Isolation and characterization of two molecular variants of myosin heavy chain from rabbit ventricle. Change in their content during normal growth and after treatment with thyroid hormone

We have prepared monoclonal antibodies specific for cardiac myosin heavy chain. These antibodies were used for the separation and characterization of the molecular variants of myosin heavy chain present in the rabbit heart. Two molecular forms of myosin heavy chain, HC alpha and HC beta, were isolated from the euthyroid rabbit heart by affinity chromatography. Their reactivity with our antibodies indicated that the primary structures of HC alpha and HC beta differ in at least four and share at least two antigenic determinants. Differences in the primary structure of HC alpha and HC beta were confirmed by analysis of the peptides produced by limited chymotryptic digestion of the two heavy chains. Thirteen peptide differences were consistently found. The HC alpha and HC beta variants are shown by immunologic analysis and in chymotryptic peptide profiles to be identical with the predominant forms of myosin heavy chain synthesized in the hearts of hyperthyroid and adult euthyroid rabbits, respectively. During development and maturation of the euthyroid rabbit heart, HC alpha comprises approximately 50% of the ventricular myosin between birth and 4 weeks of age; it diminishes to 20-30% by 8 weeks and to 10-20% by 12 weeks of age. Cardiac myosin from a 1-year-old rabbit is composed almost entirely of HC beta. Cardiac myosin from embryonic animals at 20 days gestation contained 20% HC alpha. These results show that HC alpha occurs normally in the euthyroid rabbit heart and that the relative proportions of HC alpha and HC beta depend on both the developmental stage and the thyroid state of the animal.

Regulation of the nuclear-coded peptides of yeast cytochrome c oxidase

We have analyzed the catabolite regulation of cytochrome oxidase by assaying changes in the synthesis of precursors of the nuclear-coded peptides (IV--VII) of cytochrome c oxidase in an in vitro reticulocyte cell-free system programmed with RNA isolated from cells grown in either glucose or raffinose. As a first step, we have characterized antibodies which bind to the precursors of subunits V and VI. Initial translation products for subunits IV and VII have also been tentatively identified by utilizing these antibodies. The messenger RNAs coding for the precursors of the nuclear-coded subunits fall in the expected size range of 8--15 S. Catabolite repression of the nuclear-coded oxidase peptides appears to be regulated by the abundance of their messenger RNAs. Translation of messenger RNA isolated from yeast cells grown on glucose indicates a coordinate and uniform increase in precursor synthesis during glucose derepression. In contrast, when RNA isolated from raffinose (derepressed) grown cells is used to direct cell-free translation, precursor abundance is high throughout growth, although the synthesis of some of the species changes in a complex pattern of ratio and abundance. These data indicate that the abundance of the messengers for the nuclear-coded precursors is regulated in a fashion dependent on the physiologic state of the cell.

Phospholipid accumulation during the cell cycle in synchronous cultures of the yeast, Saccharomyces cerevisiae

Phospholipid concentrations have been examined throughout successive cell cycles in synchronously growing cultures of the yeast, Saccharomyces cerevisiae. Total phospholipid phosphorus, as well as lecithin and phosphatidylethanolamine levels, exhibited stepwise increases during the cell cycle with step increments beginning just prior to new rounds of bud formation. Phosphatidylinositol and phosphatidylserine levels, on the other hand, showed what have been interpreted to be peak concentrations near the time of bud formation. Cardiolipin content varied considerably and was dependent upon the carbon source of the growth medium. Glucose-grown cells exhibited peak concentrations of cardiolipin near the time of bud formation, with marked decreases after this time. In contrast, galactose-grown synchronous cells exhibited stepwise increments in cardiolipin content, with step increases occurring near the time of new rounds of bud formation. Step or peak increases in cardiolipin, as well as all other phospholipids, were found to coincide with the time of stepwise increases in cytochrome c oxidase activity in these cells. No correlations were observed between the elaboration of mitochondrial membranes during the synchronous cell cycle and the observed patterns of phospholipid increase.

Transcriptional initiation and 5' termini of yeast mitochondrial RNA

We have used vaccinia virus guanylyltransferase to label polyphosphate-terminated yeast mitochondrial RNAs in vitro with [alpha-32P]GTP. Hybridization of RNA labeled in vitro indicates the presence of multiple transcriptional initiation sites in both grande and petite mitochondrial genomes. Agarose/urea gel electrophoresis of capped RNA suggests the existence of a precursor to the small (14 S) rRNA. In contrast, direct examination of the large (21 S) rRNA by partial ribonuclease T1 digestion reveals a complete lack of processing of the 5' end of the primary transcript of this RNA.

Molecular cloning of two fast myosin heavy chain cDNAs from chicken embryo skeletal muscle

Recombinant DNA clones containing sequences for two different types of myosin heavy chain (HC) genes from chicken embryonic skeletal muscle were constructed and analyzed. Specificity of the clones for myosin HC was demonstrated by hybrid-arrested translation, by hybridization to a 7.0-kb mRNA, and by comparison of DNA sequences with known amino acid sequences of rabbit skeletal muscle myosin HC. Restriction enzyme and electron-microscopic heteroduplex analysis showed the presence of two distinct but homologous cDNA sequences. Hybrid melting curves indicated that both types of sequences represent fast myosin HC sequences.

Purification of mitochondrial RNA polymerase from Saccharomyces cerevisiae

The RNA polymerase from the mitochondria of Saccharomyces cerevisiae has been extensively purified by Sepharose 4B, heparin Sepharose 4B phosphocellulose, and DEAE-Sephadex A-50 chromatography. The activity co-sediments with a 45,000-dalton polypeptide at 6.3 S in glycerol gradients. The activity is inhibited by antibodies to the 45,000-dalton polypeptide. The activity is not inhibited by rifampicin or alpha-amanitin. It requires Mg2+ and is inhibited by elevated ionic strength and Mn2+. The most efficient template for the RNA polymerase is poly[d(AT)], with mtDNA being the preferred natural template. The RNA polymerase transcribes mtDNA from the petite strain F11 in a nonrandom manner.

Mitochondrial transcription complex from Saccharomyces cerevisiae

A DNA protein complex has been isolated from the mitochondria of Saccharomyces cerevisiae. The complex transcribes RNA complementary to mtDNA in a nonrandom manner. The RNA polymerase activity contained in the transcription complex is not dependent on the addition of exogenous template. The activity is rendered template-dependent by autolysis and can be further purified by heparin-Sepharose 4B chromatography. The activity is inhibited by heparin, Mn2+, and increasing ionic strength. The activity requires Mg2+ and ribonucleotides. The preferred template for the template dependent activity is poly[d(AT)]. The majority of the RNA synthesized by the transcription complex from endogenous DNA is complementary to the DNA strands directing the synthesis of the large and small ribosomal RNA. In yeast the 21 S and 14S rRNA genes are widely separated, therefore the transcription of these two regions but not of the intervening regions by the transcription complex suggests the existence of at least two transcriptional promoters on the yeast mitochondrial genome.

Purification of messenger ribonucleic acids for fast and slow myosin heavy chains by indirect immunoprecipitation of polysomes from embryonic chick skeletal muscle

Fast and slow myosin heavy chain mRNAs were isolated by indirect immunoprecipitation of polysomes from 14-day-old embryonic chick leg muscle. The antibodies were prepared against myosin heavy chains purified by NaDod-SO4-polyacrylamide gel electrophoresis and were shown to be specific for fast and slow myosin heavy chains. The RNA fractions directed the synthesis of myosin heavy chains in a cell-free translation system from wheat germ. Several smaller peptides were also synthesized in lower concentrations. These probably are partial products of myosin heavy chains, since they are immunoprecipitated with antibodies to myosin heavy chains. Immunoprecipitation of the translation products with the antibodies to fast and slow myosin heavy chains showed the RNA preparations to be approximately 94% enriched for fast myosin heavy chain mRNA and approximately 84% enriched for slow myosin heavy chain mRNA with respect to myosin HC type. Peptides having slightly different mobilities on NaDodSO4-polyacrylamide gels were immunoprecipitated by antibodies to fast and slow myosin heavy chains.

Processing of precursors of 21S ribosomal RNA from yeast mitochondria

The transcription and processing of mitochondrial 21S rRNA in a petite strain of Saccharomyces cerevisiae has been examined by electron microscopic analysis of R-loop hybrids and by hybridization of labeled mitochondrial DNA probes to RNA transferred to diazobenzyloxymethyl paper. We have shown the presence of a large [5.1- to 5.4-kilobase (kb)] transcript that appears to be a precursor of mitochondrial 21S rRNA. This transcript contains sequences homologous to those of the mature 21S rRNA, to the intervening sequence present in the gene, and to additional sequences at the 3' end of the molecule. Our data suggest that this precursor of 21S rRNA is processed in two steps. The intron sequence is usually excised first, followed by removal of the extra 3' sequences. In some cases, however, the 3' extension is first removed and the intron sequence is then excised. Both pathways appear to lead to formation of the 3.1-kb mature 21S rRNA and a stable 1.2-kb intron transcript. Similar results were obtained with grande MH41-7B mitochondrial RNA by RNA transfer hybridization. We have also observed a number of additional transcripts that may be normal processing intermediates or may result from faulty cleavage-ligation during excision of the intervening sequence.

Mitochondrial proliferation in cardiac hypertrophy

Mitochondrial proliferation was studied in mature female rats following aortic constriction. Mitochondrial DNA (mtDNA) was assayed by a fluorometric method. The conditions for removal of nuclear DNA were developed and verified by assessment of molecular conformation of DNA. The mtDNA concentration in mitochondria increased 2,4, and 7 days post-operatively by 11, 72 and 117% respectively. Comparison with the rates of accumulation of cytochrome c, b, and aa3 indicates that during the first 24 hours of cardiac enlargement the inner mitochondrial components accumulate faster then mtDNA, but during the six subsequent days the rate of mtDNA increment far outstrips that of the cytochromes. These data indicate that the amount of available mtDNA templates is not the only factor regulating the transcriptional and translational processes in the enlarging myocardium. The analysis of population of replicative intermediates of mtDNA have shown dramatic decrease in the frequency of D-loops in preparations obtained from hypertrophied hearts. This observation indicates that the increase in replicative flux of mtDNA is associated with the removal of a block in the conversion of D-loops to other intermediates.

Transcription, processing, and mapping of mitochondrial RNA from grande and petite yeast

Mitochondrial RNA (mtRNA) from petite yeast strains was analyzed by electrophoresis in agarose-urea, acrylamide-urea, and agarose-methyl mercuric hydroxide gels, and by transfer to diazobenzyloxy-methyl paper and hybridization to labeled mitochondrial DNA (mtDNA). Petites contain numerous mitochondrial transcripts, including processed species like 21 S and 14 S rRNA. Petite transcripts were found to fall into three classes: 1) bands that comigrate with grande mtRNA species; 2) "group-specific" new bands found in multiple strains and coinciding with specific regions of the mitochondrial genome; and 3) "strain-specific" new bands found only in individual petite strains. A deletion map was constructed in which we used the presence or absence of the first two types of mtRNA bands in specific strains, and the restriction endonuclease map of these strains. This map confirmed the localization of 21 S and 14 S rRNA, which were mapped previously by hybridization, and also localized more than 20 additional mtRNA species. The mtRNA species were grouped in regions of the genome in a fashion that strongly suggests that many of them are precursors to fully processed mtRNA species. Hybridization experiments with grande mtRNA and cloned mtDNA fragments have shown the same kind of transcript grouping. Other hybridization experiments have demonstrated two apparent precursors to 21 S rRNA (3700 nucleotides) measuring 5500 and 4500 nucleotides. Processed tRNAs are found only in petites that contain a specific region of the genome near the P (paromomycin resistance) locus. When this region is absent, processed tRNAs are not detected, even for tRNA genes quite distant from the P locus. Since this phenotype is expressed in petites that lack mitochondrial protein synthesis, and since it maps to a specific location in the mitochondrial genome, there appears to be a mtRNA species which has a role in processing of mitochondrial tRNA.

Differential competition with cytotoxic agents: an approach to selectivity in cancer chemotherapy

An approach to increasing the selectivity of cancer chemotherapeutic agents is presented in which noncytotoxic competitive substrates are used to discern the differences in structural requirements for transport of cytotoxic agents between tumor cells and a sensitive host tissue, the hematopoietic precursor cells of the bone marrow. Examples are given for two such systems, one responsible for the transport of nucleosides and another for the transport of amino acids. Cytidine is twice as effective in reducing the toxicity of showdomycin for murine bone marrow cells in culture as it is for murine L1210 leukemia cella. Conversely, homoleucine is twice as effective in reducing the toxicity of melphalan for L1210 cells as it is for bone marrow cells. These observations can serve as a basis for the development of bone marrow protective agents and for the design of cytotoxic agents that may be preferentially transported into tumor cells.

Changes in mitochondrial DNA in cardiac hypertrophy in the rat

We studied DNA (mtDNA) replication in adult female rat hearts undergoing hypertrophy secondary to constriction of the ascending aorta. MtDNA was measured in isolated mitochondria by a fluorometric method adapted for that purpose. The conditions for removal of contaminating nuclear DNA were developed, and the purity of the mtDNA was assessed from its molecular conformation (open and closed circles) and by renaturation-kinetic analysis. The mtDNA concentration in mitochondria, expressed as micrograms of DNA per milligram of mitochondrial protein, increased 2, 4, and 7 days postoperatively by 21, 73, and 98%, respectively. Similar results were obtained when mtDNA was expressed per nonomole of cytochrome a. The population of replicative intermediates of mtDNA was analyzed by electron microscopy. In normal hearts, we observed molecular forms characteristic of animal mtDNA, such as circular monomers and dimers, catenated molecules, D-loops, expanded D-loops, and gapped molecules. D-loop frequency, which was near 50% in the mtDNA of control hearts, was markedly reduced to 5-7% in hypertrophying hearts. This result indicates that the increase in replicative flux of mtDNA is associated with the removal of a block in the conversion of D-loops to other intermediates.

Molecular aspects of cardiac hypertrophy

Despite continuous interest in cardiac hypertrophy, our knowledge of its molecular aspects is still elementary. Recently, however, several advancements of particular interest have been made: (a) Nuclei of muscle and nonmuscle cells have been separated, allowing for the first time the study of nuclear activity in specified cells (18). (b) Cardiac growth induced by pressure-overload (72) or by hormone treatment (26) has been shown to lead to myosin of altered ATPase, and strong evidence suggests that new species of myosin molecules thus appear. (c) The basis for assessment of protein synthesis and degradation has been established (46, 48). (d) Methods are being developed to supplement radioautography in evaluating cell proliferation (42, 59, 69). (e) In spontaneously hypertensive rats it has been shown that blood pressure might not be the sole factor responsible for cardiac enlargement, but that hypertrophy can be the result of genetic cardiovascular abnormality (19, 66). (f) A hypothesis relating the extent of energy utilization to the nuclear activity via NAD+ metabolism has been proposed, which allows for experimental verification (43).