The mdx mouse is an X-linked myopathic mutant, an animal model for human Duchenne muscular dystrophy. In both mouse and man the mutations lie within the dystrophin gene, but the phenotypic differences of the disease in the two species confer much interest on the molecular basis of the mdx mutation. The complementary DNA for mouse dystrophin has been cloned, and the sequence has been used in the polymerase chain reaction to amplify normal and mdx dystrophin transcripts in the area of the mdx mutation. Sequence analysis of the amplification products showed that the mdx mouse has a single base substitution within an exon, which causes premature termination of the polypeptide chain.

Amino-acid sequences derived from complementary DNAs encoding the alpha- and beta-subunits of the GABA/benzodiazepine receptor from bovine brain show homology with other ligand-gated receptor subunits, suggesting that there is a super-family of ion-channel-containing receptors. Co-expression of the in vitro-generated alpha-subunit and beta-subunit RNAs in Xenopus oocytes produces a functional receptor and ion channel with the pharmacological properties characteristic of the GABAA receptor.

When gamma-aminobutyric acid (GABA), the major inhibitory neurotransmitter in vertebrate brain, binds to its receptor it activates a chloride channel. Neurotransmitter action at the GABAA receptor is potentiated by both benzodiazepines and barbiturates which are therapeutically useful drugs (reviewed in ref. 1). There is strong evidence that this receptor is heterogeneous. We have previously isolated complementary DNAs encoding an alpha- and a beta-subunit and shown that both are needed for expression of a functional GABAA receptor. We have now isolated cDNAs encoding two additional GABAA receptor alpha-subunits, confirming the heterogeneous nature of the receptor/chloride channel complex and demonstrating a molecular basis for it. These alpha-subunits are differentially expressed within the CNS and produce, when expressed with the beta-subunit in Xenopus oocytes, receptor subtypes which can be distinguished by their apparent sensitivity to GABA. Highly homologous receptor subtypes which differ functionally seem to be a common feature of brain receptors.

The nucleotide sequence of a 3614 base-pair segment of DNA containing the sdhA gene, encoding the flavoprotein subunit of succinate dehydrogenase of Escherichia coli, and two genes sdhC and sdhD, encoding small hydrophobic subunits, has been determined. Together with the iron-sulphur protein gene (sdhB) these genes form an operon (sdhCDAB) situated between the citrate synthase gene (gltA) and the 2-oxoglutarate dehydrogenase complex genes (sucAB): gltA-sdhCDAB-sucAB. Transcription of the gltA and sdhCDAB gene appears to diverge from a single intergenic region that contains two pairs of potential promoter sequences and two putative CRP (cyclic AMP receptor protein)-binding sites. The sdhA structural gene comprises 1761 base-pairs (587 codons, excluding the initiation codon, AUG) and it encodes a polypeptide of Mr 64268 that is strikingly homologous with the flavoprotein subunit of fumarate reductase (frdA gene product). The FAD-binding region, including the histidine residue at the FAD-attachment site, has been identified by its homology with other flavoproteins and with the flavopeptide of the bovine heart mitochondrial succinate dehydrogenase. Potential active-site cysteine and histidine residues have also been indicated by the comparisons. The sdhC (384 base-pairs) and sdhD (342 base-pairs) structural genes encode two strongly hydrophobic proteins of Mr 14167 and 12792 respectively. These proteins resemble in size and composition, but not sequence, the membrane anchor proteins of fumarate reductase (the frdC and frdD gene products).

The nucleotide sequence of the aceF gene, which encodes the dihydrolipoamide acetyltransferase component (E2) of the pyruvate dehydrogenase complex of Escherichia coli Kl2, has been determined using the dideoxy chaintermination method. The aceFgene comprises 1887 base pairs (629 codons excluding the initiation codon AUG); it is preceded by a short intercistronic segment of 14 base pairs containing a good ribosomal binding site, and it is followed closely by a potential rho-independent terminator. The results extend by 1980 base pairs the previously sequenced segment of 3780 base pairs containing the structural gene (ace@ of the pyruvate dehydrogenase component (El) and they confirm that aceE and aceF are the proximal and distal genes of the ace operon.The amino terminus, carboxy-terminal sequence and amino acid composition of the acetyltransferase subunit predicted from the nucleotide sequence are in excellent agreement with previous studies with the purified protein. The predicted molecular weight ( M , = 65959) confirms experimental values derived from sedimentation equilibrium analysis and indicates that the higher values (78 000 -89 000) that have been reported are due to unusual features of the protein that lead to anomalous mobilities during sodium dodecyl sulphate/polyacrylamide gel electrophoresis and in gel filtration. The primary structure fully supports conclusions, based on limited tryptic proteolysis, that the acetyltransferase subunit possesses two heterologous domains : the lipoyl domain and the subunit binding and catalytic domain. The lipoyl domain corresponds to the amino-terminal segment of the protein. It is acidic and contains three remarkably homologous repeating units of approximately 100 amino acids, each possessing a potential lipoyl binding site and a region that is characteristically rich in alanine and proline residues. The subunit binding and catalytic domain occupies most of the residual polypeptide in the carboxy-terminal segment.The pyruvate dehydrogenase complex catalyses the oxidative decarboxylation of pyruvate to acetyl-CoA and CO, via a series of enzyme-bound intermediates [I, 21.It contains multiple copies of three cnzymic components: pyruvate dehydrogenase (E I), dihydrolipoamide acetyltransferase (E2) and lipoamide dehydrogenase (E 3). In Esclzerichiu coli the acetyltransferase component contains 24 identical polypeptide chains forming the structural core of the complex to which the El and E3 subunits are independently bound. The E2 component contains covalently-bound lipoyl cofactors and it participates in the generation of acetyl groups from hydroxyethyl-TPP-El and their transfer to coenzyme A.where TPP is thiamin pyrophosphate. The apparent molecular weight of the E2 subunit has been estimated as 78000 -89000 by sodium dodecyl sulphate/ polyacrylamide gel electrophoresis or gel-filtration in gua- nidine HCI using either purified protein [3 -91 or specificallylabelled products of phage-directed protein synthesis [lo]. However, values of 60000 -65 000 have been obtained b...