream forms; however, T. cruzi and Leishmania tarentolae contained distinct patterns of calmodulin-binding activity. Mouse serum contained no detectable binding proteins while mouse brain contained predominantly proteins of Mr 210,000, 60,000, and 49,000 which were associated with the trypanosome calmodulin probe. The developmentally regulated calmodulin-binding proteins from T. brucei were in the 10,000g pellet. We conclude that the cellular complement of calmodulin-binding proteins varies during the trypanosome life cycle.

Structural organization of multiple rat calmodulin genes

Elsewhere, we have reported the structure of a rat calmodulin gene and two distinct rat calmodulin cDNAs, pRCM1 and pRCM3. Here, I report the cloning and sequencing of the third calmodulin cDNA (pRCM4) and two additional rat calmodulin genes. The original calmodulin gene is named CaM I (pRCM1) and the newly discovered calmodulin genes are named CaM II (pRCM3) and CaM III (pRCM4). CaM II spans about 10 x 10(3) base-pairs and consisted of five exons, while CaM III spans about 7.2 x 10(3) base-pairs and consisted of six exons. One of the introns (intron 3) observed in CaM I and CaM III is lost in CaM II. Otherwise, the intron/exon organization of these genes is exactly the same. In all calmodulin genes, the first intron separates the initiation codon (ATG) from the coding region of the protein. Northern blotting showed that CaM I is transcribed primarily into 1.7 x 10(3) base-pair mRNA in various tissues examined and 4.0 x 10(3) base-pair mRNA mainly in skeletal muscle, CaM II is transcribed into 1.4 x 10(3) base-pair mRNA almost exclusively in brain and CaM III is transcribed predominantly into 2.3 x 10(3) base-pair mRNA and faintly into 1.0 x 10(3) base-pair mRNA mainly in skeletal muscle and brain. DNA sequences in the promoter-regulator regions of these genes are partly homologous but essentially distinct and possess a number of direct repeats, palindromes and feasible stem-loop structures. Together with these, I report here the structures of the third and fourth calmodulin retropseudogenes.

The cilia of Paramecium tetraurelia contain both Ca2+-dependent and Ca2+-inhibitable calmodulin-binding proteins

To identify protein targets for calmodulin (CaM) in the cilia of Paramecium tetraurelia, we employed a 125I-CaM blot assay after resolution of ciliary proteins on SDS/polyacrylamide gels. Two distinct types of CaM-binding proteins were detected. One group bound 125I-CaM at free Ca2+ concentrations above 0.5-1 microM and included a major binding activity of 63 kDa (C63) and activities of 126 kDa (C126), 96 kDa (C96), and 36 kDa (C36). CaM bound these proteins with high (nanomolar) affinity and specificity relative to related Ca2+ receptors. The second type of protein bound 125I-CaM only when the free Ca2+ concentration was below 1-2 microM and included polypeptides of 95 kDa (E95) and 105 kDa (E105). E105 may also contain Ca2+-dependent binding sites for CaM. Both E95 and E105 exhibited strong specificity for Paramecium CaM over bovine CaM. Ciliary subfractionation experiments suggested that C63, C126, C96, E95, and E105 are bound to the axoneme, whereas C36 is a soluble and/or membrane-associated protein. Additional Ca2+-dependent CaM-binding proteins of 63, 70, and 120 kDa were found associated with ciliary membrane vesicles. In support of these results, filtration binding assays also indicated high-affinity binding sites for CaM on isolated intact axonemes and suggested the presence of both Ca2+-dependent and Ca2+-inhibitable targets. Like E95 and E105, the Ca2+-inhibitable CaM-binding sites showed strong preference for Paramecium CaM over vertebrate CaM and troponin C. Together, these results suggest that CaM has multiple targets in the cilium and hence may regulate ciliary motility in a complex and pleiotropic fashion.

Transcriptional regulation of coordinate changes in flagellar mRNAs during differentiation of Naegleria gruberi amebae into flagellates

The nuclear run-on technique was used to measure the rate of transcription of flagellar genes during the differentiation of Naegleria gruberi amebae into flagellates. Synthesis of mRNAs for the axonemal proteins alpha- and beta-tubulin and flagellar calmodulin, as well as a coordinately regulated poly(A)+ RNA that codes for an unidentified protein, showed transient increases averaging 22-fold. The rate of synthesis of two poly(A)+ RNAs common to amebae and flagellates was low until the transcription of the flagellar genes began to decline, at which time synthesis of the RNAs found in amebae increased 3- to 10-fold. The observed changes in the rate of transcription can account quantitatively for the 20-fold increase in flagellar mRNA concentration during the differentiation. The data for the flagellar calmodulin gene demonstrate transcriptional regulation for a nontubulin axonemal protein. The data also demonstrate at least two programs of transcriptional regulation during the differentiation and raise the intriguing possibility that some significant fraction of the nearly 200 different proteins of the flagellar axoneme is transcriptionally regulated during the 1 h it takes N. gruberi amebae to form visible flagella.

The alpha-tubulin gene family expressed during cell differentiation in Naegleria gruberi

Genes that direct the programmed synthesis of flagellar alpha-tubulin during the differentiation of Naegleria gruberi from amebae to flagellates have been cloned, and found to be novel with respect to gene organization, sequence, and conservation. The flagellar alpha-tubulin gene family is represented in the genome by about eight homologous DNA segments that are exceptionally similar and yet are neither identical nor arrayed in a short tandem repeat. The coding regions of three of these genes have been sequenced, two from cDNA clones and one from an intronless genomic gene. These three genes encode an identical alpha-tubulin that is conserved relative to the alpha-tubulins of other organisms except at the carboxyl terminus, where the protein is elongated by two residues and ends in a terminal glutamine instead of the canonical tyrosine. In spite of the protein conservation, the Naegleria DNA sequence has diverged markedly from the alpha-tubulin genes of other organisms, a counterexample to the idea that tubulin genes are conserved. alpha-Tubulin mRNA homologous to this gene family has not been detected in amebae. This mRNA increases markedly in abundance during the first hour of differentiation, and then decreases even more rapidly with a half-life of approximately 8 min. The abundance of physical alpha-tubulin mRNA rises and subsequently falls in parallel with the abundance of translatable flagellar tubulin mRNA and with the in vivo rate of flagellar tubulin synthesis, which indicates that flagellar tubulin synthesis is directly regulated by the relative rates of transcription and mRNA degradation.

Transcriptional regulation of coordinate changes in flagellar mRNAs during differentiation of Naegleria gruberi amebae into flagellates

The nuclear run-on technique was used to measure the rate of transcription of flagellar genes during the differentiation of Naegleria gruberi amebae into flagellates. Synthesis of mRNAs for the axonemal proteins alpha- and beta-tubulin and flagellar calmodulin, as well as a coordinately regulated poly(A)+ RNA that codes for an unidentified protein, showed transient increases averaging 22-fold. The rate of synthesis of two poly(A)+ RNAs common to amebae and flagellates was low until the transcription of the flagellar genes began to decline, at which time synthesis of the RNAs found in amebae increased 3- to 10-fold. The observed changes in the rate of transcription can account quantitatively for the 20-fold increase in flagellar mRNA concentration during the differentiation. The data for the flagellar calmodulin gene demonstrate transcriptional regulation for a nontubulin axonemal protein. The data also demonstrate at least two programs of transcriptional regulation during the differentiation and raise the intriguing possibility that some significant fraction of the nearly 200 different proteins of the flagellar axoneme is transcriptionally regulated during the 1 h it takes N. gruberi amebae to form visible flagella.

Biology of Naegleria spp

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Two calmodulin genes are expressed in Arbacia punctulata. An ancient gene duplication is indicated

Calmodulin is highly conserved, and only in the sea urchin Arbacia punctulata have two distinct isotypes been reported. We have isolated and sequenced two cDNAs from a lambda gt 11 library constructed from RNA from ovary tissue of A. punctulata. One clone, designated alpha, encodes a calmodulin isotype previously designated A. It encodes an amino acid sequence that is identical with calmodulin of most vertebrates in positions 1 through 141; however, it does not encode the last seven amino acids. The other clone, designated beta, starts with an open reading frame and encodes the B form of calmodulin from position 11 through the C-terminal position 148. It has only four differences from vertebrate calmodulin, occurring at positions 78 (Asp, beta Glu), 99 (Tyr, beta Phe), 143 (Gln, beta Ala) and 147 (Ala, beta Ser). The nucleic acid sequences of the alpha and beta cDNAs differ at 46 nucleotide positions that are distributed throughout their coding sequences. We conclude that the corresponding mRNAs are not derived from post-transcriptional processing of a single gene, and we infer that they are transcribed from two non-allelic genes. The gene duplication is inferred to have occurred prior to the divergence of the vertebrates and the echinoderms. The expression of these calmodulin mRNAs in ovary tissue and eggs of a single animal differs as judged by hybridization of probes to RNA immobilized to filters.

rRNA genes of Naegleria gruberi are carried exclusively on a 14-kilobase-pair plasmid

An extrachromosomal DNA was discovered in Naegleria gruberi. The 3,000 to 5,000 copies per cell of this 14-kilobase-pair circular plasmid carry all the 18S, 28S, and 5.8S rRNA genes. The presence of the ribosomal DNA of an organism exclusively on a circular extrachromosomal element is without precedent, and Naegleria is only the third eucaryotic genus in which a nuclear plasmid DNA has been found.

mRNAs for alpha- and beta-tubulin and flagellar calmodulin are among those coordinately regulated when Naegleria gruberi amebae differentiate into flagellates

Three of four mRNAs that are specific to the differentiation of Naegleria gruberi amebae into flagellates (Mar, J., J. H. Lee, D. Shea, and C. J. Walsh, 1986, J. Cell Biol., 102:353-361) have been identified as coding for flagellar proteins. The products of these mRNAs, which are coordinately regulated during the differentiation, were identified by in vitro translation of hybrid-selected RNA followed by two-dimensional gel electrophoresis and antibody binding. Six cross-hybridizing clones complementary to a 1.7-kb RNA (class II) all selected mRNA that was translated into two alpha-tubulins. The principal in vitro product, alpha-1, comigrated with a cytoplasmic alpha-tubulin, while the minor product with a more acidic pI, alpha-2, comigrated with flagellar alpha-tubulin. While Naegleria flagellar alpha-tubulin was found to be acetylated based on its reaction with a monoclonal antibody specific to this form, we suggest that alpha-2 is not likely to arise due to acetylation in vitro but probably represents the product of a second alpha-tubulin gene. The class III clone, also complementary to a 1.7-kb RNA, selected beta-tubulin mRNA. In the course of this work it was found, using monoclonal antibodies to the alpha- and beta-subunits of tubulin, that Naegleria alpha-tubulin migrated faster than beta-tubulin on SDS-PAGE. The class IV clone, which hybridizes with a 0.5-kb RNA, selected an mRNA that was translated into a heat stable calcium-binding protein, flagellar calmodulin.

rRNA genes of Naegleria gruberi are carried exclusively on a 14-kilobase-pair plasmid

An extrachromosomal DNA was discovered in Naegleria gruberi. The 3,000 to 5,000 copies per cell of this 14-kilobase-pair circular plasmid carry all the 18S, 28S, and 5.8S rRNA genes. The presence of the ribosomal DNA of an organism exclusively on a circular extrachromosomal element is without precedent, and Naegleria is only the third eucaryotic genus in which a nuclear plasmid DNA has been found.