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Mouse satellite DNA consists of highly repetitive tandem sequences located in the centromeric heterochromatin. It is generally assumed that these simple sequences are not transcribed. We have analyzed total cellular RNA preparations from mouse liver, kidney, brain, and heart tissues at different ages for satellite transcripts. Using recombinant probes containing the major mouse satellite sequence, satellite transcripts were detected only in the heart RNA samples. These transcripts were not detected in the heart muscle of young adult animals (2 and 6 months), but then appeared at the age of 12 months and continued to increase over 2-fold up to the age of 32 months. The transcripts were resistant to DNase I and sensitive to RNases and alkaline treatment. Northern hybridization experiments showed a large and heterogeneous size range of satellite transcripts. Control studies using short-interspersed (B1 and B2) and long-interspersed (L1 and IAP) repetitive DNA sequence probes did not show a similar age-related pattern of transcription. These results indicate that satellite transcription does occur in mice but is highly tissue- and age-specific. The unique occurrence of satellite transcription only in adult and senescent heart tissue indicates age to be an important determinant of gene activity. An understanding of the regulatory mechanisms involved could lead to new insights in the biological role of satellite DNA, gene derepression of reiterated DNA sequences, and the aging processes of cardiac muscle.

When lipoprotein [a] was isolated in the presence of the proteolytic inhibitor Trasylol, its apoprotein exhibited one dominant band corresponding to a molecular weight of about 1.2 million when analyzed by electrophoresis on 3.25% sodium dodecyl sulfate-polyacrylamide gels. After chemical reduction, this band was missing but was replaced by two bands, one corresponding to a molecular weight of about 490,000 and the other to a molecular weight of about 645,000. Before treatment with reducing agents, the apolipoprotein [a] and apolipoprotein B immunoreactivities were detectable in the same electrophoretic band, but after reduction the apolipoprotein [a] was demonstrated to be separate from the apolipoprotein B. These results suggest that the apoprotein of lipoprotein [a] is composed of two subunits which are similar in molecular weight and are held together by one or more disulfide bonds. One subunit possesses apolipoprotein [a] and the other apolipoprotein B immunoreactivity. The secondary structure of the apoprotein components within lipoprotein [a] has been studied by circular dichroism and found to differ significantly from the secondary structure of the apoproteins in low density lipoproteins and high density lipoproteins. About 30% alpha-helical structure was measured in lipoprotein [a] compared to 48% in low density lipoproteins and 70% in high density lipoproteins. Lipoprotein [a] exhibited a much higher percentage of disordered structure than either of the other two lipoproteins.

We have used photoaffinity labeling of pBR322 DNA with 8-azido-3-amino-5-ethyl-6-phenylphenanthridinium chloride to demonstrate high affinity ethidium-binding sites. Plasmid equilibrated with as little as 1 drug/DNA molecule was photoactivated, freed of uncomplexed drug by ethanol precipitation, and subjected to restriction analysis. There was highly specific, rather than random, blockage of HhaI sites (d(GCGC)) at low drug concentrations. Furthermore, the same 7 new digestion fragments were generated at drug to nucleotide ratios ranging from 1:100 to 1:8000. All the new DNA fragments had chain lengths greater than the largest HhaI fragment (393 base pairs). At higher ligand concentrations closely approximating those needed for equilibrium binding studies, detection of the high affinity sites was greatly masked. Drug binding to HhaI restriction fragments which had been prepared prior to the action of drug did not induce new bands. Furthermore, the larger DNA fragments from drug-labeled plasmid were resistant to HhaI digestion over a wide range of enzyme concentrations. These findings suggest that ligand binding can be highly selective even between sites which have the same tetranucleotide sequence. Therefore, selective drug binding must be dictated not only by local base sequence preference, but also by other long range parameters.