In vivo developmental modifications of the expression of genes encoding muscle-specific enzymes in rat

Integrating genome and transcriptome profiling for elucidating the mechanism of muscle growth and lipid deposition in Pekin ducks

Muscle growth and lipid deposition are co-ordinately regulated processes. Cherry Valley Pekin duck is a lean-type duck breed with high growth rate, whereas the native Pekin duck of China has high lipid deposition. Phenotypic analysis showed that native Pekin ducks have smaller fibre diameter and larger density in the breast muscle at 3 weeks of age and higher intramuscular fat content at 6 weeks of age than those in Cherry Valley Pekin ducks. We detected 17 positively selected genes (PSGs) by comparing genes mainly involved with muscle organ development, muscle contraction, peroxisome proliferator activated receptor signalling pathway, and fatty acid metabolism. In all, 52 and 206 differentially expressed genes (DEGs) were identified in transcriptomic comparisons between the two breeds at 3 and 6 weeks of age, respectively, which could potentially affect muscle growth and lipid deposition. Based on the integration of PSGs and DEGs and their functional annotations, we found that 11 and 10 genes were correlated with muscle growth and lipid deposition, respectively. Identification of candidate genes controlling quantitative traits of duck muscle might aid in elucidating the mechanisms of muscle growth and lipid deposition and could help in improving duck breeding.

Developmental regulation of the aldolase A muscle-specific promoter during in vivo muscle maturation is controlled by a nuclear receptor binding element

During the post-natal period, skeletal muscles undergo important modifications leading to the appearance of different types of myofibers which exhibit distinct contractile and metabolic properties. This maturation process results from the activation of the expression of different sets of contractile proteins and metabolic enzymes, which are specific to the different types of myofibers. The muscle-specific promoter of the aldolase A gene (pM) is expressed mainly in fast-twitch glycolytic fibers in adult body muscles. We investigate here how pM is regulated during the post-natal development of different types of skeletal muscles (slow or fast-twitch muscles, head or body muscles). We show that pM is expressed preferentially in prospective fast-twitch muscles soon after birth; pM is up-regulated specifically in body muscles only later in development. This activation pattern is mimicked by a transgene which comprises only the 355 most proximal sequences of pM. Within this region, we identify a DNA element which is required for the up-regulation of the transgene during post-natal development in body muscles. Comparison of nuclear M1-binding proteins from young or adult body muscles show no qualitative differences. Distinct M1-binding proteins are present in both young and adult tongue nuclear extracts, compared to that present in gastrocnemius extracts.

Proximal sequences of the aldolase A fast muscle-specific promoter direct nerve- and activity-dependent expression in transgenic mice

Muscle activity is known to modulate the muscle fiber phenotype. Changes in muscle activity (normal or experimentally induced) lead to modifications of the expression status of several muscle-specific genes. However, the transcription regulatory elements involved in the adaptative response are mainly unknown. The aldolase A muscle-specific promoter, pM, is expressed in adult fast twitch muscle with a preferential expression in fast glycolytic-2B fibers. Its activity is induced during postnatal muscle maturation, suggesting a role of nerve and/or muscle activity. Indeed, denervation of gastrocnemius in newborn mice prevented the activation of the promoter in this muscle, despite the nerve-independent formation of 2B fibers. Although the nerve was necessary for pM onset during development, denervating the gastrocnemius in adults had only mild effects on pM activity. By contrast, a transgene including the pM proximal regulatory sequences that are sufficient to reproduce the 2B fiber-specific expression of the endogenous promoter was shown to be highly sensitive to both neonatal and adult denervation. Transgenes containing muscle-specific pM proximal promoter elements were used to delineate the regulatory elements involved in this response to innervation and changes in the contractile activity pattern. Nerve- and activity-dependent elements could be localized in the 130-base pair-long proximal promoter region of the human aldolase A gene.

Role of E and CArG boxes in developmental regulation of muscle glycogen phosphorylase promoter during myogenesis

Muscle glycogen phosphorylase (MGP) transcript and protein levels increase during skeletal muscle development in tandem with the products of other muscle genes responsible for glucose and glycogen metabolism. Previous studies demonstrated that a 269 bp region 5' to exon 1 of MGP is sufficient for developmental regulation in the C2C12 myogenic cell line (Froman et al., 1994). This genomic region (-209 to +60) contains four consensus E box motifs, a CArG-like sequence, and a GC-rich domain. Native MGP transcripts were not detected in pluripotent CH310T1/2 fibroblasts, but low levels of MGP mRNA were measured in CH310T1/2 cells that were stably transfected with MyoD. Three of the E box motifs in the MGP proximal promoter interacted with C2C12 nuclear proteins. However, cotransfection of the MGP promoter with myogenic regulatory factors, including MyoD and myogenin, produced less than 2-fold activation compared with 20-fold activation of the desmin promoter. Mutational analyses of the MGP promoter demonstrated that increased expression in C2C12 myotubes did not require any of the E box motifs or the CArG-like element. A small region (-76 to -68) upstream of GC-rich domain (-64 to -51) significantly reduced promoter activities in both myoblasts and myotubes. The functional studies suggest that MGP is developmentally regulated during myogenesis by alternative pathways that utilize unidentified regulatory elements or ancillary factors.

Structure and expression of the murine muscle adenylosuccinate synthetase gene

A muscle-specific isoform of adenylosuccinate synthetase (AdSS1, EC) is one of three enzymes that constitute the purine nucleotide cycle, a muscle-specific metabolic cycle. Previously, we showed that the muscle Adss1 gene was highly expressed in both skeletal muscle and heart of the adult mouse. Here we have shown that the Adss1 gene is initially activated early in embryonic development in skeletal muscle and heart precursors and is subsequently up-regulated perinatally. The earliest detectable gene expression corresponds with the establishment of the first myogenic and cardiac lineages. To allow identification of the genetic signals controlling this developmental pattern of expression, the Adss1 gene was cloned and its structure determined. Transgenic analysis has shown that 1.9 kilobase pairs of 5' flank can activate expression in skeletal muscle progenitors and direct enhanced expression to adult cardiac muscle. Sequence analysis of the promoter and 5' flanking region revealed the presence of numerous potential muscle-specific cis-regulatory elements.

Regulation of the rat muscle glycogen phosphorylase-encoding gene during muscle cell development

The muscle isozyme of glycogen phosphorylase (MGP) catalyzes the hydrolysis hydrolysis of intracellular glycogen in mammalian tissues and is produced in skeletal muscle, brain and heart. The MGP gene is developmentally and neutrally regulated in skeletal muscle, but little is known about the gene's transcriptional regulation. We have isolated and characterized the 5' flanking region of rat MGP. Truncated portions of the MGP 5' flanking region were coupled to the bacterial cat reporter gene and used in transient transfection assays in the mouse muscle C2C12 cell line. The region between -211 and +62 contained the smallest regulatory domain capable of demonstrating developmentally regulated myogenic expression in C2C12 cells. This was in contrast with findings from another investigation that transfected this cell line with human MGP [Lockyer and McCracken, J. Biol. Chem. 266 (1991) 20262-20269]. A 172-nucleotide (nt) region between -839 and -666 functioned as a potent enhancer in C2C12 cells when coupled to its cognate promoter, but not when coupled to a simian virus 40 promoter. This rat MGP enhancer region is 78% identical to a comparable region of the human MGP 5' flanking region, but contains only one putative regulatory element that has been previously identified in other muscle genes. These data suggest that rat MGP transcription in C2C12 muscle cells is modulated by a potent enhancer that utilizes novel regulatory elements.

Myf5, MyoD, myogenin and MRF4 myogenic derivatives of the embryonic mesenchymal cell line C3H10T1/2 exhibit the same adult muscle phenotype

Cells of the embryonic mesenchymal cell line C3H10T1/2 have revealed the potential that the four regulatory factors belonging to the MyoD family have to activate myogenesis. In the present study we have further investigated the myogenic phenotype of C3H10T1/2 cells stably transfected with either Myf5, MyoD, myogenin or MRF4 cDNAs. We have studied the influence of each transfected cDNA on expression of the four endogenous muscle regulatory genes and on the ability of these embryonic myogenic derivatives to express adult muscle genes. No trace of endogenous transcripts distinct from the exogenous one was found in any of the four converted populations at the myoblast stage. This indicates that cross-activation within the MyoD family does not occur at the myoblast stage in these cells. Similarly, evidence was obtained that auto- or cross-activation of the Myf5 gene occurs neither at the myoblast stage nor at the myotube stage and that no autoactivation of the MRF4 gene occurs. Our results together with previous observations indicate that in C3H10T1/2 myogenic derivatives: (1) Autoactivation at the myoblast stage is restricted to MyoD (2) Expression from each cDNA alone is sufficient to establish and maintain the myoblast phenotype (3) The endogenous Myf5 gene is not mobilized. We have also observed that endogenous transcripts for MyoD and myogenin begin to accumulate at the onset of differentiation in the four myogenic derivatives, whereas accumulation of endogenous MRF4 transcripts starts after myotubes have formed and occurs at a much lower level (100- to 500-fold lower) than in differentiated cultures of myosatellite cells.(ABSTRACT TRUNCATED AT 250 WORDS)

Modulation of embryonic and muscle-specific enolase gene products in the developing mouse hindlimb

During striated muscle development, the glycolytic enzyme enolase (EC 4.2.1.11) undergoes an isozymic transition, from the embryonic alpha alpha form towards the muscle-specific forms alpha beta and beta beta. The regulation of this transition was analyzed in mouse hindlimb muscles from embryonic day 15 (E15) to the adult stage. The quantitative modulations of the levels of the transcripts and subunits of alpha and beta enolase genes were determined. The absolute amounts of alpha and beta enolase mRNAs were estimated using in vitro synthesized transcripts as calibration standards, thus allowing an evaluation of their relative contribution at each stage examined. The muscle-specific beta enolase mRNA is already present at E15. Its level then increases and, from E17, this transcript becomes predominant. This accumulation is biphasic: a steep prenatal rise, corresponding to a net increase per fiber, accompanies the formation of secondary myofibers and the development of innervation; a second rise, beginning at postnatal day 5, is temporally correlated with the definitive specialization of the myofibers. Most of the decrease in alpha mRNA level occurs postnatally. No temporal or quantitative correlation between the up-regulation of beta mRNA and the down-regulation of alpha mRNA levels is observed throughout hindlimb muscle development. Quantitative immunoblotting analyses carried out in parallel show that the enolase isozymic transition is mainly controlled at the mRNA level.(ABSTRACT TRUNCATED AT 250 WORDS)

The proximal promoter of the aldolase A gene remains active during myogenesis in vitro and muscle development in vivo

The gene for aldolase A in mouse has been shown to be regulated by alternative promoters with attendant alternative first exons. The distal promoter/exon M functions only in muscle while the proximal promoter/exon H is active in early muscle development and in most other tissues. We have analyzed the developmental expression of M and H promoters in mouse throughout myogenesis both in vitro and in vivo. In C2C12 cells RNase protection assays revealed the M promoter is induced within 24 hr of the onset of myogenic differentiation, and both M- and H-specific mRNAs accumulate over 5 days in culture. Nuclear run-on transcription and in situ hybridization with an exon-specific probe demonstrate that the H promoter remains transcriptionally active even in differentiated myotubes. The in vitro results were then compared to similar RNase protection studies of M and H expression during muscle development in vivo. These data show a marked similarity between promoter activation and steady-state transcript accumulation in vivo and in vitro, but within a limited developmental time frame (E15 to 1 week postnatal). In situ hybridizations suggest that simultaneous transcription from both promoters may also occur early in muscle development. Furthermore, the M promoter shows no fiber-type restriction until 1 to 3 weeks postnatally, coincident with muscle maturation, while the H promoter remains transcriptionally active at all stages of development and in all fiber types.

Effect of ACTH suppression on adrenal 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA in 4-aminopyrazolopyrimidine-treated rats

4-Aminopyrazolopyrimidine (4-APP) treatments to rats for 3 days induced 2-fold increase of circulating ACTH and 11-fold increase of adrenal 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase mRNA compared to NaCl-treated controls. This in vivo model was used to study the effect of the suppression of ACTH secretion on the adrenal HMG-CoA reductase mRNA level. Dexamethasone (Dex) administration to 4-APP-treated rats caused a rapid and parallel decline of the levels of plasma ACTH and adrenal HMG-CoA reductase mRNA to 50% within 2.5 h, whereas the free and esterified cholesterol content was increased 5 and 9.4 times respectively. These changes could be counteracted by the co-administration of ACTH with Dex. Aminoglutethimide (AG) administration to 4-APP-treated rats, which increased the adrenal esterified cholesterol content (7.5 times), decreased the HMG-CoA reductase mRNA level (44%), despite plasma ACTH level remaining elevated. Moreover, the participation of newly synthesized protein(s) in the lowering of adrenal HMG-CoA reductase mRNA level induced by ACTH suppression is suggested by the fact that cycloheximide (Cyclo), when co-administered with AG, completely blocked the decrease of HMG-CoA reductase mRNA level, despite the plasma ACTH level decreasing by 68% and the free and esterified cholesterol content increasing 3.9 and 12.3 times, compared to 4-APP-treated rats. Furthermore, the specificity of these effects was established by the fact that the beta-actin mRNA level was not affected by the administration of either Dex, AG, Cyclo, or AG + Cyclo to 4-APP-treated rats.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversible extinction of insulin gene expression in insulinoma x fibroblast somatic cell hybrids

To investigate for the presence of regulator(s) repressing the expression of insulin gene in cells other than pancreatic beta cells, rat insulinoma (RIN) cells secreting insulin were hybridized with fibroblasts from various species. In RIN x L mouse fibroblast hybrids, which maintained most of the parental chromosomes, no insulin transcripts were detected. In three RIN x Indian muntjac fibroblast hybrids and one RIN x human fibroblast hybrid which had lost DNA from the fibroblast parent through subculture, expression of the insulin gene was first extinguished and then reexpressed. This suggests that negative regulator(s), present in fibroblasts, can turn off insulin gene expression in RIN x fibroblast hybrids as long as the gene(s) contributed by the fibroblast are maintained.

Abnormal expression of glycogen phosphorylase genes in regenerated muscle

Physiological and molecular biological properties of free, orthotopic grafts of rat extensor digitorum longus (EDL) muscle were determined at 28-, 42-, and 76-days postgraft. cDNA probes for the rat fetal (B), liver (L), and muscle (M) isozymes of glycogen phosphorylase were used to assay isozyme mRNA levels. Regenerating muscle grafts did not express nonmuscle phosphorylase isozymes in vivo in contrast to primary rat skeletal muscle explants in vitro. Low levels of M-phosphorylase mRNA were present at all stages of regeneration in the grafts. However, M-phosphorylase mRNA levels and activity increased markedly and nonuniformly in a subset of functionally and morphologically stabilized regenerated muscle fibers between 42- and 76-days postgraft. Biochemical, physiological, and histochemical characterization of the stabilized grafts demonstrated that all fibers present were innervated and indicated that innervation might be a necessary, but not sufficient, condition for the increase in M-phosphorylase expression. The nonuniform appearance of phosphorylase activity suggests that a differential activity profile imposed on muscle fibers by their motoneuron may govern M-phosphorylase gene expression.

Distribution and developmental transition of phosphoglycerate mutase and creatine phosphokinase isozymes in rat muscles of different fiber-type composition

Phosphoglycerate mutase and creatine phosphokinase have in mammals three isozymes (types MM, MB and BB) with similar tissue distribution and developmental transition in muscle cells. To assess whether the phenotype and the developmental switch of these isozymes differ in the diverse types of muscle fibers, the enzymatic activities and the isozyme patterns, analyzed by cellulose acetate electrophoresis, have been determined in rat soleus, extensor digitorum longus and gastrocnemius muscles during postnatal development. Both phosphoglycerate mutase and creatine phosphokinase activity increased in the three muscles, the increase in extensor digitorum longus and gastrocnemius being higher than in soleus. For the two enzymes the increase in activity was due to the progressive increment of the muscle-specific forms. It is concluded that whereas phosphoglycerate mutase and creatine phosphokinase type-B subunits are present at similar levels in both type I and type II muscle fibers, phosphoglycerate mutase and creatine phosphokinase type-M subunits exhibit much higher levels in type II fibers.

Hormonal regulation of 3-hydroxy-3-methylglutaryl coenzyme A reductase mRNA in the rat adrenal gland

We studied the effect of ACTH on 3-hydroxy-3-methylglutaryl coenzyme A (HMG-CoA) reductase enzyme. Reductase activity and reductase mass were enhanced by 22- and 6.2-fold respectively in one series of experiments, whereas in another the levels of reductase activity, reductase mass, and reductase mRNA were increased 6.6-, 3.6- and 2.2-fold respectively, following daily administration of exogenous ACTH for 3 days. Daily injection of 4-aminopyrazolopyrimidine (4-APP) to rats for 3 days increased circulating ACTH level 5.4-fold, whereas adrenal HMG-CoA reductase activity, reductase mass and reductase mRNA levels were greatly increased 36-, 10- and 16-fold, respectively. To counteract the effect of elevated plasma ACTH, dexamethasone acetate (Dex) was administered to 4-APP treated rats. At 3 h post Dex administration, plasma ACTH and corticosteroids levels were effectively decreased by 58 and 59%, respectively. The levels of adrenal HMG-CoA reductase mRNA, reductase activity and reductase mass were also diminished by 38, 31 and 40%, respectively. Our results show that rat adrenal HMG-CoA reductase can respond rapidly to hormonal changes, presumably through variations in circulating ACTH levels.

Alternative promoter usage by aldolase A during in vitro myogenesis

Aldolase A in the mouse, as in human and rat, shows tissue-specific variability of message size. In addition, in muscle tissue the mRNA size is also developmentally regulated. In order to determine whether this muscle-specific regulatory mechanism can be reproduced in vitro, we have examined the mRNA species of aldolase A isolated from mouse C2C12 myoblasts and myotubes on Northern blots and by primer extension. We show that aldolase A mRNA increases during in vitro myogenesis; that this induction is accompanied by a change in the message population; and that this change is due to activation of a muscle-specific alternative promoter.

Asynchronous regulation of muscle specific isozymes of creatine kinase, glycogen phosphorylase, lactic dehydrogenase and phosphoglycerate mutase in innervated and non-innervated cultured human muscle

Expression of muscle specific isozymes (MSIs) of creatine kinase (CK, EC 2.7.3.2), glycogen phosphorylase (GP, EC 2.4.1.1), lactate dehydrogenase (LDH, EC 1.1.1.27) and phosphoglycerate mutase (PGAM, EC 2.7.5.3) was studied both in cultured human muscle fibers which had been innervated (InnCHMFs) for 20-83 days, and in their non-innervated (non-InnCHMFs) sister control. In non-InnCHMFs, the MSI of PGAM was never detected, and there was no change in the expression of the MSI of CK during the entire period examined; the expression of MSIs of LDH and GP showed linear increase during the entire period of growth. The expression of MSIs of all 4 enzymes was significantly enhanced in InnCHMFs as compared to non-innervated control. The expression of MSIs of GP and PGAM, and to a lesser degree of LDH increased significantly in correlation with the duration of innervation; the MSI of CK increased linearly only up to 54 days of innervation and plateaued afterward. This study demonstrates: (1) innervation of cultured human muscle fibers by fetal rat spinal cord exerts a time-related maturational influence on their cellular isoenzymatic pattern; (2) to achieve induction and characteristic time-related expression of various MSIs, the requirements for neuronal influences seem to differ.

Regulation of genes for glycolytic enzymes in cultured rat hepatoma cell lines

We examined the control by hormones and culture conditions of the expression of pyruvate kinase L, aldolase B, and a liver-specific 5.4-kb mRNA species [Pichard, A. L. et al. (1985) Biochem. J. 226, 637-644] in three rat hepatoma cell lines, MH1C1, Fao and Faza. The expression level of these markers ranges from 2% (for pyruvate kinase L mRNA) to 10-12% (for 5.4-kb mRNA species) of the glucose-induced mRNA values found in rat liver. The mRNAs of the three liver-specific genes strongly decrease after treatment of the hepatoma cells with cyclic 8-bromo-AMP, cyclic dibutyryl-AMP or forkolin, pyruvate kinase L mRNA being the most sensitive to this inhibiting effect. In contrast, the concentration of pyruvate kinase L mRNA nuclear precursors is not modified by the cyclic AMP analogues, indicating that these agents do not act at the transcriptional level but, instead, probably destabilize the transcripts. Glucose or fructose does not modify the expression of these three marker genes in any of the studied cell lines. Insulin is inefficient in modifying concentrations of the mRNAs for pyruvate kinase L and aldolase B, alone or in the presence of carbohydrates. In contrast, it stimulates about fivefold the expression of the 5.4-kb mRNA species in the MH1C1 cell line; this stimulation is carbohydrate-independent. The hepatoma cell lines mimic, therefore, the effect of cyclic AMP on the inhibition in vivo of the expression of genes encoding glycolytic or lipogenic enzymes [Vaulont, S. et al. (1984) Biochem. Biophys. Res. Commun. 125, 135-147]. In contrast, the effect of carbohydrates [Munnich, A. et al. (1984) J. Biol. Chem. 259, 10228-10231] is undetectable. The insulin sensitivity of the liver-specific genes is conserved for the 5.4-kb mRNA species only, especially in the MH1C1 cell line, but not for the other investigated mRNAs, which seems to reflect a fundamental difference in the in vivo effect of insulin on these genes. Finally, S1 nuclease mapping of the start-site of pyruvate kinase L gene transcription shows that the normal site used in vivo is also used in the Fao and Faza lines while, in the MH1C1 line, it coexists with multiple aberrant upstream initiation sites.

Characterization of three optional promoters in the 5' region of the human aldolase A gene

We undertook cloning and sequencing of the 5' portion of the human aldolase A gene to elucidate the mechanisms that govern synthesis of its different mRNAs. The sequenced gene is the only active gene in human-rodent fibroblastic somatic hybrids, while the other aldolase A-related sequences are inactive. S1 mapping and primer extension analysis enabled us to demonstrate that three promoter regions were implicated in the initiation of different aldolase A mRNAs, differing only in their 5' non-coding extremities. A distal promoter, N (non-specific), governs the synthesis of a 5' non-coding region of 142 bases composed of two exons, N1 and N2, which are found in a variety of tissues. A median promoter, M (muscle), is only active in skeletal muscle, and initiates the transcription by a 5' non-coding exon of 45 bases. Finally, a proximal promoter, H (housekeeping), contained in a "G + C-rich island", permits transcription of three colinear mRNAs containing 172, 126 or 112 bases of 5' non-coding sequence; their expression seems ubiquitous. These three promoters are arranged in 1.5 X 10(3) base-pairs of DNA. Homologies between rat and human genomic sequences and the absence of homology between promoters or 5' non-coding exons of the same species exclude a recent duplication of the promoter regions.

Structure and expression of the human creatine kinase B gene

Various cDNAs for creatine kinase type B (CK-B) were isolated from human cDNA libraries using a 26-oligonucleotide guess-mer probe. One of the cDNAs appeared to be almost full-length and contained an open reading frame coding for the 381 amino acid residues of the human CK-B polypeptide. The nucleotide sequences of the translated region as well as the primary protein structure show a high degree of homology with known CK-B and CK-M sequences of other vertebrates. The level of CK-B RNA as a measure of CK-B gene activity was determined in various human tissues and cultured cells. Our results confirm that CK-B is expressed in a tissue-specific manner and give support to the previously proposed relation between CK-B gene activity and cell proliferation. Screening of genomic DNA with various cDNA regions as probes revealed that there is only one CK-B gene per haploid genome. Gene cloning and sequencing indicated that CK-B is coded for by a relatively small gene of 3.2 kb in size, which is partially overlapped by an HTF island (A. P. Bird (1986) Nature (London) 321, 557-558) with an extremely high G + C content at its 5' end.

Molecular cloning and expression of rat aldolase C messenger RNA during development and hepatocarcinogenesis

A rat brain cDNA library was screened at low stringency with an aldolase B cDNA probe corresponding to the coding sequence of the mRNA, then at high stringency with a 3' non-coding aldolase A cDNA probe. One clone, which hybridized only under the first conditions, was further characterized and used to screen the library again. Two overlapping clones, complementary to aldolase C mRNA, were obtained. They cover the 113 carboxy-terminal coding residues and the 3' non-coding region up to the poly(A) tail. Their nucleotide sequence was determined. In the coding region the overall homology with aldolase A was 67% at the nucleotide level and 76% at the protein level. With aldolase B these values were 63% and 65% respectively. The 3' non-coding region was 380 bases long and did not exhibit any homology with the untranslated 3' extension of aldolase A and B mRNAs. Southern blot analysis indicates that probably a single aldolase C gene exists per haploid genome. Aldolase C mRNA was detected at low concentration in practically all the foetal tissues and its expression markedly and rapidly decreased after birth. In brain the concentration of aldolase C mRNA remained high and stable even after birth. Aldolase C mRNA is approximately 50-fold more abundant in brain than in foetal tissues, which are the richest in messenger RNA. In the course of azo-dye hepatocarcinogenesis the aldolase C gene is re-expressed early, with a maximum at the 4th week of carcinogenic diet, which probably corresponds to the maximal proliferation of the oval cells.