Stimulation of tubulin gene transcription by deciliation of sea urchin embryos

Tetrahymena Cilia Cap is Built in a Multi-step Process: A Study by Atomic Force Microscopy

Cilia are complex and dynamic organelles that have motility and sensory functions. Defects in cilia biogenesis and function are at the origin of human ciliopathies. In motile cilia, a basal body organizes the axoneme composed of nine microtubule doublets surrounding a central pair of singlet microtubules. The distal ends of axonemal microtubules are attached to the membrane by microtubule-capping structures. Little is known about the early steps of cilium assembly. Although cilia grow and resorb from their distal tips, it remains poorly understood where and when the components of the caps are first assembled. By using Atomic Force Microscopy in tapping mode, with resolution at the nanometer range and with minimum sample manipulation, we show that Tetrahymena cilia assembly requires transient assembly of structures, composed of three components that are placed asymmetrically on an early elongating axoneme. In small uncapped axonemes the microtubule central pair was never observed. Additionally, we show that cilia cap assembly is a multi-step process in which structures of different sizes and shapes are put together in close proximity before the axoneme appears capped. We propose that the cap modifies the axoneme microtubule rate of polymerization and present a model for Tetrahymena cilia cap assembly.

Multispectral labeling of embryonic cells with lipophilic carbocyanine dyes

Incubation of hatched Strongylocentrotus purpuratus sea urchin embryos or larvae with suspensions of the carbocyanine dyes DiI, DiO, and DiD resulted in the random labeling of membranes of some ectodermal epithelial cells and blastocoelar cells, producing a range of differentially colored cells that can be tracked during development. Simultaneous application of soluble Vybrant® preparations of the three dyes resulted in similar labeling of each cell. Dye labeling of the ectoderm was nearly eliminated by deciliation and some ciliated squamous epithelial cells adjacent to labelled cells were refractory to Vybrant® dye uptake irrespective of concentration or duration of treatment, together suggesting local variation in the properties of cell membranes or cilia. Furthermore, single cells possessing distinctive morphological features were detected.

The expression of tubulin and tektin genes in dicyemid mesozoans (Phylum: Dicyemida)

Dicyemid mesozoans (Phylum Dicyemida) are endoparasites (or endosymbionts) that typically are found in the renal sac of benthic cephalopod mollusks such as octopuses and cuttlefishes. Adult dicyemids likely adhere to the renal appendage of hosts via cilia of calotte peripheral cells. These cilia seem to be continuously worn away in the interaction between the dicyemids and the epidermal cells of host renal appendages. We cloned 4 cDNAs and genes, alpha-tubulin, beta-tubulin, tektin B, and tektin C, which are thought to play a key role in ciliogenesis, from Dicyema japonicum, and studied expression patterns of these genes by whole-mount in situ hybridization. We detected coexpression of these genes in the calotte peripheral cells, but not in the trunk peripheral cells. This suggests that regeneration and turnover of cilia continuously occur in the calotte. In vermiform and infusoriform embryos, we also detected coexpression patterns of these genes, which might correlate with ciliogenesis during the embryogenesis. We also predicted the secondary structure and the coiled-coil regions of dicyemid tektins.

Adenylate kinase in sea urchin embryonic cilia

Sea urchin embryos swim by ciliary movement. Hypertonic shock causes deciliation and loss of motility. Within 2-4 h, cilia regenerate and the embryos swim again. Regeneration of cilia occurs multiple times. The adenylate kinase (AK) activity of isolated cilia was studied. A 130-kDa Sp-AK isozyme, present in sperm flagella, is also present in embryonic cilia. AK activity is responsible for approximately 93% of nonmitochondrial ATP regeneration from ADP in embryonic cilia. This is unlike sea urchin sperm flagella, where approximately 31% of the nonmitochondrial ATP regeneration is from the 130-kDa Sp-AK isozyme and approximately 69% from the flagellar creatine kinase (Sp-CK). Embryos were deciliated 1-3 times and after a 2-h period of regeneration the major ciliary axonemal proteins such as the tubulins appeared constant in amount. However, a moderate decrease in ATPase activity, and a large decrease of total AK activity, were measured. The decrease in AK activity paralleled the decrease in embryo swimming velocity. Embryos were deciliated once and cilia regeneration followed for 4 h. ATPase activity recovered to control levels by 3 h, but AK activity and swimming velocity remained lower than in controls. Detergent solubility data and kinetic experiments indicate that, in addition to the 130-kDa Sp-AK, there is at least one additional AK isozyme in embryonic cilia. Analysis of the S. purpuratus genome indicates five AK isozymes in addition to the 130-kDa Sp-AK isozyme. Decreased swimming velocity of embryos with regenerated cilia suggests that regenerated cilia are not as functionally perfect as naturally grown cilia.

Sea urchin neural alpha2 tubulin gene: isolation and promoter analysis

Expression of Talpha2 gene, during sea urchin Paracentrotus lividus development, is spatially and temporally regulated. In order to characterize this gene, we isolated the relevant genomic sequences and scanned the isolated 5'-flanking region in searching for cis-regulatory elements required for proper expression. Gel mobility shift and footprinting assays, as well as reporter gene (CAT and beta-gal) expression assays, were used to address cis-regulatory elements involved in regulation. Here we report that an upstream 5'-flanking fragment of PlTalpha2 gene drives temporal expression of reporter genes congruent with that of endogenous Talpha2 gene. The fragment contains cis-elements able to bind nuclear proteins from the gastrula stage (at which the Talpha2 gene is expressed) whose sequences could be consistent with the consensus sequences for transcription factors present in data bank.

Cellular Deflagellation

Deciliation, also known as deflagellation, flagellar autotomy, flagellar excision, or flagellar shedding, refers to the process whereby eukaryotic cells shed their cilia or flagella, often in response to stress. Used for many decades as a tool for scientists interested in the structure, function, and genesis of cilia, deciliation itself is a process worthy of scientific investigation. Deciliation has numerous direct medical implications, but more profoundly, intriguing relationships between deciliation, ciliogenesis, and the cell cycle indicate that understanding the mechanism of deciliation will contribute to a deeper understanding of broad aspects of cell biology. This review provides a critical examination of diverse data bearing on this problem. It also highlights current deficiencies in our understanding of the mechanism of deciliation.

A zebrafish vitellogenin gene (vg3) encodes a novel vitellogenin without a phosvitin domain and may represent a primitive vertebrate vitellogenin gene

By analysis of zebrafish EST (expressed sequence tag) clones from an adult cDNA library, we have identified 44 clones, about 11% of the adult EST clones, encoding vitellogenins. These vitellogenin EST clones have been derived from at least seven distinct vitellogenin genes. One of the largest vitellogenin cDNA clones, vg3, and its 5' extended clone isolated by 5' RACE (rapid amplification of cDNA ends)-PCR, have been sequenced completely. The deduced complete sequence includes a predicted mature vitellogenin of 1233 amino acids and a truncated signal peptide of 18 amino acids. Interestingly, the predicted vitellogenin has no polyserine phosvitin domain. The lack of the phosvitin domain was confirmed by isolation and sequencing of the vg3 genomic region. Phylogenetic analysis indicates that the phosvitinless vitellogenin is an intermediate between invertebrate vitellogenins and all known vertebrate vitellogenins, and thus may represent a primitive vertebrate vitellogenin. Like other vitellogenins in vertebrates, the phosvitinless vitellogenin is also synthesized mainly in the liver and weakly in the intestine.

A class of neuroD-related basic helix-loop-helix transcription factors expressed in developing central nervous system in zebrafish

Neuronal basic helix-loop-helix (bHLH) transcription factors such as neuroD and neurogenin have been shown to play important roles in neuronal development. In the present study, several distinct bHLH DNA fragments were isolated from the zebrafish genomic DNA by a pair of degenerate polymerase chain reaction (PCR) primers deduced from the conserved bHLH domains of neuroD and neurogenins. Based on the bHLH fragments, three complete neuroD-related cDNA clones, including complete coding regions, ndr1a, ndr1b, and ndr2 (ndr for neuroD related), were isolated and assembled by 5' and 3' rapid amplification of cDNA ends (RACE). A phylogenetic analysis indicated the presence of four groups of neuroD-related genes in the neuroD subfamily in vertebrates: neuroD, ndr1a/ndr1b/MATH-2, ndr2/NDRF, and neuroM/MATH3. Expression of the newly isolated neuroD-related genes was examined by reverse transcriptase (RT)-PCR and whole-mount in situ hybridization. Unlike neuroD, which was expressed broadly in primary neurons during early zebrafish development starting from 10 h postfertilization (hpf), expression of ndr1a and ndr1b started relatively late (around 22 hpf) and was restricted to the olfactory system: olfactory bulbs in the telecephalon (ndr1a and ndr1b) and olfactory organs (ndr1b) starting around 22 hpf. Although a faint ndr2 mRNA signal was detected by RT-PCR in early embryos, no ndr2 mRNA was detected by whole-mount in situ hybridization in embryos up to 72 hpf, suggesting that it is expressed rather late. Our observations suggest that the two novel neuroD-related genes, ndr1a and ndr1b, are involved in the development of the olfactory system and perhaps contribute to its functional complexity.

Deciliation: A stressful event for Paracentrotus lividus embryos

In this report, by using mono- and two-dimensional electrophoretic analysis, we demonstrate that deciliation on sea urchin embryos induces a stress response. Deciliation indeed causes not only the activation of ciliary subroutine, but also a transient decrease of bulk protein synthesis. This decrease is in agreement with our previous results on heat shock response in sea urchin, although deciliation does not induce the expression of the same main hsp set. We were able to characterize one main deciliation-stress protein of 40 kDa whose expression is transiently induced by deciliation and whose localisation is likely to be nuclear.

Expression of Reticulomyxa filosa tubulins in Pichia pastoris: regulation of tubulin pools

We expressed the alpha2- and beta2-tubulin isoforms of the giant freshwater amoeba Reticulomyxa filosa in the methylotrophic yeast Pichia pastoris. Single expression lead to little or no detectable material. Coexpression of both tubulins, however, resulted in a significant increase of expressed proteins. At the same time, the detectable internal tubulins of the host yeast cell were downregulated. This finding indicates the functionality of the expressed amoeba tubulins. Further regulation phenomena were observed on the level of equilibrium between the two R. filosa tubulin isoforms and on the level of the total tubulin pool. The P. pastoris/R. filosa system therefore seems to be an accessible system for the simultaneous study of the various mechanisms involved in tubulin regulation.

A fate map of the vegetal plate of the sea urchin (Lytechinus variegatus) mesenchyme blastula

Previous lineage tracing experiments have shown that the vegetal blastomers of cleavage stage embryos give rise to all the mesoderm and endoderm of the sea urchin larva. In these studies, vegetal blastomers were labeled no later than the sixth cleavage division (60-64 cell stage). In an earlier study we showed that single cells in the vegetal plate of the blastula stage Lytechinus variegatus embryo could be labeled in situ with the fluorescent, lipophilic dye, DiI(C18), and that cells labeled in the central region of the vegetal plate of the mesenchyme blastula primarily gave rise to homogeneous clones consisting of a single secondary mesenchyme cell (SMC) type (Ruffins and Ettensohn (1993) Dev. Biol. 160, 285–288). Our clonal labeling showed that a detailed fate map could be generated using the DiI(C18) labeling technique. Such a fate map could provide information about the spatial relationships between the precursors of specific mesodermal and endodermal cell types and information concerning the movements of these cells during gastrulation and later embryogenesis. We have used this method to construct the first detailed fate map of the vegetal plate of the sea urchin embryo. Ours is a latitudinal map; mapping from the plate center, where the mesodermal precursors reside, through the region which contains the endodermal precursors and across the ectodermal boundary. We found that the precursors of certain SMC types are segregated in the mesenchyme blastula stage vegetal plate and that prospective germ layers reside within specific boundaries. To determine whether the vegetal plate is radially symmetrical with respect to mesodermal cell fates, single blastomeres of four cell stage embryos were injected with lysyl-rhodamine dextran (LRD). The resulting ectodermal labeling patterns were classified and correlated with the SMC types labeled. This analysis indicates that the dorsal and ventral blastomers do not contribute equally to SMC derivatives in L. variegatus.

Transcriptional control of tektin A mRNA correlates with cilia development and length determination during sea urchin embryogenesis

Previous studies have shown that tektin A, one of three integral filamentous protein components of outer doublet microtubules, is synthesized in sea urchins in an amount correlating to the length of embryonic cilia initially assembled or experimentally regenerated. To investigate further the molecular mechanism for the regulation of tektin synthesis, tektin cDNA clones were used to assess mRNA levels during ciliogenesis, zinc-induced animalization, deciliation-induced regeneration and theophylline-induced elongation. Possibly involved in centriole replication, low, near-constant levels of mRNA for all three tektins are present in the unfertilized egg and during cleavage stages. Preceded by new synthesis of tektin B and C mRNAs, tektin A mRNA is up-regulated during ciliogenesis, but only tektin A mRNA levels correlate directly with ciliary length in animalized embryos; the others augment larger, non-limiting pools of tektins B and C. Tektin mRNAs decrease to steady-state levels after ciliogenesis, but are up-regulated again when the embryos are deciliated, correlating with the length of cilia to be deployed. In a species where a 3-fold ciliary length increase can be induced by theophylline treatment of zinc-arrested embryos, the mRNAs accumulate to proportionately higher levels during arrest but are not translated until induction, whereupon they decrease inversely with ciliary elongation. This suggests transcriptional control with respect to mRNA amounts but post-transcriptional control with respect to the expression of this phenotype.(ABSTRACT TRUNCATED AT 250 WORDS)

Ciliogenesis in sea urchin embryos--a subroutine in the program of development

One major milestone in the development of the sea urchin embryo is the assembly of a single cilium on each blastomere just before hatching. These cilia are constructed both from pre-existing protein building blocks, such as tubulin and dynein, and from a number of 9 + 2 architectural elements that are synthesized de novo at ciliogenesis. The finite or quantal synthesis of certain key architectural proteins is coincident with ciliary elongation and proportional to ciliary length. Upon deciliation, the synthesis of architectural proteins occurs anew, a new cilium grows, and the stores of various building blocks are replenished. This routine of coordinated ciliary gene expression may be replayed experimentally many times without delaying normal development. The ability to regenerate cilia has allowed elucidation of these various protein synthetic relationships and has led to the discovery of the pathways by which membrane-associated tubulin and axoneme-associated architectural proteins are conveyed into the highly compartmentalized growing cilium. The sea urchin embryo thus provides a very convenient model system for studies of ciliary assembly and maintenance, coordinate gene expression and membrane dynamics.

Presence of isl-1-related LIM domain homeobox genes in teleost and their similar patterns of expression in brain and spinal cord

Three novel LIM domain homeobox cDNAs encoding proteins structurally related to the Isl-1 protein were isolated from a chinook salmon pituitary cDNA library. Southern blot analysis of genomic DNA indicate that they are derived from three distinct genes, designated as isl-2a, isl-2b, and isl-3 genes. Nucleotide sequence analysis of reverse transcriptase-polymerase chain reaction amplified products reveal that the isl gene family contains two members (a and b) each of both isl-1 and isl-2 genes, and one member of isl-3 gene in the two tetraploid salmonid species, chinook salmon and rainbow trout, and only one member each of isl-1, isl-2, and isl-3 genes in the diploid zebrafish. The expression of the three isl genes in the rainbow trout were studied by reverse transcriptase-polymerase chain reaction analysis of embryonic and adult RNAs, and by in situ hybridization analysis of 8-week-old hatchlings. The transcripts of all three genes could be detected as early as 4 weeks postfertilization (the eye stage) and increased dramatically in 5-week-old embryos. In the adult, the three isl mRNAs appear to be differentially distributed in various tissues. The level of isl-1 mRNA is generally higher than those of isl-2 and isl-3 mRNAs. In situ hybridization analysis indicates that the transcripts of all three genes are localized in subsets of neurons in the brain and spinal cord. In the retina, isl-1 mRNA could be found in both the ganglion and inner nuclear layers while isl-2 and isl-3 mRNAs could only be detected in the ganglion layer. High level of isl-1 mRNA could also be found in mid-gut and interrenal organ where endocrine cells are densely populated. Based on these observations, we speculate that the three structurally related isl genes may play similar roles in cell determination and differentiation in the developing nervous system.

Overexpression of an epitope-tagged beta-tubulin in Chinese hamster ovary cells causes an increase in endogenous alpha-tubulin synthesis

A Chinese hamster beta-tubulin cDNA, engineered to express a 9 amino acid epitope from the influenza hemagglutinin antigen (HA), was transfected into Chinese hamster ovary (CHO) cells. The recombinant protein (HA beta 1-tubulin) appeared to behave normally by the following criteria: immunofluorescence indicated that HA beta 1-tubulin incorporated into all classes of interphase and spindle microtubules as well as microtubule organizing centers. The sensitivity of the cells expressing HA beta 1-tubulin to Colcemid and taxol was unchanged. A 210 kD microtubule associated protein (MAP) remained associated with microtubules that incorporate HA beta 1-tubulin. The synthesis of both endogenous beta-tubulin and HA beta 1-tubulin was repressed by colchicine. The HA beta 1-tubulin incorporated into microtubules to the same extent as the endogenous beta-tubulin, and the overall extent of microtubule assembly in transfected cells was unchanged. Finally, transfected cells had normal growth rates and morphologies. When effects on endogenous tubulin production were measured, it was found that expression of the HA beta 1-tubulin reduced the synthesis of endogenous wild-type beta-tubulin but increased the synthesis of alpha-tubulin. At steady state, a small increase in total tubulin consistent with the increased synthesis of alpha-tubulin was found. The results indicate that expression of excess exogenous beta-tubulin perturbs the synthesis of endogenous alpha-tubulin in a manner that is not easily explained by current models of tubulin regulation. The changes in tubulin synthesis along with degradation of excess tubulin subunits may reflect mechanisms that exist to ensure coordinate levels of alpha- and beta-tubulin for assembly.

Tubulin genes in the algal protist Euglena gracilis

Alpha- and beta-tubulin cDNA were selected from a Euglena lambda gt11 expression library, recloned and either sequenced (alpha-tubulin cDNA) or hybridized to Euglena RNA and DNA (alpha- and beta-tubulin cDNA). RNA for hybridization was extracted at 30 minute intervals after flagellar amputation and quantitated for cDNA binding. Unlike previous reports on most other flagellates, no net increase in either alpha- or beta-tubulin RNA could be detected during regeneration--suggesting steady state or constitutive tubulin RNA synthesis. Incubation of the cDNA with genomic DNA after restriction digestion produced patterns of hybridization consistent with the presence of one to two kinds each of the alpha- and beta-tubulin genes. The deduced amino acid sequence of the alpha-tubulin cDNA was more than 90% identical to the alpha-tubulins of Trypanosoma, Chlamydomonas, Naegleria, Tetrahymena and higher plants. The carboxy terminus of the alpha-tubulin cDNA and the previously sequenced beta-tubulin of Euglena showed greatest identity to the carboxy terminus of the tubulins from Trypanosoma brucei. The sequence data for alpha- and beta-tubulins of Euglena provides direct evidence for the similarity of two gene products from euglenas and trypanosomes and adds support to earlier suggestions that these organisms are phylogenetically related.

Regulation and evolution of the single alpha-tubulin gene of the ciliate Tetrahymena thermophila

The single alpha-tubulin gene of Tetrahymena thermophila was isolated from a genomic library and shown to encode a single protein. Comparisons of the rates of evolution of this gene with other alpha-tubulin sequences revealed that it belongs to a group of more evolutionarily constrained alpha-tubulin proteins in animals, plants, and protozoans versus the group of more rapidly evolving fungal and variant animal alpha-tubulins. The single alpha-tubulin of Tetrahymena must be used in a variety of microtubule structures, and we suggest that equivalently conserved alpha-tubulins in other organisms are evolutionarily constrained because they, too, are multifunctional. Reduced constraints on fungal tubulins are consistent with their simpler microtubule systems. The animal variant alpha-tubulins may also have diverged because of fewer functional requirements or they could be examples of specialized tubulins. To analyze the role of tubulin gene expression in regulation of the complex microtubule system of Tetrahymena, alpha-tubulin mRNA amounts were examined in a number of cell states. Message levels increased in growing versus starved cells and also during early stages of conjugation. These changes were correlated with increases in transcription rates. Additionally, alpha-tubulin mRNA levels oscillate in a cell cycle dependent fashion caused by changes in both transcription and decay rates. Therefore, as in other organisms, Tetrahymena adjusts alpha-tubulin message amounts via message decay. However the complex control of alpha-tubulin mRNA during the Tetrahymena life cycle involves regulation of both decay and transcription rates.

Transcriptional and post-transcriptional control of tubulin gene expression in Trypanosoma cruzi

We have shown that tubulin mRNA accumulation is regulated at the transcriptional level during metacyclogenesis of Trypanosoma cruzi, although the contribution of post-transcriptional mechanisms is also indicated. mRNA heterogeneity is not restricted to beta-tubulin, and differential regulation of alpha-tubulin mRNAs is observed during this stage of the parasite's life cycle. Treatment of epimastigotes with the microtubule-depolymerizing agent vinblastine resulted in growth inhibition and morphological alterations. Vinblastine also induced a rise in the pool of free tubulin subunits, concomitant with diminished tubulin synthesis and reduced mRNA levels. Tubulin gene transcription remained unaltered during vinblastine treatment, suggesting post-transcriptional control. These observations are in agreement with the autoregulatory model of tubulin gene expression described for a variety of cell types. We conclude that T. cruzi utilizes transcriptional and post-transcriptional control mechanisms for tubulin gene expression.

Different patterns of expression of beta-tubulin genes in Tetrahymena pyriformis during reciliation

The ciliate Tetrahymena pyriformis contains one alpha-tubulin (alpha TT) and two beta-tubulin (beta TT1 and beta TT2) genes. The specific expression of these genes was investigated by Northern blot hybridization using oligonucleotide probes complementary to beta TT1 and beta TT2 genes and the coding region of the alpha-tubulin gene. The three genes are expressed producing 1.8-kb mRNAs but the level of beta TT1 mRNA is much higher than that of beta TT2 mRNA. During cilia regeneration, we found that the expression patterns of the alpha TT and beta TT1 genes are similar whereas that of the beta TT2 gene is different. The alpha TT and beta TT1 transcripts reached higher values between 60-120 min after the onset of reciliation than in exponentially growing cells, while beta TT2 transcripts were maintained at low levels during the whole period. The differences in the amounts of steady-state populations of the both beta-tubulin mRNAs do not correspond to the copy number per haploid genome. These differences could result from the fact that the promoter region of beta TT2 may contain highly structured sequences which would affect the binding of the respective trans-acting factor(s). The apparent transcription rate revealed a significant increase at 15 min of reciliation which could be responsible for the high levels of alpha TT and beta TT1 transcripts in the cytoplasm between 60-120 min of reciliation. This coordinated response to cilia regeneration of the alpha TT and beta TT1 tubulin genes is also a relevant aspect of our findings. Several conserved motifs found in their promoter regions led us to think that some of them may function as cis-elements in the specific binding of nuclear protein factor(s).

Structure and expression of the polyubiquitin gene in sea urchin embryos

A cloned Lytechinus pictus cDNA has been identified, which includes seven direct repeats of a 228 bp sequence encoding ubiquitin and about 450 bp of 3' noncoding sequence. The deduced amino acid sequence is identical to that of ubiquitins of other animals (though repeats 3 and 5 each have single amino acid substitutions at different positions). Southern blot analysis revealed that the sea urchin genome contains a single copy of the polyubiquitin gene, and the number of 228 bp repeat units appears to vary from seven to ten among different alleles; no other ubiquitin coding sequences were detected. The size distribution of polyubiquitin mRNA is polymorphic among different individuals, probably corresponding to the differences in copy number of the repetitive coding sequence. The abundance of cytoplasmic polyubiquitin RNA is constant throughout embryogenesis and is similar in ectoderm, endoderm, and mesoderm cells. The constant prevalence of polyubiquitin mRNA apparently results from a balance between ontogenetic changes in its rate of synthesis and its stability in the presence of actinomycin D. Accumulation of polyubiquitin RNA was not heat shock-inducible during embryogenesis.

How is the flagellar length of mature sperm determined? II. Comparison of tubulin synthesis in spermatids between newt and Xenopus in vitro

In order to elucidate mechanisms that control flagellar length of mature sperm, we studied in synchronous cell suspension cultures flagellar growth, tubulin pool, and tubulin synthesis in round spermatids of Xenopus laevis and the newt Cynops pyrrhogaster. The average final length of flagella in Xenopus round spermatids was 35 mum, almost the same length as that in mature sperm, whereas in the newt round spermatids, the length was 210 mum, almost half that of mature sperm. Kinetics of flagellar growth showed that the rate and period of flagellar growth in the newt spermatids were two to threefold those in Xenopus spermatids. The tubulin pool size in newt spermatids was estimated to be about 10-fold greater than that in Xenopus spermatids. But even if all of the pool was used for flagellar growth, it could support only about a seventh to a tenth of the flagellar length in mature sperm in either species. Thus, the possibility that the tubulin pool primarily determines flagellar length was excluded. Since the tubulin pool size did not change throughout the culture period, the possibility that the termination of flagellar growth is due to the exhaustion of the tubulin pool was also excluded. Tubulin synthesis declined over the culture period but continued in newt spermatids longer than in Xenopus spermatids. The period of flagellar elongation almost coincided with the period of tubulin synthesis. The amount of rRNA did not decrease, excluding the possibility that the decline of tubulin synthesis was due to cytoplasmic shedding which might result in the loss of ribosomes. Tubulin synthesis and the amount of rRNA in newt spermatids was more than threefold greater than that in Xenopus spermatids, which may explain the difference in growth rates of their flagella.

Molecular cloning and expression of sea urchin embryonic ciliary dynein beta heavy chain

The determination of the structure and the expression of dynein during embryonic development are central to the understanding of dynein function. As an important first step toward these objectives, cDNAs encoding portions of sea urchin ciliary dynein were identified by antibody screening of a sea urchin cDNA expression library. Because of the complete lack of protein sequence data, it was first necessary to prove the identity of the dynein cDNAs. Of the five cDNA inserts initially cloned, one, designated P72A1, was characterized extensively. Four independent criteria demonstrated that P72A1 encoded a portion of a dynein heavy chain. (1) The beta-galactosidase-P72A1 fusion protein affinity-purified dynein-specific antibodies from crude antiserum. (2) Two other antisera to dynein, raised independently of the antiserum used to screen the cDNA library, reacted with the fusion protein. (3) A new antiserum raised against the fusion protein reacted with authentic dynein heavy chain on Western blots and stained embryonic cilia by indirect immunofluorescence microscopy. (4) Two new antisera, elicited against opposite ends of the P72A1 open reading frame, each reacted with authentic dynein heavy chain protein. Western blot analyses of dissociated dynein heavy chains revealed that P72A1 encoded a portion of the beta heavy chain. Epitope mapping experiments confirmed the identity of P72A1 as part of the beta heavy chain and also demonstrated that P72A1 encoded epitopes of the carboxyl-terminal fragment B domain of the dynein beta heavy chain. Northern blot analyses of poly(A)+ RNA revealed that P72A1 hybridized with a large RNA species ca. 12.5 kb in length. The dynein mRNA concentration increased during embryonic development. Dot blot analyses of RNA isolated at various times after embryo deciliation demonstrated that the dynein beta heavy chain mRNA accumulated rapidly in response to deciliation. The accumulation was similar to but not identical with the induction of tubulin mRNA in response to the same stimulus.

Multiple levels of regulation of tubulin gene expression during sea urchin embryogenesis

The expression of the tubulin genes during embryogenesis of the sea urchin Lytechinus pictus has been analyzed. Single strand tracer excess titrations of alpha- and beta-tubulin mRNA and RNA gel blot hybridizations indicate that tubulin mRNA remains at a constant 1.3 X 10(5) transcripts per embryo during cleavage stages, increases during ciliogenesis shortly before hatching (12 hr PF), declines until midgastrula (30-35 hr PF), and then gradually increases 3-fold to about 6 X 10(5) per pluteus larva (72 hr PF). Tubulin synthesis changes in concert with its mRNA, except that during cleavage the relative rate of tubulin synthesis increases without a corresponding increase in tubulin mRNA abundance. The relative rates of tubulin gene transcription were assayed by a run-on assay in isolated nuclei. The synthesis of alpha- or beta-tubulin RNA results in little supplementation of maternal tubulin RNA during cleavage stages, but the rate increases at least 18-fold during ciliogenesis and then gradually decreases thereafter. The accumulation of tubulin mRNA after gastrulation can be accounted for by an ontogenetic increase in tubulin RNA stability, assayed by actinomycin D chase and RNA gel blot hybridization. The rates of synthesis, stabilities, and abundances of alpha- and beta-tubulin mRNAs were similar, suggesting coordinate regulation. These observations indicate the importance of translational regulation during cleavage, transcriptional regulation during ciliogenesis, and regulation of mRNA stability by the level of unpolymerized tubulin during later development.

Microtubule formation from maternal tubulins during sea urchin embryogenesis: measurement of soluble and insoluble tubulin pools

The mass of tubulin protein in developing embryos of the sea urchin Lytechinus pictus was measured using a radiodilution immunoassay based on densitometric analysis of immunoprecipitated tubulins resolved electrophoretically. The tubulins constitute an average of 360 +/- 35 pg per egg, or 0.66% of the total protein, and there is no significant change in their concentration during embryogenesis. The masses of soluble and polymerized tubulin were measured for extracts prepared under conditions that stabilize microtubules. In eggs, a maximum of 14% of the tubulin is insoluble, and this increases throughout embryogenesis to 67% at pluteus stage (72 hr). The concentration of tubulin in eggs is at least 500 micrograms/ml, well above the critical concentration for tubulin assembly in vitro, yet microtubules have not been observed in eggs. The mass of newly synthesized tubulin, estimated from the mass of tubulin mRNA per embryo, accounts for a small fraction of the total tubulin by the end of gastrulation but for over half of the tubulin by the 72-hr pluteus stage. These observations are consistent with a model in which the declining level of unpolymerized tubulin controls the stability of tubulin mRNa, providing an autogenous regulation of the ontogenetic pattern of tubulin synthesis during sea urchin embryogenesis (Gong and Brandhorst, Development 102: 31-43).