Localization of the hsp83 transcript within a 3292 nucleotide sequence from the 63B heat shock locus of D. melanogaster

Organization and expression of the Australian sheep blowfly (Lucilia cuprina) hsp23, hsp24, hsp70 and hsp83 genes

In this study we report the isolation and characterization of a heat shock protein 70 (hsp70) gene, the hsp83 gene and two genes that encode small Hsps (Lchsp23 and Lchsp24) from the Australian sheep blowfly, Lucilia cuprina, a major agricultural pest. Phylogenetic analyses indicate that the LcHsp23 protein is the orthologue of Drosophila melanogaster Hsp23 and LcHsp24 is the orthologue of Sarcophaga crassipalpis Hsp23. Quantitative reverse-transcriptase PCR analysis showed that the basal level of Lchsp83 RNA is relatively high at all developmental stages and only moderately induced by heat shock. In contrast, Lchsp70 transcripts are present at low levels and strongly induced by heat shock at all stages. The basal levels of expression and degrees of heat induction of the Lchsp23 and Lchsp24 transcripts were more variable across the different developmental stages. Putative heat shock factor binding sites were identified in the Lchsp24, Lchsp70 and Lchsp83 gene promoters. The isolation of these hsp gene promoters will facilitate constitutive or conditional expression of a gene of interest in transgenic Lucilia.

Caenorhabditis elegans DAF-21 (HSP90) is characteristically and predominantly expressed in germline cells: spatial and temporal analysis

Three monoclonal antibodies against antigens that exist in the Caenorhabditis elegans germ line have previously been described. In the present study, a full-length mRNA for one of these antigens was isolated, and by sequencing its corresponding cDNA, it was predicted that the protein would show a high homology with the 90 kDa heat shock protein (HSP90) in other species, and with the protein of daf-21, a previously identified hsp90 homologue. The spatial and temporal distribution of the antigen (DAF-21) was analyzed in C. elegans, and the localization of daf-21 mRNA, as detected by in situ hybridization, agreed with that detected by the monoclonal antibody. Under normal conditions, daf-21 mRNA is characteristically distributed in postembryonic germ cells derived from Z2 and Z3 cells in both hermaphrodites and males. Under heat stress conditions, however, daf-21 mRNA was not only detected in germ cells, but also apparently expressed all over the body. In addition, the DAF-21 protein seemed to be localized in the perinuclear region of somatic cells.

Redundant function of Runt Domain binding partners, Big brother and Brother, during Drosophila development

The Core Binding Factor is a heterodimeric transcription factor complex in vertebrates that is composed of a DNA binding α-subunit and a non-DNA binding β-subunit. The α-subunit is encoded by members of the Runt Domain family of proteins and the β-subunit is encoded by the CBFβ gene. In Drosophila, two genes encoding α-subunits, runt and lozenge, and two genes encoding β-subunits, Big brother and Brother, have been previously identified. Here, a sensitized genetic screen was used to isolate mutant alleles of the Big brother gene. Expression studies show that Big brother is a nuclear protein that co-localizes with both Lozenge and Runt in the eye imaginal disc. The nuclear localization and stability of Big brother protein is mediated through the formation of heterodimeric complexes between Big brother and either Lozenge or Runt. Big brother functions with Lozenge during cell fate specification in the eye, and is also required for the development of the embryonic PNS. ds-RNA-mediated genetic interference experiments show that Brother and Big brother are redundant and function together with Runt during segmentation of the embryo. These studies highlight a mechanism for transcriptional control by a Runt Domain protein and a redundant pair of partners in the specification of cell fate during development.

The heat shock protein 83 (Hsp83) is required for Raf-mediated signalling in Drosophila

The heat shock protein Hsp90 has been shown to associate with various cellular signalling proteins such as steroid hormone receptors, src-like kinases and the serine/threonine kinase Raf. While the interaction between steroid hormone receptors and Hsp90 appears to be essential for ligand binding and activation of the receptors, the role of Hsp90 in Raf activation is less clear. We have identified mutations in the hsp83 gene, the Drosophila homologue of hsp90, in a search for dominant mutations that attenuate signalling from Raf in the developing eye. The mutations result in single amino acid substitutions in the Hsp83 protein and cause a dominant-negative effect on the function of the wild-type protein. We show that both wild-type and mutant forms of Hsp83 bind to the activated Drosophila Raf but the mutant Hsp83 protein causes a reduction in the kinase activity of Raf. Our results indicate that Hsp83 is essential for Raf function in vivo.

HSF recruitment and loss at most Drosophila heat shock loci is coordinated and depends on proximal promoter sequences

The heat shock response in Drosophila is primarily dependent on the binding of the heat shock transcription factor, HSF, to conserved sequences in heat shock gene promoters, the heat shock elements (HSEs). Here we examine the kinetic relationship of HSF binding to chromosomal loci and heat shock gene transcription in vivo. The features of heat shock promoters that determine the kinetics of HSF binding are also examined. Analyses of HSF association by indirect immunofluorescence with an anti-HSF antibody reveal that fluorescent signals at many loci on polytene chromosomes rapidly increase and then gradually decrease as heat shock time progresses. While overall amounts of fluorescent signal vary from locus to locus, the patterns of acquisition and loss of HSF at most loci are coordinated with only one identified exception. Immunostaining with an anti-RNA polymerase II antibody indicates that the kinetics of RNA polymerase II accumulation on the heat shock loci are similar to those of HSF. Furthermore, nuclear run-on assays confirm that the major heat shock genes are coordinately transcribed during the attenuation period. In contrast, the kinetics of HSF association with HSE "polymers" in a transgenic fly strain are not coordinated with those of endogenous loci. The addition of core promoter sequences to one of the HSEs found in the polymer restores coordinate HSF binding, suggesting that the kinetic patterns of HSF binding depend on a core promoter located near the HSEs. Finally, the distribution of the heat shock protein HSP70 is examined for its role in regulating the attenuated response of HSF to heat shock.

The heat shock genes in the Drosophila montium subgroup: chromosomal localization and evolutionary implications

The hsp70, hsp83, hsromega, and the small heat shock protein genes were mapped on the polytene chromosomes of six species, representative of the geographical distribution of the Drosophila montium subgroup of the melanogaster species group. In addition, based on hybridization conditions, the putative locus of the hsp68 gene is given. In contrast to the situation in the melanogaster subgroup species, the hsp70 locus is single in the montium species. The hsp83, hsromega and the small hsp loci are also single in the montium genomes studied here, a common feature of all Drosophila species. Among the hsp genes studied, the small hsp genes and the hsromega-homologous sequences exhibit a higher degree of divergence between the melanogaster and the montium subgroups. Our results support the idea that the montium subgroup species has a genome organization closer to that of the common ancestor compared with the melanogaster subgroup species.

Identification of a novel Drosophila melanogaster heat-shock gene, lethal(2)denticleless [l(2)dtl], coding for an 83-kDa protein

In this study, we describe the identification of a novel Drosophila melanogaster (Dm) gene, l(2)dtl, characterized by elevated expression under heat-shock (HS) conditions. It encodes a protein of 83 kDa with no homology to known members of the HSP90 family and other proteins. Gene l(2)dtl is located on the right arm of the second chromosome at locus 59F5, close to the tumor suppressor gene l(2)tid, a homolog of the dnaJ encoding a chaperone strongly conserved in evolution. In the following, we present the sequence of l(2)dtl, the putative protein it encodes, and its molecular localization in a closely interspaced gene cluster consisting of at least four nested genes spanning an approximately 10-kb genomic interval. Furthermore, we present the temporal expression of l(2)dtl in the wild type under normal and HS conditions, and describe the isolation and the phenotype of eight embryonic lethal l(2)dtl mutants.

Precise limitation of concerted evolution to ORFs in mosquito Hsp82 genes

Two Hsp82 genes were isolated from the malaria vector Anopheles albimanus in a single lambda phage clone. The two genes are in a head-to-head arrangement separated by approx. 0.9 kbp. Northern hybridizations and 5' RACE demonstrate that both genes are transcribed, have moderate levels of constitutive transcription, and are also heat-inducible with maximum transcript accumulation occurring after 40 degrees C heat shocks. Both genes have typical heat-shock promoters and conserved intron boundaries in the untranslated leaders. The open reading frames are 99.6% identical differing in only nine silent nucleotide positions in the 2166 bp ORFs. However, precisely outside the ORFs, the flanking DNA of the two genes shows no evidence of common derivation. The high degree of identity between the two ORFs appears to be a result of gene conversion occurring by a process similar to that previously suspected in the A. albimanus Hsp70 genes and several D. melanogaster genes arranged as palindromes. This process probably involves a stem-loop intermediate and is restricted in extent by flanking sequence divergence. These Hsp82 genes clearly demonstrate the extreme precision with which gene conversion can lead to protein-coding-region homogeneity yet allow flanking DNA divergence.

The role of tumor rejection antigens in host antitumor defense mechanisms

BACKGROUND

Normal cells undergo contact inhibition of growth when their surface molecules interact. Tumor cells, however, have undergone a mutation that prevents this arrest of growth upon contact inhibition and allows constant growth. Thus, growth inhibition fails to occur despite the interaction of surface molecules. In recent years a subgroup of these surface molecules has been of interest to cancer investigators. This subgroup has been termed the tumor rejection antigens (TRAs). As the name implies, these are specific to the tumor of origin and may direct the immune system of the host to target the tumor cells and kill them.

METHODS

A literature search was carried out on TRAs to ascertain the current thinking on the subject.

RESULTS

Initial studies of TRAs have revealed that some of them may be heat shock proteins (HSPs). In particular, grp96, a number of the HSP90 family, has been implicated. More recent studies, however, have shown that HSPs alone may not be immunogenic but may act as carrier proteins for tumor specific peptides.

CONCLUSION

Such findings have led to speculation that HSPs or their associated peptides may have a role in the diagnosis and/or treatment of specific cancers. Immunotherapy and bispecific antibodies in particular are areas in which HSPs may prove to be useful.

The SR protein B52/SRp55 is essential for Drosophila development

B52, also called SRp55, is a 52-kDa member of the Drosophila SR protein family of general splicing factors. Escherichia coli-produced B52 is capable of both activating splicing and affecting the alternative splice site choice in human in vitro splicing reactions. Here we report the isolation of a B52 null mutant generated by remobilizing a P element residing near the B52 gene. The resulting deletion, B52(28), is confined to the B52 gene and its neighbor the Hrb87F gene. Second-instar larvae homozygous for the deletion are deficient in both B52 mRNA and protein. The B52 null mutant is lethal at the first- and second-instar larval stages. Germ line transformation of Drosophila flies with B52 genomic DNA rescues this lethality. Thus, B52 is an essential gene and has a critical role in Drosophila development. Larvae deficient in B52 are still capable of splicing the five endogenous pre-mRNAs tested here, including both constitutively and alternatively spliced genes. Therefore, B52 is not required for all splicing in vivo. This is the first in vivo deficiency analysis of a member of the SR protein family.

The SR protein B52/SRp55 is essential for Drosophila development

B52, also called SRp55, is a 52-kDa member of the Drosophila SR protein family of general splicing factors. Escherichia coli-produced B52 is capable of both activating splicing and affecting the alternative splice site choice in human in vitro splicing reactions. Here we report the isolation of a B52 null mutant generated by remobilizing a P element residing near the B52 gene. The resulting deletion, B52(28), is confined to the B52 gene and its neighbor the Hrb87F gene. Second-instar larvae homozygous for the deletion are deficient in both B52 mRNA and protein. The B52 null mutant is lethal at the first- and second-instar larval stages. Germ line transformation of Drosophila flies with B52 genomic DNA rescues this lethality. Thus, B52 is an essential gene and has a critical role in Drosophila development. Larvae deficient in B52 are still capable of splicing the five endogenous pre-mRNAs tested here, including both constitutively and alternatively spliced genes. Therefore, B52 is not required for all splicing in vivo. This is the first in vivo deficiency analysis of a member of the SR protein family.

Phosphorylation of RNA polymerase II C-terminal domain and transcriptional elongation

The carboxy-terminal domain (CTD) of the large subunit of RNA polymerase II is essential in vivo, and is found in either an unphosphorylated (IIa) or hyperphosphorylated (IIo) form. The Drosophila uninduced hsp70 and hsp26 genes, and the constitutively expressed beta-1 tubulin and Gapdh-2 genes, contain an RNA polymerase II complex which pauses after synthesizing a short transcript. We report here that, using an in vivo ultraviolet crosslinking technique and antibodies directed against the IIa and IIo forms of the CTD, these paused polymerases have an unphosphorylated CTD. For genes containing a 5' paused polymerase, passage of the paused RNA polymerase into an elongationally competent mode in vivo coincides with phosphorylation of the CTD. Also, the level of phosphorylation of the CTD of elongating polymerases is shown not to be related to the level of transcription, but is promoter specific.

Translational regulation of the heat shock response

All organisms from bacteria to man respond to an exposure to higher than physiological temperatures by reprogramming their gene expression, leading to the increased synthesis of a unique set of proteins termed heat shock proteins (hsps). The hsps function as molecular chaperones in both normal and stressed cells. The rapid and efficient synthesis of hsps is achieved as a result of changes occurring at gene transcription, RNA processing and degradation, and mRNA translation. With regard to the translational regulation, the emerging picture is that the two key steps of polypeptide chain initiation, namely mRNA binding and Met-tRNA(i) binding to ribosomes, are regulated in heat-shocked mammalian cells. In Drosophila, mRNA binding is regulated by a structural feature of the leader of heat shock mRNAs and by the inactivation of eukaryotic initiation factor- (eIF-) 4F. No clear evidence for changes in Met-tRNA(i) binding has been obtained yet.

A molecular screen for polar-localised maternal RNAs in the early embryo of Drosophila

Localised, maternally synthesised RNAs and proteins play an important role in an early animal embryogenesis. In Drosophila, genetic screens have recovered a number of maternal effect loci that encode localised products in the embryo. However, only a third of Drosophila's genes have been genetically mutated. Consequently, we conducted a molecular screen for polar-localised RNAs in the early Drosophila embryo in order to identify additional maternal molecules that carry out spatially restricted functions during early embryogenesis. Total RNA was purified from anterior or posterior poles cut off early Drosophila embryos. These RNAs were used to construct directionally cloned anterior and posterior cDNA libraries which were used in a differential screen for cDNAs representing maternal RNAs localised to one or other pole of the embryo. Five such clones were identified, representing cyclin B RNA, Hsp83 RNA, 28S ribosomal RNA, mitochondrial cytochrome c oxidase subunit one RNA and mitochondrial 16S large ribosomal RNA. Mutations in the loci encoding these RNAs have not been recovered in genetic screens, confirming that our molecular approach complements genetic strategies for identifying maternal molecules that carry out spatially restricted functions in the early embryo. We consider the possible biological significance of localisation of each of these species of transcripts as well as the mechanism of their localisation, and discuss the potential use of our cDNA libraries in screens for rarer localised RNAs.

Dynamic Hsp83 RNA localization during Drosophila oogenesis and embryogenesis

Hsp83 is the Drosophila homolog of the mammalian Hsp90 family of regulatory molecular chaperones. We show that maternally synthesized Hsp83 transcripts are localized to the posterior pole of the early Drosophila embryo by a novel mechanism involving a combination of generalized RNA degradation and local protection at the posterior. This protection of Hsp83 RNA occurs in wild-type embryos and embryos produced by females carrying the maternal effect mutations nanos and pumilio, which eliminate components of the posterior polar plasm without disrupting polar granule integrity. In contrast, Hsp83 RNA is not protected at the posterior pole of embryos produced by females carrying maternal mutations that disrupt the posterior polar plasm and the polar granules--cappuccino, oskar, spire, staufen, tudor, valois, and vasa. Mislocalization of oskar RNA to the anterior pole, which has been shown to result in induction of germ cells at the anterior, leads to anterior protection of maternal Hsp83 RNA. These results suggest that Hsp83 RNA is a component of the posterior polar plasm that might be associated with polar granules. In addition, we show that zygotic expression of Hsp83 commences in the anterior third of the embryo at the syncytial blastoderm stage and is regulated by the anterior morphogen, bicoid. We consider the possible developmental significance of this complex control of Hsp83 transcript distribution.

Dynamic Hsp83 RNA localization during Drosophila oogenesis and embryogenesis

Hsp83 is the Drosophila homolog of the mammalian Hsp90 family of regulatory molecular chaperones. We show that maternally synthesized Hsp83 transcripts are localized to the posterior pole of the early Drosophila embryo by a novel mechanism involving a combination of generalized RNA degradation and local protection at the posterior. This protection of Hsp83 RNA occurs in wild-type embryos and embryos produced by females carrying the maternal effect mutations nanos and pumilio, which eliminate components of the posterior polar plasm without disrupting polar granule integrity. In contrast, Hsp83 RNA is not protected at the posterior pole of embryos produced by females carrying maternal mutations that disrupt the posterior polar plasm and the polar granules--cappuccino, oskar, spire, staufen, tudor, valois, and vasa. Mislocalization of oskar RNA to the anterior pole, which has been shown to result in induction of germ cells at the anterior, leads to anterior protection of maternal Hsp83 RNA. These results suggest that Hsp83 RNA is a component of the posterior polar plasm that might be associated with polar granules. In addition, we show that zygotic expression of Hsp83 commences in the anterior third of the embryo at the syncytial blastoderm stage and is regulated by the anterior morphogen, bicoid. We consider the possible developmental significance of this complex control of Hsp83 transcript distribution.

Characterization of two maize HSP90 heat shock protein genes: expression during heat shock, embryogenesis, and pollen development

We have isolated two genes from Zea mays encoding proteins of 82 and 81 kD that are highly homologous to the Drosophila 83-kD heat shock protein gene and have analyzed the structure and pattern of expression of these two genes during heat shock and development. Southern blot analysis and hybrid select translations indicate that the highly homologous hsp82 and hsp81 genes are members of a small multigene family composed of at least two and perhaps three or more gene family members. The deduced amino acid sequence of these proteins based on the nucleotide sequence of the coding regions shows 64-88% amino acid homology to other hsp90 family genes from human, yeast, Drosophila, and Arabidopsis. The promoter regions of both the hsp82 and hsp81 genes contain several heat shock elements (HSEs), which are putative binding sites for heat shock transcription factor (HSF) commonly found in the promoters of other heat shock genes. Gene-specific oligonucleotide probes were synthesized and used to examine the mRNA expression patterns of the hsp81 and hsp82 genes during heat shock, embryogenesis, and pollen development. The hsp81 gene is only mildly heat inducible in leaf tissue, but is strongly expressed in the absence of heat shock during the pre-meiotic and meiotic prophase stages of pollen development and in embryos, as well as in heat-shocked embryos and tassels. The hsp82 gene shows strong heat inducibility at heat-shock temperatures (37-42 degrees C) and in heat shocked embryos and tassels but is only weakly expressed in the absence of heat shock. Promoter-GUS reporter gene fusions made and analyzed by transient expression assays in Black Mexican Sweet (BMS) Maize protoplasts also indicate that the hsp82 and hsp81 are regulated differentially. The hsp82 promoter confers strong heat-inducible expression of the GUS reporter gene in heat-treated cells (60- to 80-fold over control levels), whereas the hsp81 promoter is only weakly heat inducible (5- to 10-fold over control levels).

83-kilodalton heat shock proteins of trypanosomes are potent peptide-stimulated ATPases

A Crithidia fasciculata 83-kDa protein purified during a separate study of C. fasciculata trypanothione synthetase was shown to have ATPase activity and to belong to the hsp90 family of stress proteins. Because no ATPase activity has previously been reported for the hsp90 class, ATP utilization by C. fasciculata hsp83 was characterized: this hsp83 has an ATPase kcat of 150 min-1 and a Km of 60 microM, whereas the homologous mammalian hsp90 binds ATP but has no ATPase activity. Crithidia fasciculata hsp83 undergoes autophosphorylation on serine and threonine at a rate constant of 3.3 x 10(-3) min-1. Similar analysis was performed on recombinant Trypanosoma cruzi hsp83, and comparable ATPase parameters were obtained (kcat = 100 min-1, Km = 80 microM, kautophosphorylation = 6.3 x 10(-3) min-1). The phosphoenzyme is neither on the ATPase hydrolytic pathway nor does it affect ATPase catalytic efficiency. Both C. fasciculata and T. cruzi hsp83 show up to fivefold stimulation of ATPase activity by peptides of 6-24 amino acids.

Heat shock response in Drosophila

Major alterations in genetic activity have been observed in every organism after exposure to abnormally high temperatures. This phenomenon, called the heat shock response, was discovered in the fruit fly Drosophila. Studies with this organism led to the discovery of the heat shock proteins, whose genes were among the first eukaryotic genes to be cloned. Several of the most important aspects of the regulation of the heat shock response and of the functions of the heat shock proteins have been unraveled in Drosophila.

DNA sequence requirements for generating paused polymerase at the start of hsp70

RNA polymerase II is transcriptionally engaged but paused approximately 25 nucleotides from the start site of the hsp70 gene of Drosophila melanogaster in uninduced (non-heat-shocked) flies. Here, we identify regions of the hsp70 promoter that are required for formation of this paused polymerase. Various hsp70 promoter sequences are substituted for promoter sequences of a yolk protein gene, yp1, which, in males, is normally not expressed and has no paused polymerase. Run-on assays with nuclei of male transgenic flies are used to measure the level of paused polymerase on the hybrid genes. Sequences that reside upstream of the hsp70 TATA element, when fused upstream of the yp1 TATA element, specify the formation of a paused polymerase on the 5' end of this hybrid gene. Within this region are multiple copies of the GAGA element, which is known to bind a constitutively expressed factor. This element appears to play a role in generating the pause. Also, in the absence of much of this upstream region, hsp70 sequences in the vicinity of the transcriptional start and pause site participate in specifying the pause. Deletions of the pause site reduce the level of paused polymerase but do not lead to constitutive transcription. However, a connection between transcription and pausing is seen. The level of paused polymerase on the various hybrid hsp70-yp1 promoters correlates with the promoter's potential to direct heat-induced transcription.

Occurrence of oligopurine.oligopyrimidine tracts in eukaryotic and prokaryotic genes

A program to analyse the length and frequency distribution of specific base tracts in genomic sequences is described. The frequency of oligopurine.oligopyrimidine tracts (R.Y. tracts) in a data base of 163 transcribed genes is analysed and compared. The complete genomes of SV40 virus, N. tobacum chloroplast, yeast 2 micron plasmid, bacteriophage lambda, plasmid pBR322 and the E. coli lac operon are also analyzed. A highly significant overrepresentation of oligopurine and oligopyrimidine tracts is observed in all eukaryotic genes examined, as well as in the chloroplast genome. The overrepresentation is evident in all gene subregions of the chloroplast, in the following order: intergenic regions, 3' downstream and 5' upstream (promoter), 5' and 3' untranslated, introns and coding regions. In genes coding for basic proteins, oligopurine rather than oligopyrimidine tracts are found on the coding stand. In prokaryotic genes only the longest R.Y. tracts (greater than or equal to 12) are found in excess, and are concentrated near regulatory regions. While a structural role for R.Y. tracts is most likely in intergenic regions, a functional role, as initiation sites for strand separation, is proposed for regulatory gene regions.

Effect of heat shock on gene expression of Aedes albopictus cells infected with Mayaro virus

Three major Mayaro virus proteins of 62, 50 and 34 kDa were detected in Aedes albopictus cells after 48 h postinfection at 28 degrees C. When the infected cells were shifted from 28 to 37 degrees C for 90 min (heat shock conditions), the synthesis of two major heat shock proteins (HSP) 82 and 70 kDa was induced concomitantly with strong inhibition of virus and normal protein synthesis. Total cellular RNA was isolated from mock and infected cells incubated at 28 degrees C or under heat shock. Northern blot analysis with HSP genomic probes from Drosophila sp showed that (1) the probe for HSP 82 hybridized with an RNA of 2.6 kb present only in heat-shocked cells, (2) the HSP 70 probe hybridized with RNA species of 2.5 kb, present only in RNA from heat-shocked cells. These results showed that Mayaro virus was not able to alter the reprogrammation of gene expression induced by heat shock in A. albopictus cells.

Microbial stress proteins

There is general agreement that a function, perhaps the major function, of stress proteins under normal physiological conditions is to help assembly and disassembly of protein complexes and to catalyse protein-translocation processes. It remains unclear, however, as to what role these processes play in stressed cells. It could be that cells under stress produce abnormal, misfolded or otherwise damaged proteins and that increased synthesis of stress proteins is required to counter protein modifications. A role for stress proteins in recovery of cells from stress, as opposed to a role in helping cells to withstand a lethal stress, is thus suggested. The intracellular location of stress proteins, in the unstressed and stressed cell, is worthy of further studies. Members of the hsp70 family are associated with the cytosol, mitochondria and endoplasmic reticulum. There is evidence, particularly from studies on mammalian cells (Tanguay, 1985; Welch and Mizzen, 1988; Arrigo et al., 1988), that following stress hsps migrate to various cellular compartments and subsequently delocalize after stress. However, there is little comparable data from microbial systems for this phenomenon (e.g. Rossi and Lindquist, 1989). The question as to the role of stress proteins in the transient acquisition of thermotolerance remains to be answered. It is insufficient to equate the kinetics of stress-protein synthesis with acquisition of thermotolerance. Quantitative data on the amount of stress protein present at various times, including the recovery period, is required. The demonstration that microbial stress proteins are important antigenic determinants of micro-organisms causing major debilitating diseases in the world is an exciting observation. Studies on the interplay of pathogen and host, both carrying similar antigenic hsp determinants, will be a challenging area for future research. It is likely that E. coli and Sacch. cerevisiae, with their well-established biochemical and genetic properties, will continue to be the experimental systems of choice for studies on stress proteins. On the other hand, it is encouraging that studies on other micro-organisms have expanded in the past few years and have made substantial contributions towards our understanding of the stress response. The ubiquitous nature of the stress response and the remarkable evolutionary conservation of the stress proteins continue to be attractive areas for research.

Cloning and analysis of a human 86-kDa heat-shock-protein-encoding gene

An 86-kDa heat-shock-protein-encoding (hsp86) cDNA probe permitted to identify, in whole genomic human DNA, two EcoRI fragments of 2.6 and 5.3 kb. These two fragments, as well as an homologous phage lambda VIII1 harboring about 19 kb of human DNA, were isolated from genomic libraries. Sequence analysis revealed that three different genomic hsp86 sequences had been cloned, one of them being the 5' half of a functional gene. This gene contains several introns, as compared to the entire Hsp86-encoding sequence found in lambda VIII1, which represents a processed pseudogene. Cloned hsp86 promoter, with its TATA-box and a heat-shock element upstream at nt positions -25 and -75, respectively, was functional, as verified by fusion to the bacterial chloramphenicol acetyltransferase-encoding gene and its transient expression in vivo. The typical hsp86-type heat-shock regulation was observed, i.e., significant basal activity associated with an inducibility at elevated temperatures. Furthermore, accurate and efficient in vitro transcription was initiated at this hsp86 promoter, resulting in expression of the hsp86 gene, as well as the unrelated sequences.

Heat-shock protein 83 of Leishmania mexicana amazonensis is an abundant cytoplasmic protein with a tandemly repeated genomic arrangement

The 83-kDa heat-shock protein HSP83 is a highly abundant protein in Leishmania amastigotes and promastigotes exposed to elevated temperature. Antibodies against this protein were obtained by immunization with a synthetic peptide derived from a conserved region. These antibodies recognized both the denatured and the native form of the molecule and were used for immunofluorescence analysis. These experiments, together with analysis by cell fractionation, show that HSP83 is distributed in the cytoplasm of Leishmania parasites. The gene for HSP83 in Leishmania mexicana amazonensis has been cloned from a genomic library, and molecular characterization shows it is present in several copies of 4-kb repeats arranged in tandem.

The 86-kilodalton antigen from Schistosoma mansoni is a heat-shock protein homologous to yeast HSP-90

We report the sequence of a cDNA clone encoding an 86-kDa polypeptide antigen (p86) from Schistosoma mansoni. Fusion proteins made in Escherichia coli are recognized by human infection sera. The reading frame of this antigen is highly homologous to those of the large heat-shock proteins of Saccharomyces cerevisiae (HSP90) and Drosophila melanogaster (HSP83). mRNA encoding p86 increases in response to heat shock of adult worms, as does HSP70. Comparisons of the sequences of HSP70 and HSP83 homologues show that these two families of heat-shock proteins are not significantly related except for the last four amino acid residues, which are Glu-Glu-Val-Asp in every case. This sequence is not found at the carboxy terminus of any other protein in the current databases.

Isolation and functional analysis of chicken 90-kDa heat shock protein gene promoter

We report the nucleotide sequence of a 2652 bp derived from a chicken 90-kDa heat shock protein (hsp 90) genomic clone. This fragment contains 890 bp of the 5' flanking region and 1762 bp of structural gene sequence encoding the first 85 amino acids of the protein. The start site of transcription was determined by primer extension and RNase mapping. Two introns have been identified. The first intron presents two features in common with the unique intron of the hsp 83 of drosophila: its location just before the ATG initiation codon and its length of approximately 1.3 Kb. The 5' flanking region contains a TATAA element, a CCAAT box and several putative cis-regulatory elements that might account for the basal level of expression and developmental regulation of the gene. Functional analyses show that hsp 90 gene expression is constitutive and heat inducible and that a full heat shock response requires the cooperativity of two distinct blocks of overlapping heat shock response elements.

Heat shock and developmental regulation of the Drosophila melanogaster hsp83 gene

In contrast to the hsp70 gene, whose expression is normally at a very low level and increases by more than 2 orders of magnitude during heat shock, the hsp83 gene in Drosophila melanogaster is expressed at high levels during normal development and increases only severalfold in response to heat shock. Developmental expression of the hsp83 gene consists of a high level of tissue-general, basal expression and a very high level of expression in ovaries. We identified regions upstream of the hsp83 gene that were required for its developmental and heat shock-induced expression by assaying beta-galactosidase activity and mRNA levels in transgenic animals containing a series of 5' deletion and insertion mutations of an hsp83-lacZ fusion gene. Deletion of sequences upstream of the overlapping array of a previously defined heat shock consensus (HSC) sequence eliminated both forms of developmental expression of the hsp83 gene. As a result, the hsp83 gene with this deletion mutation was regulated like that of the hsp70 gene. Moreover, an in vivo polymer competition assay revealed that the overlapping HSC sequences of the hsp83 gene and the nonoverlapping HSC sequences of the hsp70 gene had similar affinities for the factor required for heat induction of the two heat shock genes. We discuss the functional similarity of hsp70 and hsp83 heat shock regulation in terms of a revised view of the heat shock-regulatory sequence.

Heat shock and developmental regulation of the Drosophila melanogaster hsp83 gene

In contrast to the hsp70 gene, whose expression is normally at a very low level and increases by more than 2 orders of magnitude during heat shock, the hsp83 gene in Drosophila melanogaster is expressed at high levels during normal development and increases only severalfold in response to heat shock. Developmental expression of the hsp83 gene consists of a high level of tissue-general, basal expression and a very high level of expression in ovaries. We identified regions upstream of the hsp83 gene that were required for its developmental and heat shock-induced expression by assaying beta-galactosidase activity and mRNA levels in transgenic animals containing a series of 5' deletion and insertion mutations of an hsp83-lacZ fusion gene. Deletion of sequences upstream of the overlapping array of a previously defined heat shock consensus (HSC) sequence eliminated both forms of developmental expression of the hsp83 gene. As a result, the hsp83 gene with this deletion mutation was regulated like that of the hsp70 gene. Moreover, an in vivo polymer competition assay revealed that the overlapping HSC sequences of the hsp83 gene and the nonoverlapping HSC sequences of the hsp70 gene had similar affinities for the factor required for heat induction of the two heat shock genes. We discuss the functional similarity of hsp70 and hsp83 heat shock regulation in terms of a revised view of the heat shock-regulatory sequence.

Sex-specific control of Drosophila melanogaster yolk protein 1 gene expression is limited to transcription

The sex of Drosophila melanogaster is determined by a hierarchy of genes. The ultimate targets of this regulatory hierarchy are the genes encoding terminal differentiation products of one sex. For one of the best-characterized target genes, that encoding female-specific yolk protein 1 (YP1), sex-specific transcriptional controls have been clearly demonstrated. In addition, sex-specific posttranscriptional controls were suggested from experiments in which YP1 RNA was induced in males with hormones. To determine whether males can efficiently process and translate a transcript which is normally found only in females, we used a non-sex-specific promoter, the hsp70 gene promoter, to drive YP1 gene transcription in germ line transformed males. The efficiency of expression of the YP1 gene at levels of RNA splicing, translation, and protein secretion in these males was compared with that in wild-type females. These experiments show that there are no sex-specific posttranscriptional controls operating to limit the production of secreted YP1 in males. Promoters containing different numbers of heat shock elements were tested for their ability to drive YP1 gene transcription in males. These results show that incompatibility between the hsp70 gene heat shock elements and the YP1 gene promoter can be overcome by increasing the amount of hsp70 gene sequence up or downstream of the TATA box. In the course of this study, two vectors useful for placing genes under heat shock regulation were constructed. One of these vectors is designed so that the heat-induced transcript produced is the "authentic" primary transcript; it should be useful for studies of posttranscriptional regulation.

The intracellular location of yeast heat-shock protein 26 varies with metabolism

An antibody highly specific for heat-shock protein (hsp)26, the unique small hsp of yeast, and mutants carrying a deletion of the HSP26 gene were used to examine the physical properties of the protein and to determine its intracellular distribution. The protein was found in complexes with a molecular mass of greater than 500 kD. Thus, it has all of the characteristics, including sequence homology and induction patterns, of small hsps from other organisms. When log-phase cells growing in glucose were heat shocked, hsp26 concentrated in nuclei and continued to concentrate in nuclei when these cells were returned to normal temperatures for recovery. However, hsp26 did not concentrate in nuclei under a variety of other conditions. For example, in early stationary-phase cells hsp26 is induced at normal growth temperatures. This protein was generally distributed throughout the cells, even after heat shock. Similarly, in cells genetically engineered to synthesize hsp26 in the presence of galactose, hsp26 did not concentrate in nuclei, with or without a heat shock. To determine if the failure of hsp26 to concentrate in the nucleus of these cells was due to the fact that the protein had been produced at 25 degrees C or to a difference in the physiological state of the cell, we investigated the distribution of the heat-induced protein in cells grown under several different conditions. In wild-type cells grown in galactose or acetate and in mitochondrial mutants grown in glucose or galactose, hsp26 also failed to concentrate in nuclei with a heat shock. We conclude that the intracellular location of hsp26 in yeast depends upon the physiological state of the cell and not simply upon the presence or absence of heat stress. Our findings may explain why previous investigations of the intracellular localization of small hsps in a variety of organisms have yielded seemingly contradictory results.

The non-activated glucocorticoid receptor: structure and activation

Glucocorticoid hormone receptors are present in the soluble fraction of target cell homogenates as large entities (Mr approximately 300,000) that are unable to interact with DNA. These large complexes contain an Mr approximately 94,000 steroid- and DNA-binding polypeptide, in association with an Mr approximately 90,000 non-ligand-binding entity, which has been identified as a heat shock protein, hsp90. This protein has been purified to near homogeneity as a component of the non-activated receptor complex. Characterization of the purified protein revealed its presence as a dimer in the large receptor form. Dissociation of the receptor-hsp90 complex can be induced by heat treatment only when ligand is bound to the receptor, as demonstrated by specific DNA-binding assay and sucrose gradient ultracentrifugation, hsp90 represents ca 1% of total proteins in rat liver cytosol, and milligram amounts were purified using a combination of high performance ion exchange and gel permeation chromatography. Monospecific antibodies were raised in rabbits. They were found to precipitate the intact non-activated glucocorticoid receptor, as well as the Mr approximately 27,000 steroid-binding fragment of the receptor generated by trypsin treatment, indicating that hsp90 interacts with the steroid-binding domain of the glucocorticoid receptor. Finally, translation of glucocorticoid receptor mRNA in reticulocyte lysate yields a protein which also interacts with hsp90 and binds to DNA only after ligand-binding and heat treatment. Thus, the glucocorticoid receptor is synthesized in a non-activated form also in vitro.

Heat-shock proteins, Hsp84 and Hsp86, of mice and men: two related genes encode formerly identified tumour-specific transplantation antigens

Mouse cDNA clones have been isolated with the help of Drosophila melanogaster 82-kDa heat-shock protein (Hsp82)-coding sequences as hybridization probe. Sequencing of the overlapping mouse clones reveals a long open reading frame (ORF) that encodes a polypeptide of 83.3 kDa which shows about 80% similarity to the respective Drosophila Hsp82 amino acid sequence. The N-terminal half of this cDNA cross-hybridizes to a different class of mouse cDNA clones indicating a related gene. Northern blot hybridization experiments reveal a 2.6-kb poly(A)+RNA when probed with the hsp84 clone and a 2.85-kb signal with the hsp84-related cDNA. The amino acid sequences deduced from the contiguous ORF of the hsp84 and the hsp84-related cDNA coincide with the N-terminal sequence of formerly identified 84-kDa and 86-kDa tumour-specific transplantation antigens (Ullrich et al., 1986). In addition, the amino acid composition of the putative 84-kDa mouse Hsp described here is very similar to that of the 84-kDa tumour antigen described by Ullrich et al. (1986). Both observations corroborate the assumption that these Hsps are identical to the described 84-kDa and 86-kDa tumour-specific transplantation antigens. Using these mouse hsp gene clones as hybridization probes we also isolated the corresponding pair of human cDNA clones. Comparison of the respective sequences reveals a strong evolutionary constraint on these two genes in mouse and man.

Translation of unspliced transcripts after heat shock

Severe heat shocks block the splicing of intervening sequences from messenger RNA precursors. The RNA's that accumulate after a severe heat shock have normal transcription start sites and are uncut at both their 5' and 3' splice junctions. Some of these unspliced transcripts leave the nucleus and enter the pool of cytoplasmic messenger RNA. Translation of these RNA's proceeds into their intervening sequences, resulting in the production of abnormal proteins. Thus, the repression of normal transcription, which usually accompanies the heat shock response, may protect the cell from the large-scale synthesis of abnormal RNA's and aberrant proteins.

Sex-specific control of Drosophila melanogaster yolk protein 1 gene expression is limited to transcription

The sex of Drosophila melanogaster is determined by a hierarchy of genes. The ultimate targets of this regulatory hierarchy are the genes encoding terminal differentiation products of one sex. For one of the best-characterized target genes, that encoding female-specific yolk protein 1 (YP1), sex-specific transcriptional controls have been clearly demonstrated. In addition, sex-specific posttranscriptional controls were suggested from experiments in which YP1 RNA was induced in males with hormones. To determine whether males can efficiently process and translate a transcript which is normally found only in females, we used a non-sex-specific promoter, the hsp70 gene promoter, to drive YP1 gene transcription in germ line transformed males. The efficiency of expression of the YP1 gene at levels of RNA splicing, translation, and protein secretion in these males was compared with that in wild-type females. These experiments show that there are no sex-specific posttranscriptional controls operating to limit the production of secreted YP1 in males. Promoters containing different numbers of heat shock elements were tested for their ability to drive YP1 gene transcription in males. These results show that incompatibility between the hsp70 gene heat shock elements and the YP1 gene promoter can be overcome by increasing the amount of hsp70 gene sequence up or downstream of the TATA box. In the course of this study, two vectors useful for placing genes under heat shock regulation were constructed. One of these vectors is designed so that the heat-induced transcript produced is the "authentic" primary transcript; it should be useful for studies of posttranscriptional regulation.

Heat shock but not other stress inducers leads to the disruption of a sub-set of snRNPs and inhibition of in vitro splicing in HeLa cells

Splicing of pre-mRNA in HeLa cells exposed to various stress response inducers has been investigated. In vivo, intron-containing transcripts of the hsp27 gene accumulate in cells stressed by heat or sodium arsenite. In vitro analysis, however, reveals a differential effect of stress on splicing: nuclear extracts from cells exposed to a severe heat shock are incapable of splicing an exogenously supplied substrate while splicing is not perturbed in extracts treated with sodium arsenite, the amino acid analog canavinine or ethanol. Pretreatment of cells with a mild heat shock prior to a severe heat shock protects the splicing apparatus and allows splicing to proceed unimpeded. Analyses of the splicing defect in extracts from heat-shocked cells show that the inhibition of splicing cannot be accounted for by changes in the major RNA and protein components of small nuclear ribonucleoprotein particles (snRNPs) or in a previously described heat-labile factor that is essential for in vitro splicing. Fractionation of small nuclear ribonucleoprotein particles from heat-shock extracts by native polyacrylamide gel electrophoresis reveals dramatic changes in certain particles, most noticeably in a U4/U5/U6 snRNP complex and the U2 snRNP. Alterations in these particles are accompanied by the assembly of labeled pre-mRNA transcript into aberrant splicing complexes that differ from those formed in normal extracts.

Purification and characterization of the 90-kDa heat-shock protein from mammalian tissues

The 90-kDa heat-shock protein (HSP90) has been purified from mammalian tissues, mouse liver and porcine brain, with a good yield by a new method involving hydrophobic chromatography. Mouse liver HSP90 and porcine brain HSP90 were compared with mouse lymphoma HSP90 which was purified from T lymphoma cell line, L5178Y, by a modification of the previously reported method. These three HSP90s were indistinguishable from one another in amino acid composition, one-dimensional peptide mapping, elution pattern of proteolytic fragments (trypsin- or V8-protease-cleaved) in reverse-phase high-performance liquid chromatography, reactivity with the antibody against mouse T lymphoma HSP90 and the ability to bind to F-actin. The amino acid sequences of three portions (total 47 amino acid residues) of lymphoma HSP90 were determined and they were homologous to those of the corresponding portions of Drosophila HSP83A and yeast HSP90. These results suggest that HSP90 is a highly conserved protein during evolution.

Two-step purification and N-terminal amino acid sequence analysis of the rat Mr 90,000 heat shock protein

A fast and efficient method for the isolation of the rat Mr approximately 90,000 heat shock protein is presented, comprising a two-step high-performance anion-exchange and gel-permeation column chromatography. The Mr approximately 90,000 protein was purified to electrophoretic homogeneity in high yield (up to 2 mg per 10g of normal rat liver) in 4-5 h. The purified protein was recognized on protein immunoblots by monospecific rabbit antibodies raised against the rat glucocorticoid receptor-associated Mr approximately 90,000 non-ligand-binding protein. The N-terminal sequence of the first 25 amino acids of the purified protein showed a high degree of similarity with Mr approximately 90,000 heat shock proteins from calf, human, Drosophila, and yeast.

The Caenorhabditis elegans hsp70 gene family: a molecular genetic characterization

We have isolated genomic clones representing six distinct members of the Caenorhabditis elegans 70-kDa heat-shock protein gene (hsp70) family. Each member exists as a single copy element in the C. elegans genome. Transcripts of four of the hsp70 genes have been detected by Northern-blot analysis. One member, hsp70C, appears to be a heat-shock-cognate hsp70 gene (hsc70) since its transcription is developmentally regulated and is not increased in response to heat shock. Transcripts of another gene, hsp70A, are abundant in control worms and are also increased (two- to six-fold) upon heat shock. Nucleotide sequencing of genomic and cDNA clones of hsp70A reveals that it is highly homologous to Drosophila and yeast heat-shock-inducible and heat-shock-cognate hsp70 genes. Three DNA elements homologous to the heat-shock promoter, 5'-C--GAA--TTC--G-3' are located upstream from the Hsp70A-coding region. We find that hsp70A contains three introns, one of which is in a similar position with an intron in the Drosophila hsc1 and hsc2 genes. Finally, utilizing strain-specific restriction fragment length differences, we have mapped the chromosomal position of hsp70A to the far right of chromosome IV.

Sequence analysis of the nonsteroid binding component of the calf uterine estrogen receptor

Microbore reversed-phase high performance liquid chromatography has been utilized to fractionate and purify a number of tryptic peptides generated from the 90K nonsteroid binding component of the calf uterine estrogen receptor. Sequence analysis was performed on six peptides yielding 78 unique amino acid assignments, this corresponds to approximately 10% of the molecule. These peptides share sequence similarities with three heat shock proteins, Drosophila hsp 83 (83% homologous), yeast hsp 90 (55%) and chicken hsp 108 (32%). The amino acid composition of the protein indicates a prevalence of charged amino acid residues.

Differential patterns of transcript accumulation during human myogenesis

We evaluated the extent to which muscle-specific genes display identical patterns of mRNA accumulation during human myogenesis. Cloned satellite cells isolated from adult human skeletal muscle were expanded in culture, and RNA was isolated from low- and high-confluence cells and from fusing cultures over a 15-day time course. The accumulation of over 20 different transcripts was compared in these samples with that in fetal and adult human skeletal muscle. The expression of carbonic anhydrase 3, myoglobin, HSP83, and mRNAs encoding eight unknown proteins were examined in human myogenic cultures. In general, the expression of most of the mRNAs was induced after fusion to form myotubes. However, several exceptions, including carbonic anhydrase and myoglobin, showed no detectable expression in early myotubes. Comparison of all transcripts demonstrated little, if any, identity of mRNA accumulation patterns. Similar variability was also seen for mRNAs which were also expressed in nonmuscle cells. Accumulation of mRNAs encoding alpha-skeletal, alpha-cardiac, beta- and gamma-actin, total myosin heavy chain, and alpha- and beta-tubulin also displayed discordant regulation, which has important implications for sarcomere assembly. Cardiac actin was the only muscle-specific transcript that was detected in low-confluency cells and was the major alpha-actin mRNA at all times in fusing cultures. Skeletal actin was transiently induced in fusing cultures and then reduced by an order of magnitude. Total myosin heavy-chain mRNA accumulation lagged behind that of alpha-actin. Whereas beta- and gamma-actin displayed a sharp decrease after initiation of fusion and thereafter did not change, alpha- and beta-tubulin were transiently induced to a high level during the time course in culture. We conclude that each gene may have its own unique determinants of transcript accumulation and that the phenotype of a muscle may not be determined so much by which genes are active or silent but rather by the extent to which their transcript levels are modulated. Finally, we observed that patterns of transcript accumulation established within the myotube cultures were consistent with the hypothesis that myoblasts isolated from adult tissue recapitulate a myogenic developmental program. However, we also detected a transient appearance of adult skeletal muscle-specific transcripts in high-confluence myoblast cultures. This indicates that the initial differentiation of these myoblasts may reflect a more complex process than simple recapitulation of development.

An unusual split Drosophila heat shock gene expressed during embryogenesis, pupation and in testis

Gene 2, one of the seven heat shock genes from locus 67B of Drosophila melanogaster, is transcribed into two polyadenylated RNAs having different developmental profiles of expression. The smaller transcript, of about 560 nucleotides, is expressed from mid-embryogenesis to the first two larval stages and again at the beginning of pupation. The larger transcript, 780 nucleotides, contains an additional 5' exon, accounting for its larger size. It is detected in pupae and adults, is male-specific and is localized in the testes. Heat shock does not affect the abundance of these two transcripts but induces the accumulation of a third RNA species of about 2000 nucleotides. This heat-shock RNA has the same cap site as the embryonic transcript, while its 3' portion entirely includes the neighbouring hsp22 gene. It appears, therefore, that in this case, heat shock alters the normal transcription termination process. By contrast to most heat shock genes, gene 2 contains several micro introns. One long open reading frame common to the three transcripts encodes a putative polypeptide of 111 amino acid residues. No homology was found with the other small heat shock genes of locus 67B.

Differential patterns of transcript accumulation during human myogenesis

We evaluated the extent to which muscle-specific genes display identical patterns of mRNA accumulation during human myogenesis. Cloned satellite cells isolated from adult human skeletal muscle were expanded in culture, and RNA was isolated from low- and high-confluence cells and from fusing cultures over a 15-day time course. The accumulation of over 20 different transcripts was compared in these samples with that in fetal and adult human skeletal muscle. The expression of carbonic anhydrase 3, myoglobin, HSP83, and mRNAs encoding eight unknown proteins were examined in human myogenic cultures. In general, the expression of most of the mRNAs was induced after fusion to form myotubes. However, several exceptions, including carbonic anhydrase and myoglobin, showed no detectable expression in early myotubes. Comparison of all transcripts demonstrated little, if any, identity of mRNA accumulation patterns. Similar variability was also seen for mRNAs which were also expressed in nonmuscle cells. Accumulation of mRNAs encoding alpha-skeletal, alpha-cardiac, beta- and gamma-actin, total myosin heavy chain, and alpha- and beta-tubulin also displayed discordant regulation, which has important implications for sarcomere assembly. Cardiac actin was the only muscle-specific transcript that was detected in low-confluency cells and was the major alpha-actin mRNA at all times in fusing cultures. Skeletal actin was transiently induced in fusing cultures and then reduced by an order of magnitude. Total myosin heavy-chain mRNA accumulation lagged behind that of alpha-actin. Whereas beta- and gamma-actin displayed a sharp decrease after initiation of fusion and thereafter did not change, alpha- and beta-tubulin were transiently induced to a high level during the time course in culture. We conclude that each gene may have its own unique determinants of transcript accumulation and that the phenotype of a muscle may not be determined so much by which genes are active or silent but rather by the extent to which their transcript levels are modulated. Finally, we observed that patterns of transcript accumulation established within the myotube cultures were consistent with the hypothesis that myoblasts isolated from adult tissue recapitulate a myogenic developmental program. However, we also detected a transient appearance of adult skeletal muscle-specific transcripts in high-confluence myoblast cultures. This indicates that the initial differentiation of these myoblasts may reflect a more complex process than simple recapitulation of development.