Regulation of myosin synthesis by thyroid hormone: relative change in the alpha- and beta-myosin heavy chain mRNA levels in rabbit heart

The expression of mRNAs for two cardiac myosins has been examined in the ventricles of hypo- and hyperthyroid rabbits by means of cloned cDNA sequences corresponding to the mRNAs of the alpha- and beta-myosin heavy chains (HCs). The temporal change in the relative levels of the alpha- and beta-HC mRNAs after 3,5,3'-triiodothyronine (T3) treatment of hypothyroid rabbits was determined by nuclease S1 mapping. In the hypothyroid state, only HC beta-mRNA was expressed in the ventricles. The HC alpha-mRNA was first detectable 4 h after administration of T3 (200 micrograms/kg) to hypothyroid animals. By 12 h, HC alpha-mRNA represented 20% of total myosin mRNA, increasing to 50% by 24 h and to about 90% by 72 h. The relationship between the relative mRNA levels and relative synthesis rates of the myosin HCs was evaluated in 5-6-week-old normal and thyrotoxic rabbits. Myosin synthesis rates were determined by labeling of protein in vivo with [3H]leucine. The V1 (HC alpha) and V3 (HC beta) isomyosins were separated by affinity chromatography with monoclonal antibodies, and the HCs were isolated electrophoretically. In a normal euthyroid group of animals and in animals 12 and 24 h after administration of 200 micrograms of 3,5,3',5'-tetraiodothyronine/kg, the relative mRNA levels and relative synthesis rates of the alpha- and beta-HCs were not significantly different. Our results show that, first, thyroid hormone causes a rapid accumulation of HC alpha-mRNA and loss of HC beta-mRNA and, second, in normal and thyrotoxic rabbits, the relative synthesis rates of HC alpha and HC beta reflect the relative abundance of the alpha- and beta-HC mRNAs.(ABSTRACT TRUNCATED AT 250 WORDS)

Analysis of cloned mRNA sequences encoding subfragment 2 and part of subfragment 1 of alpha- and beta-myosin heavy chains of rabbit heart

Two cardiac myosin heavy chain cDNA clones, pMHC alpha 252 and pMHC beta 174, were constructed using rabbit ventricular mRNA isolated from adult thyrotoxic and normal hearts, respectively. The complete DNA sequences of the 2.2- and 1.4-kilobase inserts of pMHC beta 174 and pMHC alpha 252, respectively, were obtained. The 736 amino acids specified by pMHC beta 174 begin 439 (1.3 kilobases) residues from the heavy chain NH2 terminus and include a 400-amino acid segment of subfragment 1 and the entire subfragment 2 region. Clone pMHC alpha 252 encodes 465 amino acids encompassing all of subfragment 2 and a portion of light meromyosin. Comparison of these two clones revealed extensive sequence overlap which included 1107 nucleotides specifying a 369-amino acid segment corresponding to subfragment 2. Within this region 78 (7%) base and 32 (8.7%) amino acid mismatches were noted. These differences were clustered within discrete regions, with the subfragment 1/subfragment 2 junctional region being particularly divergent. Structural differences between pMHC alpha 252 and pMHC beta 174 indicate that these two clones represent two similar but distinct myosin heavy chain genes whose expression is responsible for ventricular myosin heavy chain isoforms alpha and beta, respectively. The derived amino acid sequences of both clones exhibit extensive homology (greater than 81%) with sequences obtained by direct analysis of adult rabbit skeletal muscle myosin heavy chain protein. The sequences corresponding to the subfragment 2 region are consistent with an alpha-helical conformation with a characteristic 7-residue periodicity in the linear distribution of nonpolar amino acids. Conversely, subfragment 1 sequences specified by pMHC beta 174 suggest a folded highly irregular structure.

Variability of blood lead concentrations during infancy

As part of a study of early childhood development, more than 200 children had their blood lead concentrations (PbB) determined semiannually during the first 2 yr of life. These children were selected from 11,837 consecutive births surveyed for umbilical cord PbB at Boston Lying-In Hospital. Candidate subjects were drawn from the highest, lowest, and middle deciles of PbB. The mean PbB was 7.2 +/- 5.3 (standard deviation) micrograms/dl at birth and did not change appreciably with age. However, the average change in an individual's PbB every 6 months was 4 micrograms/dl, which was several fold in excess of the analytical reproducibility. Only 25% of the children in the highest category at birth were in the highest category at 2 yr of age. Approximately 40% of the children remained in their immediately previous PbB tertile category. A stochastic description of these patterns of change fits the data. Our results should caution investigators who might wish to rely on a single determination to categorize children with PbB.

Initiation of transcription of the yeast mitochondrial gene coding for ATPase subunit 9

We have determined transcriptional initiation sites for the ATPase subunit 9 gene on the yeast mitochondrial genome. Using S1 nuclease mapping, in vitro capping of primary transcripts with GTP and guanylyl transferase, and in vitro transcription analysis with purified mitochondrial RNA polymerase, we find the major site of transcriptional initiation to be at a point 630 nucleotides upstream of the coding region for the gene. In addition, we find much lower levels of initiation at a second site 78 nucleotides downstream of the first. Both initiation sites occur at the same position within a nonanucleotide sequence which we have previously found associated with initiation of rRNA synthesis. This work further supports the notion that this nonanucleotide sequence is an integral component of mitochondrial promoters and indicates that the same RNA polymerase is used for transcription of both mRNA and rRNA in yeast mitochondria.

Identification of multiple transcriptional initiation sites on the yeast mitochondrial genome by in vitro capping with guanylyltransferase

We have studied transcriptional initiation in the mitochondria of the yeast Saccharomyces cerevisiae by analyzing mitochondrial transcripts from grande and petite yeast after labeling in vitro with vaccinia virus guanylyltransferase and [alpha-32P]GTP. This procedure labels triphosphate-terminated RNA which arises from transcriptional initiation. Exploiting the extremely low GC content (18%) of yeast mitochondrial DNA, we digested the in vitro capped transcripts with the G-specific ribonuclease T1; this resulted in 27 oligonucleotides varying in size from 2 to 51 nucleotides. RNA from 14 overlapping petites was analyzed and 20 transcripts were localized by deletion mapping. Nineteen oligonucleotides were sequences and 13 were identified and precisely localized by comparison with known DNA sequences. In all cases, transcription is initiated at a consensus nonanucleotide sequence which can be considered part of the yeast mitochondrial promoter. We identified initiation sites for the 21 S and 14 S rRNAs; the phenylalanine, f-methionine, and glutamic tRNAs; two sites for the OLI-1 gene; and three for the ori (rep) regions. Most promoters appear to give rise to very long multigene primary transcripts. Examples are multigene transcripts for the glutamic tRNA and COB genes and for the OLI-1, serine tRNA, and Var genes. Since the consensus nonanucleotide sequences at the ori regions are similar to those at other transcriptional initiation sites, it is likely that the same RNA polymerase primes DNA replication and gene transcription.

Identification of a single transcriptional initiation site for the glutamic tRNA and COB genes in yeast mitochondria

We have identified a single transcriptional initiation site for the glutamic tRNA and COB (cytochrome b) genes by using the complementary techniques of in vitro capping of RNA and in vitro transcription. In the capping reaction, mitochondrial RNA is labeled with [alpha-32P]GTP by vaccinia virus guanylyltransferase. This reaction is specific for the 5' ends of RNA retaining the terminal triphosphate of transcriptional initiation. Exploiting the extremely low G+C content (18%) of yeast mitochondrial DNA, we digested in vitro capped transcripts from various petite deletion mutants with the G-specific RNase T1. By petite deletion mapping, a capped transcript giving rise to a 51-base RNase T1-generated oligonucleotide was localized near the glutamic tRNA gene. When the sequence of this oligonucleotide was determined, it perfectly matched the DNA sequence 391 base upstream of the glutamic tRNA. Purified yeast mitochondrial RNA polymerase initiated transcription in vitro at the same site as shown by the sequence of the 33-base oligonucleotide product of the reaction performed in the absence of CTP. Initiation starts at a nonanucleotide sequence previously implicated in yeast mitochondrial transcriptional initiation. Because there is no evidence of an initiation site in the 1,050 bases between the glutamic tRNA and COB genes, the two genes are likely to be transcribed together. Further evidence of a long common transcript was provided by RNA blot hybridization.

Cloned mRNA sequences for two types of embryonic myosin heavy chains from chick skeletal muscle. II. Expression during development using S1 nuclease mapping

We have examined the expression of two embryonic myosin HC mRNAs using two cDNA clones (110 and 251) which we have previously constructed from RNA isolated from 14-day-old embryonic chick skeletal muscle. Sequence divergence in the 3' nontranslated regions enabled us to analyze the differential expression of the mRNAs corresponding to the two clones using the S1 nuclease mapping procedure. Clone 251 mRNA is expressed primarily in embryonic fast muscle, where its transcripts appear to be the predominant species. This mRNA is minimally expressed in the posthatching period, but it is not detected in adult leg and breast muscle. Messenger RNA for clone 110 is also primarily expressed in embryonic fast muscle. However, in the posthatching and adult stages of development, it continues to be expressed at a low level in leg muscle but not in breast muscle. The differential expression of these mRNAs during development strongly indicates that they correspond to two different genes coding for embryonic myosin HCs. Other myosin HC mRNAs which were partially homologous to the clone 110 or 251 mRNAs were also identified by S1 nuclease mapping. Using the probes from these two clones, a minimum of four other developmentally expressed forms were detected. Two of these correspond to "neonatal" myosin HCs, while the other two code for different adult myosin HCs present in leg and in breast muscle, respectively. The results therefore suggest a much greater diversity of myosin HC mRNAs expressed during development than previously reported.

Cloned mRNA sequences for two types of embryonic myosin heavy chains from chick skeletal muscle. I. DNA and derived amino acid sequence of light meromyosin

Two myosin heavy chain cDNA clones (251 and 110), constructed from chick embryonic skeletal muscle mRNA, were subjected to extensive DNA sequence analysis. A complete description of the DNA sequence of clone 251 was obtained. This 1.5-kilobase pair cDNA sequence specified the COOH-terminal 439 amino acids of the myosin heavy chain, and included the entire 3' nontranslated region. The translated and 3' nontranslated sequences were purine- (64%) and AT-(71%) rich, respectively. The derived amino acid sequence of clone 251 correlated well with sequences obtained by direct amino acid sequencing of adult rabbit back muscle myosin heavy chain protein (87% homology), as well as with cloned myosin heavy chain sequences from other species. Comparison of clone 251 with a partial DNA sequence of clone 110 revealed significant structural differences both in the translated, and 3' nontranslated regions. This data indicates that these two clones represent two distinct myosin heavy chain genes. The protein sequence specified by clone 251 corresponds to the light meromyosin portion of the myosin heavy chain rod. These sequences, like other myosin heavy chain rod sequences, are alpha-helical and exhibit 7- and 28-residue periodicities in the linear distribution of nonpolar, and basic and acidic amino acids, respectively.

Stringent requirement for Ca2+ in the removal of Z-lines and alpha-actinin from isolated myofibrils by Ca2+-activated neutral proteinase

Treatment of isolated myofibrils with Ca2+-activated neutral proteinase (CANP) results in specific removal of Z-line and of alpha-actinin. To investigate the ionic requirement for these processes, we measured Z-line removal by phase-contrast and interference microscopy and alpha-actinin removal by sodium dodecyl sulphate/polyacrylamide-gel electrophoretic analysis of myofibrillar proteins. The proteolytic digestion of native purified proteins was measured directly on polyacrylamide gels and by the fluorescamine technique. We found that the removal of Z-line and alpha-actinin as well as the release of proteolytic degradation products from isolated myofibrils by CANP occur only in the presence of Ca2+; Sr2+, Ba2+, Mn2+, Mg2+, Co2+ and Zn2+ are all ineffective. In contrast with this stringent requirement for Ca2+, the proteolytic activity of CANP measured with denatured casein, native and denatured haemoglobin, native actin and tropomyosin also occurs in the presence of other bivalent cations, in the following order: Ca2+ greater than Sr2+ greater than Ba2+. These data suggest that only Ca2+ can produce the conformational change in myofibrils that renders them susceptible to the action of CANP, whereas its proteolytic activity is stimulated by several bivalent ions.

Identification of initiation sites for heavy-strand and light-strand transcription in human mitochondrial DNA

The initiation sites for heavy (H) and light (L) strand transcription in HeLa cell mitochondrial DNA have been investigated by mapping experiments utilizing in vitro "capped" mitochondrial RNA molecules or nascent RNA chains. Mitochondrial poly(A)-containing RNA molecules were labeled at their 5' ends with [alpha-32P]GTP and guanylyltransferase ("capping" enzyme) and mapped on the mitochondrial genome by DNA transfer hybridization and S1 nuclease protection experiments. A mapping site for the capped 5' ends was found on the H strand very near to the 5' terminus of the 12S rRNA gene, and another site was found on the L strand very near to the 5' terminus of the 7S RNA coding sequence. In parallel experiments, the 5' ends of the nascent chains isolated from mitochondrial DNA transcription complexes were similarly mapped very near to the 5' termini of the 12S rRNA gene and of the 7S RNA coding sequence. The in vitro capped RNA molecules and the nascent chains thus presumably identify the same transcriptional initiation sites on the H strand and the L strand. The occurrence of a second possible initiation site for H-strand transcription 90-110 nucleotides upstream of that described above--i.e., 20-40 nucleotides upstream of the tRNAPhe gene--had been previously indicated by a mapping analysis of the nascent RNA chains and has been confirmed in the present work. The presence of two initiation sites for H-strand transcription can be correlated with other types of evidence that point to two different transcription events leading to the synthesis of a polycistronic molecule corresponding to the almost entire H strand and to the synthesis of the rRNA species.

Analysis of transcriptional initiation of yeast mitochondrial DNA in a homologous in vitro transcription system

We have developed an in vitro transcription system for yeast mitochondrial rRNA genes. Using highly purified yeast mitochondrial RNA polymerase and bacterial plasmids carrying DNA segments containing the mitochondrial rRNA sites of transcriptional initiation, we have been able to demonstrate correct initiation of transcription in vitro. By directly sequencing the transcription products, we show that transcription in vitro of both the 14S and 21S rRNAs is initiated at precisely the same site as it is in vivo. Transcription of the rRNA genes is highly sensitive to ionic strength and RNA polymerase concentration. Additional factors or modified conditions may be necessary to permit accurate transcription of mitochondrial protein genes.

Molecular cloning of mRNA sequences for cardiac alpha- and beta-form myosin heavy chains: expression in ventricles of normal, hypothyroid, and thyrotoxic rabbits

We have isolated cDNA clones from thyrotoxic (pMHC alpha) and normal (pMHC beta) adult rabbit hearts. Restriction map analysis and DNA sequence analyses show that, although there is strong homology between overlapping regions of the two clones, they are distinctly different. The two clones exhibited 78-83% homology between the derived amino acid sequences and those determined by direct amino acid sequence analysis of rabbit fast skeletal muscle myosin heavy chains. The clones specify a segment of the myosin heavy chain corresponding to subfragment 2 and the COOH-terminal portions of subfragment 1. Nuclease S1 mapping was used to compare transcription of the two clones with expression of the alpha and beta forms of myosin heavy chains in the ventricles of thyrotoxic, hypothyroid (propylthiouracil-treated), and normal rabbits. Thyrotoxic ventricles contained only pMHC alpha transcripts whereas hypothyroid ventricles contained exclusively pMHC beta transcripts. These data correlate well with the presence of alpha- and beta-form myosin heavy chains. In the normal young adult rabbit, pMHC beta transcripts predominate, agreeing with the known beta form/alpha form ratio of 4:1. We therefore conclude that pMHC alpha and pMHC beta contain sequences of the alpha- and beta-form myosin heavy chain genes, respectively.

Transcriptional initiation and processing of the small ribosomal RNA of yeast mitochondria

We have identified the nucleotide at which transcription initiates on the yeast mitochondrial small (14 S) rRNA gene by sequencing of RNA labeled at the 5' initiating triphosphate with vaccinia virus guanylyltransferase [alpha-32P]GTP (in vitro capping reaction). Initiation occurs within the stem of a 12-base palindromic repeat. The initiation sequence has homology with the large (21 S) ribosomal RNA initiation sequence that has been previously determined. We have also sequenced the 5' and 3' ends of the mature 14 S rRNA after labeling with T4 polynucleotide kinase and RNA ligase, respectively. These sequences demonstrate that about 80 nucleotides are cleaved from the 5' end of a precursor to produce the mature 14 S rRNA. This cleavage is imprecise in that the processing occurs at one of five adjacent nucleotides 77 to 81 nucleotides downstream from the 5' initiation site. The 3' ends of this precursor and the mature 14 S rRNA are unique and identical.

Species correlations between cardiac isomyosins. A comparison of electrophoretic and immunological properties

Structural relationships between cardiac isomyosins were analyzed in 10 species using native-gel electrophoresis and radioimmunoassay. In the rat and rabbit, three types of ventricular isomyosin, V1, V2, and V3, were identified by electrophoresis. Monoclonal antibodies specific for the heavy chains of either type V1 or type V3 isomyosin in the rat and rabbit were used for comparison of immunological relationships between atrial and ventricular myosins in other species. Normal guinea pig ventricular myosin reacted with both anti-V2 and anti-V3 antibodies, but only a single myosin band was detected in this species by electrophoresis. When thyrotoxic cardiac hypertrophy was induced in guinea pigs, there was a decrease in myosin reactivity with the anti-V3 antibody and an increase in anti-V1 reactivity. This change in immunological reactivity indicated a change in proportions of two cardiac isomyosins in the guinea pig ventricle even though no myosin heterogeneity was detected by electrophoresis. In six other species including Xenopus, chicken, dog, pig, beef, and human, only a single band of myosin was detected by electrophoresis, and each myosin reacted only with the anti-V3 antibody. In the mouse, three types of ventricular myosin were also detected by electrophoresis. However, unlike V1 isomyosin of the rat and rabbit, mouse V1 isomyosin reacted equally with both anti-V1 and anti-V3 antibodies. In conclusion, we have identified highly conserved epitopes in cardiac myosin, which were found to specifically occur on either the high Ca2+-ATPase type V1 isomyosin or the lower ATPase type V3 ventricular isomyosin in most of the species examined.

The biogenesis and regulation of yeast mitochondria RNA polymerase

Yeast mitochondrial RNA polymerase is a nuclear-coded protein of approximately 90,000 daltons comprised of two 45,000-dalton subunits of pI 6.9 to 7.0. To investigate the nature of the initial translation product of the RNA polymerase, we have analyzed those products of a cell-free translation system directed by yeast RNA that are immunoreactive with antibodies to the 45,000-dalton peptide of polymerase. A precursor of one or more of the subunits of the polymerase, 2,000 daltons later than the mature product, has been characterized using immunoreaction, immunocompetition, and peptide digestion. The role of transcription of the polymerase gene in catabolite repression of mitochondrial development has been investigated by analyzing the changes in cell-free synthesis of the RNA polymerase precursor during glucose and raffinose growth. The results indicate an increase in precursor synthesis and probably in the corresponding transcript abundance during glucose derepression. In contrast, the precursor is present at high levels until stationary phase during raffinose growth. These data indicate the involvement of increased transcription of the polymerase gene in the process of derepression.